New Quota System¶

https://blueprints.launchpad.net/cinder/+spec/count-resource-to-check-quota

Cinder quotas have been a constant pain for operators and cloud users. This spec proposes a new quota system to resolve inconsistencies in tracking quota usage.

Problem description¶

Cinder’s current quota system is based on reservations and commits/rollbacks. When the API receives an operation that consumes quota, the request is validated and Cinder checks that there’s enough quota to perform the operation, and finally creates reservations to ensure that this quota is not consumed by other operations. Then when the operation finishes these reservations are committed as used resources or rolled back if the operation failed.

The current usage and reservations for resources, e.g. number of volumes or gigabytes, are tracked in the database as a counter in the quota_usages table, so if for whatever reason this counter doesn’t match the real usage, then users may become unable to create new resources or able to create resources beyond their project limits.

There are many possible reasons why resource tracking can become out of sync in Cinder, including bugs in the code and services dying during an operation.

As a workaround for quotas becoming out of sync, the Cinder service has code to make reservations expire after a given time and also has the possibility of resynching the quotas with a certain frequency.

Besides the impact on operators and users, the current quota implementation also has an impact on developers, because the system of reserve/commit and resource counting in the database has the downside of having to be very thorough in order to always keep track of everything, making the quota system a very manual and tedious process to code, which in turn results in hard to find bugs, since we don’t know at which point the counting went wrong.

Low level implementation details of the current quota system are also present everywhere in Cinder, and almost every single area of the code needs to be aware of the low level implementation details, making the code very verbose and obscuring the high level logic.

As an example, we can look at the code of the accept method of the transfer api in file cinder/transfer/api.py, where, at the time of this writing, out of the 106 lines of code that constitute the method, 70 are quota related!

Use Cases¶

A cloud administrator wants to prevent system capacities from being exhausted without notification so it limits quota systems based on the deployments capabilities.
A cloud administrator wants to limit how many resources each department can consume.
A Cinder contributor wants to add a new feature that creates or destroys resources, so it needs to write code to manage the quota.

Proposed change¶

This spec proposes a new quota system where most low level quota details will be hidden from developers working on features, simplifying feature development and the chances of introducing new bugs.

This quota system will support 2 different drivers:

StoredQuotaDriver: This will be similar to the old system, using counters in the DB, but instead of doing reservations and commits/rollbacks for every resource modification, it will only do reservations for the very few operations that really need it to keep track of the resources while the operation is in progress.
DynamicQuotaDriver: This driver will no longer store usage and resource tracking in the database table (quota_usages) and instead dynamically calculates each quota check based on the resources that exist in the database.

Calculations will be counting for resources (e.g. snapshots) or sum of values for sizes (e.g. gigabytes).

Just like the StoredQuotaDriver driver this will use reservations as little as possible.

The reason for having 2 drivers instead of a single one is because there are trade-offs with each of the drivers, and the default will be the DynamicQuotaDriver driver, for reasons explained later in the Performance Impact section.

The changes will try, as much as possible, to avoid over engineering the solution focusing on the 2 new drivers and current cinder and not solve all cases for potential different drivers that will probably never come.

Quota limits¶

In the original implementation of quota classes (quota_classes database table) it was mentioned the possibility of supporting different classes besides the existing default, and being able to pass it via the context, but more than 9 years after its implementation neither Nova nor Cinder support it, so this new quota system and drivers will just focus on the default quota class, which will be referred as system wide defaults, global defaults, global limits, or just defaults since the term is easier to understand.

Limits from the quotas table will be referred as per project limits.

The effective quota limits that apply to a specific project after considering the global and per project limits will be referred simply as quota limits.

The way quota limits are calculated will be kept as they are now:

System wide quota limit defaults are stored in the quota_classes table under the records that have the default value in their class_name column.
Per project quota limit overrides that replace the system wide limits are optional and will be stored in the quotas table.
A hard_limit value of -1 indicates that there are no limits.

Resources¶

The new quota system will not introduce or remove any of the existing quota resources, so available resources for the quota limits will still be: volumes, volumes_<volume-type>, snapshots, snapshots_<volume-type>, gigabytes, gigabytes_<volume-type>, backups, backup_gigabytes, groups, and per_volume_gigabytes. And quota usage will report in-use and reserved values for all existing limits except the per_volume_gigabytes, since there cannot be any usage for it.

Reservation values will be stored in the delta field of the reservations table just like they are today.

For the DynamicQuotaDriver these values will be dynamically added, grouping by resource for non deleted rows belonging to the specific project. On the other hand the StoredQuotaDriver will track the sum in the reserved field of the quota_usages table.

Both drivers will adhere to the following rules when reporting in-use values:

volumes and volumes_<volume-type> quotas must match the number of non deleted rows in the volumes table, with the use_quota field set to true plus the sum of positive values from the reservations table where the resource matches volumes or volumes_<volume-type>, in both cases only those values belonging to the specific project.
snapshots and snapshots_<volume-type> quotas must match the number of non deleted rows in the snapshots table, with the use_quota field set to true, and belonging to the specific project.
gigabytes and gigabytes_<volume_type> quotas must match the sum of the size of the quotable volumes (as defined above) incremented in the sum of the volume_size values of the quotable snapshots (as defined above) when the no_snapshot_gb_quota configuration option is set to false (default value) plus the sum of positive values from the reservations table where the resource matches gigabytes or gigabytes_<volume-type>, in both cases only those values belonging to the specific project.
groups quota must match the number of non deleted rows in the groups table.
backups quota must match the number of non deleted rows in the backups table, and backup_gigabytes must match the sum of their size values.
per_volume_gigabytes is a quota limit that doesn’t need any kind of calculation.

Mechanism¶

The new quota system will rely heavily on database transactions and database row locking using the SELECT FOR UPDATE SQL statement to control parallel operations and ensure quota limits are honored and all database changes happen or they are automatically rolled back.

A high level view of how this mechanism would work is:

Start a transaction
Get current quota limits creating a lock on those rows
Check operation doesn’t go over quota
Create the resource on the database or make reservations
Finish the transaction releasing the lock

The lock will only happen on the rows of the resources we are interested in, allowing operations on other projects and resources to be executed in parallel. For example, quota checks to create a volume will lock rows for volumes, volumes_<volume-type>, gigabytes, and gigabytes_<volume-type>, so cinder will be able to check for the quota to create a backup since that only requires backup and backup_gigabytes.

The new system will leverage Python context manager functionality and the Oslo DB transaction context provider available in the Cinder RequestContext (context.session) to facilitate the sharing of the transaction/session between different areas of the Cinder code.

This will allow developers to write cleaner code, for example when creating a volume, the create method of the Cinder Volume Oslo object will have to check that it can create 1 volume, that will consume additional gigabytes and that the size of the volume doesn’t exceed the largest volume size allowed, so the code will be something like this:

with self.quota_check(self._context, self.volume_type.id,
                      vol_gbs=self.size,
                      vol_qty=1,
                      vol_size=self.size):

   db_volume = db.volume_create(self._context, updates)

The quota_check is a property in the Volume OVO that returns a context manager that ensures quota limits are honored. Returned context manager depends on whether the volume consumes quota or not, returning a noop if it doesn’t and returning a context manager provided by the quota driver if it does.

The quota driver context manager starts a DB session/transaction in the provided context so the volume_create call will use that same session to create the volume record, and the transaction will be finalized when the code exits the context manager, thus ensuring that no other operations check the quota until the volume has been created.

From a developer’s point of view all this will be hidden, because at a higher level all they need to do is create the Volume OVO and the quota will be automatically checked. As an example this is the code in the create volume flow (cinder/volume/flows/api/create_volume.py):

volume = objects.Volume(context=context, **volume_properties)
volume.create()

In an effort to abstract the quota system implementation and hide its details from most of the code, the code interfacing with the driver directly will no longer use the resource names such as gigabytes and volumes_<volume-type>, instead the parameters that will be used for the volume and snapshot context manager checker are:

vol_qty: Delta on the number of volumes that will be consumed within the checker context manager. The quota system internal name for this is volumes in the database.
vol_type_vol_qty: Delta on the number of volumes for the specific volume type that will be consumed within the checker context manager. Defaults to the value of vol_qty since that’s the most common case. The quota system internal name for this is volumes_<volume-type> in the database.
vol_gbs: Delta on the number of volume gigabytes that will be consumed within the checker context manager. The quota system internal name for this is gigabytes in the database.
vol_type_gbs: Delta on the number of volume gigabytes for the specific volume type that will be consumed within the checker context manager. Defaults to vol_gbs since that’s the most common case. The quota system internal name for this is gigabytes_<volume-type> in the database.
snap_qty: Delta on the number of snapshots that will be consumed within the checker context manager. The quota system internal name for this is snapshots in the database.
snap_type_qty: Delta on the number of snapshots for the specific volume type that will be consumed within the checker context manager. Defaults to the value of snap_qty since that’s the most common case. The quota system internal name for this is snapshots_<volume-type> in the database.
snap_gbs: Delta on the number of snapshot gigabytes that will be consumed within the checker context manager. Will end up using the quota system internal name of gigabytes if the no_snapshot_gb_quota configuration option is set to false (default) or will be disregarded if set to true.
snap_type_gbs: Delta on the number of snapshot gigabytes for the specific volume type that will be consumed within the checker context manager. Defaults to the value of snap_gbs since that’s the most common case. The quota system internal name for this is gigabytes_<volume_type> in the database if the no_snapshot_gb_quota configuration option is set to false or will be disregarded if set to true.
vol_size: Total volume size when creating or extending it, in the internally the quota system uses the per_volume_gigabytes quota limit to check this value.

This change may seem worthless, but it has its value, because it abstracts the implementation details of the snapshots and volumes sharing the same quota size limits which provides:

Cleaner code since snapshot creation or transfer of a volume with snapshots doesn’t need to know about the no_snapshot_gb_quota configuration option.
If we want to add, in the future, snapshot specific quota limits - snapshot_gigabytes and snapshot_gigabytes_<volume-type>- we’ll be able to do so without affecting any of the Cinder code with the sole exception of the quota driver itself.

Reservations¶

For the new quota system the reservation commit and rollback operations will be grouped into a single context manager that handles both cases. Committing and rolling back reservations have different meanings for the 2 drivers.

For the DynamicQuotaDriver these are noop operations, since checks use the DB values every time and the database has already been modified in the same transaction that the reservations are removed. On the other hand the StoredQuotaDriver needs to modify the in_use and reserved counters in the quota_usages table accordingly to the operation.

As mentioned before, reservations will only be necessary for specific operations, to be exact on 3 operations: extend, transfer, and retype.

Each of these operations have different reasons for requiring reservations:

Extend: Until the operations completes, the size field of the volume in the database must be kept as it is to reflect its real value, but we need to reserve the additional gigabytes, for gigabytes and gigabytes_<volume_type> quotas, during the operation so we don’t go over quota due to other concurrent operations. If the operation completes successfully the size of the volume will be increased and the reservations will be committed.
Transfer: Under normal circumstances accepting a transfer would not require the use of a reservation, as we should be able to check the quota and do the database changes to accept the transfer in the same transaction. Unfortunately the SolidFire and VMDK drivers need to make some changes in their backend on transfer, so the volume service has to make a driver call.

We cannot keep the database locked while the driver call completes, as it can take some time and we don’t want to prevent the API from processing other operations.

That is why reservations will be created before calling the driver and cleared after accepting the resources.

In terms of reservations, transfers are complex for the StoredQuotaDriver, because when completing one it needs to modify 2 different projects. One to increase counters and the other to decrease them, so higher levels will need to make 2 different calls for 2 different projects, one with positive and one with negative numbers and negative numbers should ignore quota usage and limits.

When storing reservations for transfer of volumes with snapshots they have to be stored separately in case someone restarts the service after changing the no_snapshot_gb_quota_toggled configuration option as detailed in the vol_snap_check_and_reserve_cm method below.
Retype: When doing a retype the API needs to reserve gigabytes_<dest-volume-type> and volumes_<dest-volume-type> until the operation is completed as well as create negative reservations for gigabytes_<source-volume-type and volumes_<source-volume-type>.

This consumes volumes and gigabytes on both types until the operation completes for the following reasons:
- If the retype fails we will continue consuming volumes and gigabytes on the source volume type, but if we “released” that usage when we started the operation we may find that there is no longer enough quota available for the volume to stay there. This is the main reason.
- Even if the retype succeeds Cinder doesn’t know the reasons why the cloud administrator has set the quota limits, so freeing the source gigabytes and volumes as soon as the retype starts means that if a new volume for the source type is created during the retype Cinder will be exceeding the quota for that volume type.
This is the only operation where a race condition can happen, though it’s a corner case. It can happen if we are adding a new quota limit, global or per project, to a volume type resource (e.g. volume_<volume_type>) that didn’t have any limit in the database while at the same time we are doing volume retypes to that same volume type. This race should fall within reasonable expectations, as one would argue that the limit was added right after the retype already passed the quota check.

It is possible that while doing an operation on a resource the code flow doesn’t complete in an unexpected way leaving leftover reservations in the database, for example:

A coding bug in Cinder that leaves the volume in an unexpected status.
Service kill.
Node restart or shutdown.

For these situations the new quota system will add code to the os-reset_status REST API action on volumes to automatically clear any reservations that the volume may have when the status is changed, which is what happens when a volume is stuck in extending, retyping, etc. This way there is no need to wait until the reservation expires and the operator can do the cleanup in an easy way without needing additional API calls.

On volume deletion the code will also clear any existing reservations on the specific volume.

To facilitate the cleanup of these reservations the volume’s id will be used as the uuid field for all the reservations, instead of creating a random one, regardless of the value of the resource field in the reservations table.

Both drivers will create reservations the same way to facilitate switching the drivers without having usage numbers go out of sync.

Changing configuration¶

There are 2 Cinder configuration options that are crucial for the new quota system to operate correctly: quota_driver and no_snapshot_gb_quota.

The no_snapshot_gb_quota configuration option is used to determine whether snapshots should be counted towards the volume quota or not, so this is not something we want to be counting in some places and not counting in others; we want a consistent behaviour through all the Cinder services, which means that they must have the same value.

Currently Cinder has no way of enforcing the same value for the no_snapshot_gb_quota, and what’s worse, it cannot even know when the current quota calculations have become invalid because this configuration option has changed (Bug #1952635).

This is something we definitely don’t want in the quota system, and with the new quota system we have bigger problems, because it’s not only no_snapshot_gb_quota that can be changed, but also quota_driver, and changing the quota driver means that a quota system may need to recalculate things to ensure that it starts operating with the correct quota assumptions. For example when changing from the DynamicQuotaDriver to the StoredQuotaDriver all the counters in the DB will be wrong, so the StoredQuotaDriver needs to calculate the counters before it can start working or the whole quota system will not operate correctly.

These configuration options are not the kind of things that are frequently changed, and we expect most deployments to never have to change them at all, but Cinder should still provide a way for them to be safely changed since one of the cases we expect to happen is a deployment outgrowing the usefulness of the DynamicQuotaDriver and running into performance issues. In that case they will want to switch to the StoredQuotaDriver.

To support changing configuration option changes to the quota system there are 3 things that the new quota system needs to be able to do:

Detect changes in configuration options.
Signal drivers that the no_snapshot_gb_quota configuration option has changed and for driver to react to this change.
Signal drivers that they were not the quota driver that was running on the last start and they should see if they need to do some calculations.

To detect changes to these configuration options, a new global_data table will be created to store the currently used configuration values. This table will be used to signal quota drivers when things have changed.

A system administrator will have to follow these steps to change any of these 2 configuration options:

Stop all Cinder services.
Change the cinder.conf file in all the nodes where a Cinder service is running.
Run a cinder-manage command to apply the changed options.
Restart Cinder services.

The cinder-manage command will not only trigger the quota system recalculations, but it will also make the appropriate DB changes in the global_data table to reflect the new configuration options that are in effect.

Since we cannot allow Cinder services to run with mismatching configuration options they will fail to start if the quota configuration options from the database don’t match the one that the service has. This will prevent system administrators making an error and only realize it after their whole system has some crazy quotas.

Please see the Changing configuration alternatives for other possible mechanism to the one proposed here.

Interface¶

Here is the proposed interface for the new quota system drivers:

NAME¶

Unique string of maximum 254 ASCII characters that identifies the driver.

init¶

def __init__(self, driver_switched, no_snapshot_gb_quota_toggled):

Initialization method for the quota driver where the driver_switched parameter indicates whether the last run was done using the same Quota driver or if a different one was used and this is the first run with this one.

This is important because switching to the StoredQuotaDriver from the DynamicQuotaDriver means that in-use and reserved counters need to be recalculated since they could be out of sync or missing altogether.

This effort is going to focus on only supporting these 2 quota drivers and avoid unnecessary complexity, because if we wanted to support other kind of drivers that were not based on the Cinder database we would need to add a more complex mechanism, since the cinder code would need to notify drivers when the limits are changed in the DB and there would need to be a way for Cinder to request information from the old quota driver, such as current reservations, when switching.

The interface can be enhanced if a future quota driver finds it insufficient.

The no_snapshot_gb_quota_toggled parameter indicates whether the option has changed since the last run. This is important for the StoredQuotaDriver that would need to recalculate in-use and reserved counters. This is something that doesn’t work correctly right now.

Drivers can block the Cinder database when synchronizing when the driver has been switched or the snapshot quota configuration option has been toggled, because the driver will only be called with any of the parameters set to True on a single service in the deployment and the quota will not be in use at that time by any other service.

resync¶

def resync(self, context, project_id):

This is only relevant for the StoredQuotaDriver, and is intended to allow the cinder-manage command request a recalculation of quotas for a specific project or for the whole deployment.

set_defaults¶

def set_defaults(self, context, **defaults):

Set system wide default limits.

The keys for the keyword arguments defaults are in the internal form of the quota system, that is to say, they will be gigabytes and not vol_gbs.

This will be a common implementation for both database based quota drivers, where it modifies the record if it exists and creates it if it doesn’t.

set_limits¶

def set_limits(self, context, project_id=None, **limits):

Set project specific limits.

The keys for the keyword arguments limits are in the internal form of the quota system, that is to say, they will be gigabytes and not vol_gbs.

This will be a common implementation for both database based quota drivers, where it modifies the record if it exists and creates it if it doesn’t.

clear_limits_and_defaults_cm¶

def clear_limits_and_defaults_cm(self, context,
                                 project_id=None, type_name=None):

This context manager removes, on exit, all existing per project limits, for when a project is deleted, or all type specific global defaults and per project limits, for when a type is deleted.

Parameters project_id and type_name will be used used as filter in the deletion. So if only project_id is provided then only per project entries will be deleted (in the db driver those from the quotas table), and if only the type_name is provided then only gigabytes_<type-name>, volume_<type-name> and snapshots_<type-name> resources will be removed but for per-project (quotas db table) and global (quota_classes table).

If an error occurs within the context manager the limits and defaults should not be cleared.

This will be a common implementation for both database based quota drivers.

type_name_change_cm¶

def type_name_change_cm(self, context, old_name, new_name,
                        project_id=None):

Context manager to make necessary modification, on enter, to system wide defaults and per project limits to account for a volume type name change.

This will rename gigabytes_<old_name>, volume_<old_name> and snapshots_<old_name> to gigabytes_<new_name>, volume_<new_name> and snapshots_<new_name> respectively in all tables.

The database change to the volume type name is called within this context manager to ensure that the quota defaults and limits stay in sync with the volume type name and we don’t change one but not the other.

This will be a common implementation for both database based quota drivers.

get_defaults¶

def get_defaults(self, context, project_id=None):

Returns system wide defaults for quota limits. If project_id is not None then volume types quota resources (volumes_<volume-type>, gigabytes_<volume-type>, and snapshots_<volume-type>) will be filtered based on the project’s visibility of the volume types, if project_id is None then all defaults will be returned regardless of the is_public value of the volume types.

In terms of volume type visibility, a project can view all public volume types and private ones where it has permissions (entries in the volume_type_projects table).

System wide defaults are stored in the database in the quota_classes table with the default value on the class_name.

Returned data is a dictionary mapping resources to their hard limits, and must include all volume type resources even if there is no record in the database.

In the following example of returned data the gigabytes_lvmdriver-1, volumes_lvmdriver-1, and snapshots_lvmdriver-1 are not present in the database:

{
 "per_volume_gigabytes": -1,
 "volumes": 10,
 "gigabytes": 1000,
 "snapshots": 10,
 "backups": 10,
 "backup_gigabytes": 1000,
 "groups": 10,
 "gigabytes___DEFAULT__": -1,
 "volumes___DEFAULT__": -1,
 "snapshots___DEFAULT__": -1,
 "gigabytes_lvmdriver-1": -1,
 "volumes_lvmdriver-1": -1,
 "snapshots_lvmdriver-1": -1
}

This will be a common implementation for both database based quota drivers.

get_limits_and_usage¶

def get_limits_and_usage(self, context, project_id, usages=True):

Get all effective quota limits for a specific project, and optionally quota usage, for a specific project. If project_id is None the one from the context will be used.

Volume types quota resources (volumes_<volume-type>, gigabytes_<volume-type>, and snapshots_<volume-type>) will be filtered based on the project’s visibility of the volume types.

A project can view all public volume types and private volume types where it has permissions (entries in the volume_type_projects table).

A quota limit values defined in the quotas table overrides global values from the quota_classes.

Returned data will always be a dictionary (or defaultdict), but the contents will depend on whether we are getting quota usage or not. Just like the get_defaults method this returns all volume type resources even if there is no record in the database.

{
 "per_volume_gigabytes": -1,
 "volumes": 8,
 "gigabytes": 1000,
 "snapshots": 10,
 "backups": 10,
 "backup_gigabytes": 1000,
 "groups": 10,
 "gigabytes___DEFAULT__": -1,
 "volumes___DEFAULT__": -1,
 "snapshots___DEFAULT__": -1,
 "gigabytes_lvmdriver-1": -1,
 "volumes_lvmdriver-1": -1,
 "snapshots_lvmdriver-1": -1
}

With quota usage returned value will look like this:

{
 'per_volume_gigabytes': {'limit': -1, 'in_use': 0, 'reserved': 0},
 'volumes': {'limit': 8, 'in_use': 1, 'reserved': 0},
 'gigabytes': {'limit': 1000, 'in_use': 1, 'reserved': 0},
 'snapshots': {'limit': 10, 'in_use': 0, 'reserved': 0},
 'backups': {'limit': 10, 'in_use': 0, 'reserved': 0},
 'backup_gigabytes': {'limit': 1000, 'in_use': 0, 'reserved': 0},
 'groups': {'limit': 10, 'in_use': 0, 'reserved': 0},
 'gigabytes___DEFAULT__': {'limit': -1, 'in_use': 0, 'reserved': 0},
 'volumes___DEFAULT__': {'limit': -1, 'in_use': 0, 'reserved': 0},
 'snapshots___DEFAULT__': {'limit': -1, 'in_use': 0, 'reserved': 0},
 'gigabytes_lvmdriver-1': {'limit': -1, 'in_use': 1, 'reserved': 0},
 'volumes_lvmdriver-1': {'limit': -1, 'in_use': 1, 'reserved': 0},
 'snapshots_lvmdriver-1': {'limit': -1, 'in_use': 0, 'reserved': 0}
}

group_check_cm¶

def group_check_cm(self, context, qty=1, project_id=None):

Context manager to check group quota upon context entering.

Raises QuotaError if quota usage would go over the quota limits upon adding qty new groups.

Effective quota limits are determined based on the project’s quota limits (hard_limit for the groups resource in the quotas table) if defined or the global defaults (in the quota_classes table) otherwise.

The project is determined by the project_id parameter or the context’s project_id if the optional project_id parameter value is None.

The context manager must ensure that there are no race conditions with concurrent calls to group_check_cm within different threads and processes in the node as well as across different nodes.

For the database driver this can be achieved using a SELECT FOR UPDATE on the groups quota limit which blocks other requests until the context manager exists.

Users of this context manager should try to keep the code within the context manager to a minimum to allow higher concurrency.

For the DB driver, the context manager will start a database transaction/session, making it available in the session attribute of the provided context, and this transaction will be committed if the code enveloped by the context manager completes successfully, but if an exception is raised in the enveloped code then the transaction will be rolled back. So this context manager not only checks the quotas but also provides a transaction context.

An example of using this context manager within the create method of the Group Oslo Versioned Object:

with quota.driver.group_check_cm(self._context, qty=1):

    db_groups = db.group_create(self._context,
                                updates,
                                group_snapshot_id,
                                source_group_id)

group_free¶

def group_free(self, context, gbs, qty=1, project_id=None):

Context manager to free group quotas upon context exiting. The DB row soft deletion of groups will be enclosed by this call.

This is only relevant for the StoredQuotaDriver that needs to decrease its counters.

backup_check_cm¶

def backup_check_cm(self, context, gbs, qty=1, project_id=None):

Context manager to check backup quotas upon context entering.

Raises QuotaError if quota usage would go over the quota limits upon adding qty backups or gbs backup gigabytes.

Effective quota limits are determined based on the project’s quota limits (hard_limit for the backups and backup_gigabytes resources in the quotas table) if defined or the global defaults (in the quota_classes table) otherwise.

The project is determined by the project_id parameter or the context’s project_id if the optional project_id parameter value is None.

The context manager must ensure that there are no race conditions with concurrent calls to backup_check_cm within different threads and processes in the node as well as across different nodes.

For the database driver this can be achieved using a SELECT FOR UPDATE on the backups and backup_gigabytes quota limits which blocks other requests until the context manager exists.

Users of this context manager should try to keep the code within the context manager to a minimum to allow higher concurrency.

An example of using this context manager within the create method of the Backup Oslo Versioned Object:

with quota.driver.backup_check_cm(self._context, qty=1, gbs=self.size):

    db_backup = db.backup_create(self._context, updates)

backup_free¶

def backup_free(self, context, gbs, qty=1, project_id=None):

Context manager to free backup quotas upon context exiting. The DB row soft deletion of the backup will be enclosed by this call.

This is only relevant for the StoredQuotaDriver that needs to decrease its counters.

vol_snap_check_and_reserve_cm¶

def vol_snap_check_and_reserve_cm(self, context, type_id, type_name=None,
                                  project_id=None,
                                  *,
                                  uuid=None,
                                  vol_gbs=0, vol_qty=0,
                                  vol_type_gbs=None, vol_type_vol_qty=None,
                                  snap_gbs=0, snap_qty=0,
                                  snap_type_gbs=None, snap_type_qty=None,
                                  vol_size=0):

Context manager that, upon entering, checks volume and snapshot quotas and optionally makes reservations.

Volumes and snapshot are tightly coupled resources, since a snapshot cannot exist without a parent volume, so their quota checks are handled jointly in the same method.

Raises QuotaError if quota usage would go over the quota limits upon consuming provided resources:

vol_qty number of volumes being reserved.
vol_gbs additional volume gigabytes.
vol_type_vol_qty volumes of the specified volume type. Defaults to the value of vol_qty.
vol_type_gbs additional volume gigabytes of the specified volume type. Defaults to the value of vol_gbs.
snap_qty snapshots.
snap_gbs additional snapshot gigabytes.
snap_type_qty snapshots of the specified volume type. Defaults to the value of snap_qty.
snap_type_gbs additional snapshot gigabytes of the specified volume type. Defaults to the value of snap_gbs.

Unlike the vol_gbs, vol_type_gbs, snap_gbs, and snap_type_gbs parameters, the vol_size is not an increment over existing consumption, but an absolute value representing the total size of the volume. And the context manager also raises a QuotaError exception if it is greater than the per_volume_gigabytes limit.

Effective quota limits are determined based on the project’s quota limits volumes, volumes_<volume-type>, snapshots, snapshots_<volume-type>, gigabytes, gigabytes_<volume_type>, and per_volume_gigabytes if defined in the quotas table or the global defaults defined in the quota_classes table otherwise.

The project is determined by the project_id parameter or the context’s project_id if the optional project_id parameter value is None.

The volume type name (type_name) is necessary to perform quota checks, but the method can query this information based on the type_id. Due to current Cinder behavior (where a type can be changed to private even when projects have volumes) then the quota driver needs to confirm that the project still has access to it.

Volumes and snapshots are currently the only resources that can have reservations, and this method automatically creates them when a uuid is provided. This uuid must be of the primary resource for the operation, that is to say that if we are transferring a volume with all its snapshots the reservations will pass the volume’s uuid.

Both drivers must use different entries for volume and snapshot gigabyte reservations because the no_snapshot_gb_quota_toggled configuration option may be changed and the service restarted before a transfer is accepted, and the StoredQuotaDriver will need to make a decision both when recalculating (if driver has changed) and on transfer accept.

This context manager must ensure that there are no race conditions with concurrent calls to vol_snap_check_and_reserve_cm within different threads and processes in the node as well as across different nodes.

For the database driver this can be achieved using a SELECT FOR UPDATE on the volumes, volumes_<volume-type>, snapshots, snapshots_<volume-type>, gigabytes and gigabytes_<volume_type> quota limits which blocks other volume and snapshot requests until the context manager exists.

Users of this context manager should try to keep the code within the context manager to a minimum to allow higher concurrency.

When creating reservations the context manager must ensure that they are cleaned up if an exception is raised within the context manager. For the DB driver the context manager will start a database transaction/session, making it available in provided context, and will commit everything on normal context manager exit and roll everything back, including the reservations, when an exception is raised.

An example of using this context manager within the create method of the Volume Oslo Versioned Object:

with self.quota_check(self._context, self.volume_type.id,
                      volume_type and volume_type.name,
                      vol_gbs=self.size,
                      vol_qty=1,
                      vol_size=self.size):

    db_volume = db.volume_create(self._context, updates)

Where quota_check is a property that takes into account whether the volume uses quota or not:

@property
def quota_check(self):
    if self.get('use_quota', True):
        return quota.driver.vol_snap_check_and_reserve
    return self.nullcontext

vol_snap_free¶

def vol_snap_free(self, context, type_id, type_name=None, project_id=None,
                  *,
                  vol_gbs=0, vol_qty=0,
                  vol_type_gbs=None, vol_type_vol_qty=None,
                  snap_gbs=0, snap_qty=0,
                  snap_type_gbs=None, snap_type_qty=None):

Context manager to free volume and snapshot quotas upon context exiting.

This is only relevant for the StoredQuotaDriver that needs to decrease its counters.

reservations_clean_cm¶

def reservations_clean_cm(self, context, resource_uuid, commit=True):

Context manager that cleans all reservations, committing or rolling back, for the given uuid on exit.

The uuid is the “primary” uuid of the operation and it won’t be a different uuid for each resource that has been reserved. E.g. when accepting a volume transfer with its snapshots, all reservations will use the volume’s id.

For the DynamicQuotaDriver this is mostly just deleting the entries from the database, but for the StoredQuotaDriver it needs to adjust the in-use and reserved counters.

These counters may be from different projects, for the transfer of volumes, so the context’s project_id will be ignored.

The DynamicQuotaDriver driver must also take into account the no_snapshot_gb_quota_toggled configuration option when committing a transfer, because the snapshot reservations are stored in different row entries in case the option is changed and the service rebooted before a transfer is accepted.

Differences¶

There are some differences between the new and old system that are worth highlighting:

Resource consumption rules stated in the Resources section of this spec are absolute, so it doesn’t matter if a volume becomes in error status because scheduling failed or because the driver call on the volume service failed. If there is a quotable database record, then it will be counted towards the quota.
Negative reservations, created when retyping a volume, won’t be taken into account in usage calculations, because like we mentioned before we want the volumes_<volume-type> and gigabytes_<volume-type> of the source type to still be consumed while we do the operations, since we don’t know if we’ll succeed or not, and on failure we would need to consume them again.
The new quota system drops the illusion that Cinder can support multiple ORM systems and accepts the fact that Cinder is tightly coupled with SQLAlchemy and MySQL/InnoDB (this is not new, there is already a patch proposed that removes the intermediate layer, so instead of having all the quota code in cinder/db/sqlalchemy/api.py it will be under cinder/quota, including all the database queries.

This approach has the downside of having DB code in multiple places, with potential code duplication, but on the other hand it has the great benefit of having the quota code contained in fewer files and using less memory for custom quota drivers (currently the standard quota driver file is always loaded even if it’s not instantiated).
All deployments will use the default quota class instead of supporting the already deprecated configuration file quota limits.
The new quota system fixes a number of existing bugs, so there are some undesired behaviors that change:
- Now listing quota limits and quota usages won’t show private types the project doesn’t have access to (bug #1576717).
- A limit of 0 will be shown if a project has resources for a type we no longer have access to because it was made private after the resource was created (related bug #1952456). This can also happen if an admin creates a volume for a private type that the project doesn’t have access to.

Limitations¶

Spec is aimed at these 2 drivers, so additional drivers may not be easy to add. Though it shouldn’t be a problem if these 2 work as expected.
There is a bottleneck in concurrent code execution, because the code locks on the system wide defaults, which are common to all projects. So, even if the critical section code enveloped by the check context managers is very small, it will still limit to only 1 context entering at a time for the whole deployment for the given quota limits. As an example, if we are concurrently creating 100 volumes in as many projects, they will be happening mostly in parallel, but once they reach the point to check quota limits and create the DB record they will be serialized.
Race condition on the retype operation as explained in the reservations section.

Alternatives¶

Work with what we have¶

Some of the alternatives include:

Carefully go through the Cinder code looking for potential error causes and fixing them.
Refactor existing quota code to move part of the Quota Python logic into database queries.
Refactor existing code to reduce spillage of the quota system implementation details all over the code and reduce the usage of reservations to only the strictly necessary cases.

These alternatives have the same underlying issue as the current implementation, where it would be hard to tell whether we have resolved all the issues or not, and upon encountering another out of sync case in a deployment we would be, once again, in a position where we cannot tell how we reached that point.

Unified Limits¶

Another alternative is to use the KeyStone Unified Limits. At first glance this may seem like a perfect solution since it:

Allows for a unified limit system in all OpenStack (once all projects implement it).
Supports different enforcement models, including hierarchies.

But upon closer inspection it’s not without its disadvantages:

While Glance and Nova implemented its usage in the Yoga release this still cannot be considered a proven solution since there have not been enough time for users to actually evaluate it.
The Unified Limits system does not have any mechanism to prevent race conditions between concurrent operations. So we’ll have to implement our own mechanism that needs to work across all the Cinder services. It can be with a DLM, some database locking, or how Nova is going to do it, which is to check the limit, commit claim, then check limit again and revert if over usage is detected. The Nova mechanism means that we are always doing a double check and sometimes a revert, and we can even get a false failure due to a double race condition on the checks (2 concurrent requests pass the initial check and then both fail on the confirmation check, whereas only 1 of them on its own would have succeeded).
The oslo.limit project will fail a limit check if the limit has not been previously registered in KeyStone, which is the opposite of how our quota system currently behaves, as it assumes unlimited (-1). This means that Cinder will either have to manage the registrations of the limits when we create or destroy volume types, when a project is given access to a volume type, when a volume type’s public status changes, etc. or to force operators to manage all this on their own. A more reasonable alternative would be to modify the oslo.limit project to support alternative behavior on non defined values.
It will be slower since we have to call an external service, KeyStone, for the limit check, which has to check the user making the call, go to the database, etc. And because each of the resources that are checked requires its own REST API call to KeyStone

This could be improved in Keystone to allow multiple simultaneous checks.
Hierarchical support using the Strict Two Level enforce mechanism isn’t implemented in oslo.limit

Fix bottleneck¶

As mentioned before, in the proposed quota system there is a bottleneck in the concurrent code execution due to the DB locking, because it is locking using entries from the quotas table which are shared among all projects.

To resolve this bottleneck using the DB locking we would need to duplicate the system wide defaults. These entries can be duplicated in the quotas table or in the quota_classes table.

If they are duplicated into the quotas table, then a new column would need to be added (is_default) to flag the contents as being defaults or not. Because when a global default from the quota_classes table is changed it would need to be changed in the quotas table records that have the default values but not on the records that have been explicitly set, even if they have the same value as the default.

If the quota_classes table is used, then we would store the project_id into the name column, which means that we would have trouble in the future if we ever wanted to fully implement the Quota Classes concept. Though this is unlikely given how long it has been since the table was created and how the concept has never been implemented.

When setting a global default quota limit we would need to remove the restriction on the name column being default when using the quota_classes and if we used the quotas table we would need an additional query besides the one to the default quota_classes record, as we would need to update non deleted records from the quotas table for that resource that have the is_default value set to true.

Another thing to consider is that Cinder doesn’t know beforehand what projects exist, so it will also need to dynamically duplicate the global default quota records if they are not present when the first operation on a project is called. This can be done efficiently, only incurring into additional queries on the first request for a project, by assuming the values exist and querying with locking on them, and only if the result is missing the values we go and duplicate the global defaults.

This dynamic duplication is also tricky, because we don’t want to have races with a global quota limit update request or with other operation that triggers the same duplication. These 2 races can be prevented with a SELECT ... FOR UPDATE on the quota_classes table.

At the time of writing, we are hoping that the bottleneck is not significant enough to warrant the extra effort of removing it. If time proves us wrong we can go and implement one of these or other solution.

Changing configuration alternatives¶

The Changing configuration subsection of the Proposed change section presented the chosen mechanism to change quota_driver and no_snapshot_gb_quota configuration options, but those are not the only possibilities.

This subsection presents 2 alternatives to make changes to the options and ensure that all Cinder services run with the same configuration option values:

Build a complex system to orchestrate the change on running services: Signal the change to all Cinder services and make sure they complete ongoing quota operations before signaling the quota driver that it needs to do recalculations, then signal services that they have to reload the quota driver and finally continue their operation.

Implementing this is quite complex, starting with how difficult is to make sure that there are no services that have missed the notification: A service may have a temporary loss of connection to RabbitMQ or the DB.

We also have the difficulty of hitting pause in all running operations among others.
Only allow changing the configuration option when all services are down. Cinder services would be smart enough to detect on start that the configuration has changed and confirm that they are the only service that is currently running and can proceed to tell the quota driver that it needs to do the recalculations.

We find multiple challenges when trying to make Cinder smart enough to detect that there are no other services running:
- We have no way of knowing if a Cinder API service is running or not because they don’t issue DB heartbeats and they don’t receive any RPC calls via RabbitMQ. We can make them issue DB heartbeats and we may even make them listed on a message queue for RabbitMQ messages.
- When starting all Cinder services at the same time they need to avoid racing to tell the driver to recalculate the quotas. This can be resolved locking for update a DB row to prevent the race and allowing only 1 service to do the calculation.
- Even if all services are brought down, the DB heartbeat for the services won’t timeout for a while, so Cinder will have to wait until those heartbeats time out. This introduces an unnecessary delay on the Cinder restart.
- Spurious/temporary network issues that may make Cinder think that there are no running services. This is actually the biggest issues.

Since changing the quota configuration options is not something that’s going to be frequently changed, we actually expect it to be change once at most, passing the responsibility of doing it right to the system administrator seems like the best choice.

In any case this is not something that has to be like that forever. If this behavior becomes a real problem we can change it in the future.

Data model impact¶

The new quota system will place greater importance on the database queries and reduce the Python code, so the database needs to be ready to perform counting queries efficiently.

The main change to ensure efficiency will be adding the proper indexes to resource tables. The index that we’ll need to have is one on the project_id and deleted columns for the following tables (that currently don’t have them):

volumes
snapshots
backups
groups

This changes will bring additional benefits to the Cinder service, because right now listing resources on a deployment with many projects or with many deleted resources is not efficient, because the database has to go through all the resources to filter out non delete rows that belong to a specific project (bug #1952443).

Existing quota_usages tables will no longer be used, and it will be removed on the next release after the rollout of the new quota system.

The reservations table will remain in use, although only for a couple of operations.

Additionally we’ll need to track the current quota driver that is being used as well as the no_snapshot_gb_quota configuration option to be able to tell the quota drivers if they have changed.

For this purpose the proposal is to create a new table that can store global cinder information.

Table global_data will have the following fields:

created_at: When this key-value pair was created
updated_at: When this key-value pair was last updated
key: String value describing the value. For example no_snapshot_gb_quota or quota_driver.
value: String with the value of the key. For example true or StoredQuotaDriver.

REST API impact¶

There will be no REST API impact, because we currently only expose the usage and reservations (quota_usages table) through a listing API that we’ll still be able to provide with the current quota driver interface, and we still have reservations (even if fewer operations use them) so that information still remains relevant and we don’t need to remove it from the response.

Security impact¶

None.

Active/Active HA impact¶

None.

Notifications impact¶

No notification impact, since no new operations are added or removed.

Other end user impact¶

End users of the Cinder service should not have significant impact, except for how the quota is counted.

Current behavior can be erratic on how quota is counted, as it will depend on how and where things fail, so we can have volumes in ERROR status that have been counted towards quota and others that have not.

With this new approach the quota consumption rules will be very straightforward, users/admins just need to list resources and add all that have the consumes_quota field set to true to check if usage is correct.

This stable behavior, can have a positive impact on a deployment regardless of how services are going down, since end users will not be able to go over their allowed quota and be forced to clean failed resources instead of being able to leave them be.

Performance Impact¶

Some preliminary code was prototyped for the volume creation and get usage operations to evaluate the performance of the different quota drivers: the old, the new StoredQuotaDriver, and the new DynamicQuotaDriver.

The results showed that the new StoredQuotaDriver system was twice as fast as the old code in both operations, and the DynamicQuotaDriver was slower than the StoredQuotaDriver, as expected, but faster than the old one until there are around 26000 resources per project.

So the DynamicQuotaDriver is less likely to be out of sync with reality because it doesn’t store fixed values, but the StoredQuotaDriver has better performance, and that’s the reason why both drivers are going to be implemented, to allow system administrators decide which one is better for them.

The default driver will be DynamicQuotaDriver to prioritize the usage values always stay in sync, and large deployments or those looking for best performance will be able to use the StoredQuotaDriver.

Deployments may even start with one quota system and then switch to the other if necessary.

Other deployer impact¶

As soon as the new code is deployed and executed the new quota system will be used, there will be no backward compatibility support for old quota code.
Deployments using a custom external quota driver will no longer be able to start. This should not be a problem as we believe there is nobody using a custom driver.
During rolling upgrades the quota system will be more fragile than usual, and users may be able to go over quota.
New quota system will no longer have an internal brute force cleaning mechanism of quotas, the volume state change API will be used to clean reservations, and the cinder-manage quota sync command will be used for the StoredQuotaDriver, so the following configuration options will be deprecated and will no longer have any effect: reservation_expire, reservation_clean_interval, until_refresh, and max_age.
Configuration option use_default_quota_class will be deprecated, because all deployments will use the default quota class instead of supporting the already deprecated configuration file quota limits (related bug #1609937).

Developer impact¶

There should be a positive impact on Cinder developers, since the code should be more readable without all the quota code in between the higher level logic, and adding new code should not require touching the quota manually.

The new code will probably break the cinderlib project, so changes to the project will also be necessary.

Implementation¶

Assignee(s)¶

Primary assignee:: Gorka Eguileor (geguileo)
Other contributors:: Rajat Dhasmana (whoami-rajat)

Work Items¶

As discussed in the PTG/mid-cycle this work may be split in 2 phases that may be implemented in different releases:

Phase 1: `DynamicQuotaDriver`¶

Deprecate configuration options and log warnings for deployments that are using custom quota drivers.
Add required indexes to volumes, snapshots, backups, and groups tables.
Add missing backup and backup_gigabytes default quota limits to the quota_classes table.
Remove deprecated consistencygroups resources from the quota_classes, quotas, quota_usages and reservations table.
Write the DynamicQuotaDriver database quota driver.
Make the following operations use the new quota driver:
- Create volume
- Delete volume
- Manage volume
- Extend volume
- Retype volume
- Transfer volume
- Create snapshot
- Delete snapshot
- Manage snapshot
- Backup create
- Backup restore
- Group create
- Group delete
Remove code for the old quota driver.
Make the cinder-manage quota sync and check be noop.
Write the DynamicQuotaDriver database quota driver unit tests.
Update existing unit tests.
Write initial documentation and mention that a more efficient driver will be coming in the future.

Phase 2: `StoredQuotaDriver`¶

Write the StoredQuotaDriver database quota driver.
Write the StoredQuotaDriver database quota driver unit tests.
Update the cinder-manage quota sync and check commands.
Add the cinder-manage quota change command.
Do basic manual performance comparison of old and new quota system.
Add support for the new quota system to cinderlib with a noop quota driver and use it as the default value of the quota_driver configuration option.
Update documentation.

Dependencies¶

The database engine cannot lock on non-existent rows, so the new code needs the database to hold default quota limit records for all the basic resources in the quota_classes table. So this new code depends on us ensuring that the backups and backup_gigabytes records are present in the database and we should also have the consistencygroups removed since they haven’t been used for a long time (bug #1952420).

Testing¶

Besides some manual testing that will be performed to do some basic performance comparison between the old and the new quota system, most of the testing will be focused on the testing of the SQL queries.

Currently supported database engines are InnoDB and SQLite, and the second one has some limitations and quirks that may make testing some of the queries difficult or impossible, so some of the unit tests will be skipped on SQLite.

We may explore the possibility of running a tempest job that checks the quota usage after finishing the tempest run and reports if it has become out of sync.

Documentation Impact¶

The Cinder quota documentation will be updated to reflect how resources will be tracked now, to contain description and use cases of the different quota drivers, as well as the procedure to change the quota driver driver and the no_snapshot_gb_quota_toggled configuration option.

References¶

In the Yoga PTG it was accepted that the quota system will be flaky during rolling upgrades.
WIP patch showing how the new quota system could look like

Related bugs:

Backup creation quota warning in logs: bug #1952420.
Remove default_quota_class configuration option: bug #1609937
Warnings when creating volumes and snapshots with a type that doesn’t have values in quota_classes: bug #1435807.
Inneficient listing of resources: bug #1952443
Showing quotas for private types: bug #1576717.
Incorrect limit for private volume types: bug #1952456
Incorrect usage when changing no_snapshot_gb_quota: bug #1952635

New Quota System

New Quota System¶

Problem description¶

Use Cases¶

Proposed change¶

Quota limits¶

Resources¶

Mechanism¶

Reservations¶

Changing configuration¶

Interface¶

NAME¶

__init__¶

resync¶

set_defaults¶

set_limits¶

clear_limits_and_defaults_cm¶

type_name_change_cm¶

get_defaults¶

get_limits_and_usage¶

group_check_cm¶

group_free¶

backup_check_cm¶

backup_free¶

vol_snap_check_and_reserve_cm¶

vol_snap_free¶

reservations_clean_cm¶

Differences¶

Limitations¶

Alternatives¶

Work with what we have¶

Unified Limits¶

Fix bottleneck¶

Changing configuration alternatives¶

Data model impact¶

REST API impact¶

Security impact¶

Active/Active HA impact¶

Notifications impact¶

Other end user impact¶

Performance Impact¶

Other deployer impact¶

Developer impact¶

Implementation¶

Assignee(s)¶

Work Items¶

Phase 1: DynamicQuotaDriver¶

Phase 2: StoredQuotaDriver¶

Dependencies¶

Testing¶

Documentation Impact¶

References¶

Cinder Specs 0.0.1.dev653

init¶

Phase 1: `DynamicQuotaDriver`¶

Phase 2: `StoredQuotaDriver`¶