Glance VMware store to support multiple datastores

https://blueprints.launchpad.net/glance/+spec/vmware-store-multiple-datastores

Glance, when configured to use VMware store, stores the images in a virtual storage container called datastore, identified by the configuration option vmware_datastore_name. The size of the datastore can only be increased to a certain limit which also depends upon the underlying physical storage.

The capacity of the store is thus bound by the size of a single datastore and poses serious scalability issues.

Problem description

Currently the number of images stored by glance, when configured to use VMware store, is limited by the capacity of a single datastore. The size of a datastore has an upper bound [1].

e.g The maximum datastore size is 64TB when backed by VMFS-5 filesystem. While the same for a datastore with NFS may depend on the storage array vendor. While this is the theoretical maximum based on the supported filesystem, datastores are also sized based on the underlying physical storage, including internal and external devices and networked storage. With networked storage the choice of storage protocol (iSCSI, FC, FCoE) also play a vital role in deciding the optimal size. Thus, the capacity of a datastore used in a datacenter varies by deployment.

Apart from the capacity, there may be a performance bottleneck when many compute nodes concurrently access a single datastore to download images.

Proposed change

To allow adding more capacity and improving performance, we suggest the use of multiple datastores for storing images.

There is only one major aspect of the proposed change: - Datastore Selection.

1) Datastore Selection:

This change proposes to select a datastore based on the priority given to it by the operator and the capacity to accommodate the image. A new configuration option (say vmware_datastores) will be added that would enable the operator to specify multiple datastores along with their relative weights. In case of equal priority the selection will be based on the maximum freespace available on the datastore. This approach is very similar to filesystem_store_datadirs used by filesystem store.

Example: say vmware_datastores is configured with nfs_datastore, iscsi_datastore, ssd_datastore with weights 100, 100 and 200 respectively. Here, ssd_datastore will be the preferred datastore if it can accommodate the image data being added. Otherwise, nfs_datastore or iscsi_datastore will be chosen based on maximum freespace available.

Alternatives

1) The datastore selection can be done through round-robin/randomized strategy instead of priority-based. This would be ideal if all datastores are of same type and perform similarly. But since datastores are of different types, the strategy is best driven by operator input. e.g SSD backed datastores may be faster that networked datastores. While its possible to determine the type of datastore, implicit assumptions cannot be made.

Data model impact

Does not have any impact on glance data model.

REST API impact

None

Security impact

This spec only adds a selection logic to the existing functionality and hence does not have any security impact.

Notifications impact

None

Other end user impact

None

Performance Impact

The use of multiple datastores will reduce concurrent operations on a single datastore.

Other deployer impact

This proposal will allow administrators to configure multiple datastores to save the image data in VMware storage backend. If enabled, it will override the existing configuration option vmware_datastore_name that allows to specify a single datastore. If disabled, the store will fallback to vmware_datastore_name option. Thus, the change is backwards compatible. We will deprecate the vmware_datastore_name option going forward.

New Multi Strings configuration option in glance-api.conf

vmware_datastores Optional. Default: Not set.

A datastore along with its datacenter path and a weight. If the datastore is a member of a datastore cluster, then the name of the cluster should also be included. Otherwise, the cluster name can be omitted. This option can be specified multiple times to specify multiple datastores. Thus, the required format becomes:

vmware_datastores =

<datacenter_path>:<datastore1>:<weight>

vmware_datastores =

<datacenter_path>:<datastore_cluster_name>:<datastore2>:<weight>

The weights are used to establish the relative priorities of the specified datastores. Thus, they can be any arbitrary integer values.

Examples:
vmware_datastores =

datacenter1:nfs_datastore:2

vmware_datastores =

datacenter1:ssd_datastore:3

vmware_datastores =

datacenter1:backup_datastore:1

In the above example, ssd_datastore will be given highest priority, followed by nfs_datastore and backup_datastore.

vmware_datastores =

datacenter1:nfs1_datastore:100

vmware_datastores =

datacenter1:nfs2_datastore:100

vmware_datastores =

datacenter1:backup_datastore:50

In this example, the nfs datastores will be given equal priority, followed by the backup_datastore. With equal priority, the contention between nfs datastores is resolved by the maximum freespace.

Note:- If the datacenter path or datastore name contains a colon (:) symbol, it must be escaped with a backslash.

Developer impact

None

Implementation

Assignee(s)

Primary assignee:

smurugesan (sabari)

Other contributors:

None

Reviewers

Core reviewer(s):

nikhil-komawar arnaudleg

Other reviewer(s):

rgerganov

Work Items

  1. Implement new config option in VMware store driver.

  2. Implement datastore selection logic in the VMware store driver.

  3. Implement unit tests.

  4. Change glance-api sample conf in glance repository.

  5. Add a deprecation warning for vmware_datastore_name.

  6. Update the documentation.

Dependencies

  • oslo.vmware has introduced a Datastore object which will be used in this implementation.

  • oslo.vmware has a vim_util module that has some low-level utility methods to interact with the vmware api’s. This is required to parse api responses.

Testing

  • Tempest tests are not required.

Documentation Impact

  • Document new configuration options.