Configure maximum number of volumes to attach¶
Currently, there is a limitation in the libvirt driver restricting the maximum number of volumes to attach to a single instance to 26. Depending on virt driver and operator environment, operators would like to be able to attach more than 26 volumes to a single instance. We propose adding a configuration option that operators can use to select the maximum number of volumes allowed to attach to a single instance.
We’ve had customers ask for the ability to attach more than 26 volumes to a single instance and we’ve seen launchpad bugs opened from users trying to attach more than 26 volumes (see References). Because the supportability of any number of volumes depends heavily on which virt driver is being used and the operator’s particular environment, we propose to make the maximum configurable by operators. Choosing an appropriate maximum number will require tuning with the specific virt driver and deployed environment, so we expect operators to set the maximum, test, tune, and adjust the configuration option until the maximum is working well in their environment.
Operators wish to be able to attach a maximum number of volumes to a single instance, with the ability to choose a maximum well-tuned for their environments.
When a user attempts to attach more than 26 disk devices with the libvirt
driver, the attach fails in the
reserve_block_device_name method in
nova-compute, which is eventually called by the
attach_volume method in
reserve_block_device_name method calls
self.driver.get_device_name_for_instance to get the next available device
name for attaching the volume. If the driver has implemented the method, this
is where an attempt to go beyond the maximum allowed number of disk devices to
attach, will fail. The libvirt driver fails after 26 disk devices have been
attached. Drivers that have not implemented
appear to have no limit on the maximum number of disk devices. The default
get_device_name_for_instance is located in the
nova.compute.utils module. Only the libvirt driver has provided its own
reserve_block_device_name method is a synchronous RPC call (not cast).
This means we can have the configured allowed maximum set differently per
nova-compute and still fail fast in the API if the maximum has been exceeded
during an attach volume request.
For a server create, rebuild, evacuate, unshelve, or live migrate request, if
the maximum has been exceeded, the server will go into the
ERROR state and
the server fault message will indicate the failure reason.
Note that the limit in the libvirt driver is actually on the total number of disk devices allowed to attach to a single instance including the root disk and any other disks. It does not differentiate between volumes and other disks.
We propose to add a new configuration option
[compute]max_disk_devices_to_attach IntOpt to use to configure the maximum
allowed disk devices to attach to a single instance per nova-compute. This way,
operators can set it appropriately depending on what virt driver they are
running and what their deployed environment is like. The default will be
unlimited (-1) to keep the current behavior for all drivers except the libvirt
The configuration option will be enforced in the
get_device_name_for_instance methods, using the count of the number of
already attached disk devices. Upon failure, an exception will be propagated to
nova-api via the synchronous RPC call to nova-compute, and the user will
receive a 403 error (as opposed to the current 500 error).
Other ways we could solve this include: choosing a new hard-coded maximum only for the libvirt driver or creating a new quota limit for “maximum disk devices allowed to attach” (see the ML thread in References).
Data model impact¶
REST API impact¶
Other end user impact¶
Other deployer impact¶
Deployers will be able to set the
configuration option to control how many disk devices are allowed to be
attached to a single instance per nova-compute in their deployment.
- Primary assignee:
- Other contributors:
Add a new configuration option
Modify (or remove) the libvirt driver’s implementation of the
get_device_name_for_instancemethod to accomodate more than 26 disk devices
Add enforcement of
Add handling of the raised exception in the API to translate to a 403 to the user, if the maximum number of allowed disk devices is exceeded
The new functionality will be tested by new unit and functional tests.
The documentation for the new configuration option will be automatically included in generated documentation of the configuration reference.
Optional section intended to be used each time the spec is updated to describe new design, API or any database schema updated. Useful to let reader understand what’s happened along the time.