Limit Volume Copy Bandwidth Per Backend¶
Currently, cinder has a global configuration to limit a volume copy bandwidth to mitigate slow down of instances volume access during large image copies. ( https://blueprints.launchpad.net/cinder/+spec/limit-volume-copy-bandwidth )
This spec proposes to make this config variable per backend.
The current global config has some issues.
When multiple backends exist, we cannot assign different bandwidth limit. For instance, it is desirable to allow large bps for SSD than for HDD.
If volume copy runs concurrently, bandwidth larger than the bps limit is consumed. From the viewpoint of QoS (to keep bandwidth for access from instances), we should limit total volume copy bandwidth per backend.
Make the cinder.conf parameter ‘volume_copy_bps_limit’ writable in each backend section. (If volume_copy_bps_limit is specified in DEFAULT section as existing design, that value is used as a global default. This keeps backwards compatibility.)
The bps limit is divided if multiple volume copy operations run concurrently. For example, if volume_copy_bps_limit is set to 100MB/s, and 2 copies are running at the same time in a backend, each copy can use up to 50MB/s. If one of the copies finished, the other copy’s limit is updated to 100MB/s.
On initialization of VolumeDriver class, a ‘Throttle’ class object is created and stored in ‘_throttle’ member for each backend driver object, that manages the total bandwidth allowed and volume copy operations in flight.
It should be passed to copy_volume() as following, so that it can set/update the bandwidth limit.
In copy_volume method, Throttle class method is called to setup cgroups (or some other QoS feature, when the Throttle class is overridden).
Data model impact¶
REST API impact¶
Other end user impact¶
When volume copy I/O bandwidth is limited, it takes more time to complete volume copy. Users are required to balance between volume copy performance and interference to instances performance.
Other deployer impact¶
This feature is disabled by default. Users who want to use this feature need to set ‘volume_copy_bps_limit’ in cinder.conf.
A ‘_throttle’ member is added to VolumeDriver class, which should be passed to volume_utils.copy_volume() and related function. Otherwise, bandwidth limit will be ignored.
In Linux environments, ‘_throttle’ object will be implemented using cgroups. Drivers for non-Linux environments (e.g. Windows) can override the initialization of ‘_throttle’ to use another method for throttling.
- Primary assignee:
Use ‘backend_driver.configuration.volume_copy_bps_limit’ etc. if ‘CONF.volume_copy_bps_limit’ is not specified.
Replace current setup_blkio_cgroup() with new ‘Throttle’ class
Pass the Throttle object from the backend drivers to copy_volume()
With volume_copy_bps_limit = 100MB/s for a fake backend driver:
start a volume copy, then the bps limit is set to 100MB/s
start a second volume copy, then the limit is updated to 50MB/s for both
finish one of the copies, then the limit is resumed to 100MB/s
The cinder client documentation about volume_copy_bps_limit in cinder.conf should address that copy bps limit can be specified for each backend driver by writing this config in the backend section.