Merge "Doc: Add notes on create_cloned_volume locking"
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@ -139,3 +139,31 @@ reference, are:
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- `cinder/volume/drivers/nec/cli.py`
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- `cinder/volume/drivers/nec/volume_helper.py`
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- `cinder/volume/drivers/netapp/dataontap/nfs_base.py`
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Notes on Driver Locking
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-----------------------
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From the volume manager flow, create_cloned_volume() happens to be called
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with a lock that prevents concurrent calls to clone from the same volume
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at the same time.
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This is done by the cinder/volume/manager.py create_volume() code::
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elif source_volid is not None:
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locked_action = "%s-%s" % (source_volid, 'delete_volume')
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and subsequent COORDINATOR.get_lock() call.
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This seems to have been intended to prevent a volume from being deleted
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while being used as the source of a volume clone, the fact that it
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prevents concurrent clone operations is a side effect.
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This means that a driver that cannot correctly handle concurrent clone
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operations from the same volume will work for normal clone operations,
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but then fail when a clone operation is performed as part of cloning
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from the image-volume cache, or cloning from a Cinder backend of Glance.
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(See https://bugs.launchpad.net/cinder/+bug/1851512 for an example.)
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It should be assumed that, at some point, this locking behavior will be
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changed to allow concurrent clone calls.
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