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Re: [Qemu-devel] [PATCH for-2.7 v2 07/17] rbd: Implement image locking

From: Jason Dillaman
Subject: Re: [Qemu-devel] [PATCH for-2.7 v2 07/17] rbd: Implement image locking
Date: Tue, 26 Apr 2016 10:42:43 -0500

On Sun, Apr 24, 2016 at 7:42 PM, Fam Zheng <address@hidden> wrote:
> On Fri, 04/22 21:57, Jason Dillaman wrote:
>> Since this cannot automatically recover from a crashed QEMU client with an
>> RBD image, perhaps this RBD locking should not default to enabled.
>> Additionally, this will conflict with the "exclusive-lock" feature
>> available since the Ceph Hammer-release since both utilize the same locking
>> construct.
>> As a quick background, the optional exclusive-lock feature can be
>> enabled/disabled on image and safely/automatically handles the case of
>> recovery from a crashed client.  Under normal conditions, the RBD
>> exclusive-lock feature automatically acquires the lock upon the first
>> attempt to write to the image and transparently transitions ownership of
>> the lock between two or more clients -- used for QEMU live-migration.
> Is it enabled by default?

Starting with the Jewel release of Ceph it is enabled by default.

>> I'd be happy to add a new librbd API method to explicitly expose acquiring
>> and releasing the RBD exclusive lock since it certainly solves a couple
>> compromises in our current QEMU integration.
> Does the API do enable/disable or acquire/relase? Could you explain the
> differences between it and rbd_lock_exclusive?

There is already an API for enabling/disabling the exclusive-lock
feature (and associated CLI tooling).  This would be a new API method
to explicitly acquire / release the exclusive-lock (instead of
implicitly acquiring the lock upon first write request as it currently
does in order to support QEMU live-migration).

The rbd_lock_exclusive/rbd_lock_shared are essentially advisory locks.
Nothing stops a client from accessing the image if it doesn't attempt
to acquire the lock (even if another client already has the image
locked exclusively).  It also doesn't support automatic recovery from
failed clients.


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