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Date: Mon, 25 Apr 2016 08:42:50 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH for-2.7 v2 07/17] rbd: Implement image
 locking
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To: dillaman@redhat.com
Cc: qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, Jeff Cody <jcody@redhat.com>, Markus Armbruster <armbru@redhat.com>, Max Reitz <mreitz@redhat.com>, den@openvz.org, pbonzini@redhat.com, John Snow <jsnow@redhat.com>

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?

> 
> 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?

Fam