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Date: Fri, 25 May 2012 13:17:04 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] Block job commands in QEMU 1.2 [v2,
	including support for replication]
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Eric Blake <eblake@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Ori Mamluk <omamluk@zerto.com>, Luiz Capitulino <lcapitulino@redhat.com>

Il 25/05/2012 11:43, Stefan Hajnoczi ha scritto:
> On Thu, May 24, 2012 at 03:41:29PM +0200, Paolo Bonzini wrote:
>> Persistent dirty bitmap
>> =======================
>>
>> A persistent dirty bitmap can be used by management for two reasons.
>> When mirroring is used for continuous replication of storage, to record
>> I/O operations that happened while the replication server is not
>> connected or unavailable.
> 
> For incremental backups we also need a dirty bitmap API.  This allows
> backup software to determine which blocks are "dirty" in a snapshot.
> The backup software will only copy out the dirty blocks.
> 
> (For external snapshots the "dirty bitmap" is actually the qemu-io -c
> map of is_allocated clusters.  But for internal snapshots it would be a
> diff of image metadata which results in a real bitmap.)

Perhaps that be simply a new qemu-img subcommand?  It should be possible
to run it while the VM is offline.  Then the file that is produced could
be fed to blockdev-dirty-enable.

Paolo

> So it seems like a dirty bitmap API will be required for continuous
> replication (so the server can find out what it missed) and for
> incremental backup.  If there is commonality here we should work
> together so the same API can do both.
> 
> Stefan
>