Re: [Qemu-devel] Block job commands in QEMU 1.2 [v2, including support for replication]

[Stefan Hajnoczi]

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.)

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