Re: [Qemu-devel] drive-backup

[Fam Zheng <famz@redhat.com>]

On Thu, 02/25 08:49, Stefan Priebe - Profihost AG wrote:
> 
> Am 22.02.2016 um 23:08 schrieb John Snow:
> > 
> > 
> > On 02/22/2016 03:21 PM, Stefan Priebe wrote:
> >> Hello,
> >>
> >> is there any chance or hack to work with a bigger cluster size for the
> >> drive backup job?
> >>
> >> See:
> >> http://git.qemu.org/?p=qemu.git;a=blob;f=block/backup.c;h=16105d40b193be9bb40346027bdf58e62b956a96;hb=98d2c6f2cd80afaa2dc10091f5e35a97c181e4f5
> >>
> >>
> >> This is very slow with ceph - may be due to the 64k block size. I would
> >> like to check whether this is faster with cephs native block size of 4mb.
> >>
> >> Greets,
> >> Stefan
> >>
> > 
> > It's hardcoded to 64K at the moment, but I am checking in a patch to
> > round up the cluster size to be the bigger of (64k,
> > $target_cluster_size) in order to make sure that incremental backups in
> > particular never copy a fraction of a cluster. As a side-effect, the
> > same round-up will happen for all modes (sync=top,none,full).
> > 
> > If QEMU is aware of the target cluster size of 4MB, this would
> > immediately jump the copy-size up to 4MB clusters for you.
> > 
> > See: https://lists.nongnu.org/archive/html/qemu-devel/2016-02/msg02839.html
> 
> Thanks for your patches and thanks for your great answer. But our
> problem is not the target but the source ;-) The target has a local
> cache and don't care about the cluster size but the source does not.
> 
> But it works fine if we change the default cluster size to 4MB. So it
> has point us to the right direction.

Does it mean overriding the copy granularity is what you want to do?
Asking because drive-mirror has a "granularity" parameter for this purpose, if
so it should probably be considered for drive-backup as well.

Fam