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Re: [Qemu-block] [Qemu-devel] drive-backup

From: Stefan Priebe - Profihost AG
Subject: Re: [Qemu-block] [Qemu-devel] drive-backup
Date: Thu, 25 Feb 2016 08:49:48 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

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.


> Otherwise, after my trivial fix, you should find cluster_size to be a
> mutable concept and perhaps one that you could introduce a runtime
> parameter for if you could convince the necessary parties that it's
> needed in the API.
> You'd have to be careful in the case of incremental that all the various
> cluster sizes work well together:
> - Granularity of bitmap (Defaults to cluster size of source, or 64K if
> unknown. can be configured to be arbitrarily larger.)
> - Cluster size of source file (For qcow2, defaults to 64k)
> - Cluster size of target file
> - Cluster size of backup routine (Currently always 64K)
> I think that LCM(source_cluster_size, target_cluster_size,
> backup_cluster_size) = MAX(A, B, C) will always be a safe minimum.
> Bitmap granularity is more flexible, and it is more efficient when the
> bitmap granularity matches the backup cluster size, but it can cope with
> mismatches. If the bitmap is smaller or larger than the backup cluster
> size, it generally means that more data that is clean is going to be
> transferred across the pipe.
> (Hmm, I wonder if it's worth checking in code to adjust a bitmap
> granularity after it has been created so people can easily experiment
> with performance tweaking here.)
> Let me know if any of my ramble sounds interesting :)
> --John

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