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Re: backup_calculate_cluster_size does not consider source
From: |
Dietmar Maurer |
Subject: |
Re: backup_calculate_cluster_size does not consider source |
Date: |
Wed, 6 Nov 2019 14:34:05 +0100 (CET) |
> On 6 November 2019 14:17 Max Reitz <address@hidden> wrote:
>
>
> On 06.11.19 14:09, Dietmar Maurer wrote:
> >> Let me elaborate: Yes, a cluster size generally means that it is most
> >> “efficient” to access the storage at that size. But there’s a tradeoff.
> >> At some point, reading the data takes sufficiently long that reading a
> >> bit of metadata doesn’t matter anymore (usually, that is).
> >
> > Any network storage suffers from long network latencies, so it always
> > matters if you do more IOs than necessary.
>
> Yes, exactly, that’s why I’m saying it makes sense to me to increase the
> buffer size from the measly 64 kB that we currently have. I just don’t
> see the point of increasing it exactly to the source cluster size.
>
> >> There is a bit of a problem with making the backup copy size rather
> >> large, and that is the fact that backup’s copy-before-write causes guest
> >> writes to stall. So if the guest just writes a bit of data, a 4 MB
> >> buffer size may mean that in the background it will have to wait for 4
> >> MB of data to be copied.[1]
> >
> > We use this for several years now in production, and it is not a problem.
> > (Ceph storage is mostly on 10G (or faster) network equipment).
>
> So you mean for cases where backup already chooses a 4 MB buffer size
> because the target has that cluster size?
To make it clear. Backups from Ceph as source are slow.
That is why we use a patched qemu version, which uses:
cluster_size = Max_Block_Size(source, target)
(I guess this only triggers for ceph)
- Re: backup_calculate_cluster_size does not consider source, Stefan Hajnoczi, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Max Reitz, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Wolfgang Bumiller, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Max Reitz, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Dietmar Maurer, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Max Reitz, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Max Reitz, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Dietmar Maurer, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Max Reitz, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source,
Dietmar Maurer <=
- Re: backup_calculate_cluster_size does not consider source, Max Reitz, 2019/11/06
- Re: backup_calculate_cluster_size does not consider source, Vladimir Sementsov-Ogievskiy, 2019/11/06
Re: backup_calculate_cluster_size does not consider source, Dietmar Maurer, 2019/11/06