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Re: backup_calculate_cluster_size does not consider source

From: Max Reitz
Subject: Re: backup_calculate_cluster_size does not consider source
Date: Wed, 6 Nov 2019 14:52:03 +0100
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On 06.11.19 14:34, Dietmar Maurer wrote:
>> 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.

Yep, but if the target would be another ceph instance, the backup buffer
size would be chosen to be 4 MB (AFAIU), so I was wondering whether you
are referring to this effect, or to...

> That is why we use a patched qemu version, which uses:
> cluster_size = Max_Block_Size(source, target)


The main problem with the stall I mentioned is that I think one of the
main use cases of backup is having a fast source and a slow (off-site)
target.  In such cases, I suppose it becomes annoying if some guest
writes (which were fast before the backup started) take a long time
because the backup needs to copy quite a bit of data to off-site storage.

(And blindly taking the source cluster size would mean that such things
could happen if you use local qcow2 files with 2 MB clusters.)

So I’d prefer decoupling the backup buffer size and the bitmap
granularity, and then set the buffer size to maybe the MAX of source and
target cluster sizes.  But I don’t know when I can get around to do that.

And then probably also cap it at 4 MB or 8 MB, because that happens to
be what you need, but I’d prefer for it not to use tons of memory.  (The
mirror job uses 1 MB per request, for up to 16 parallel requests; and
the backup copy-before-write implementation currently (on master) copies
1 MB at a time (per concurrent request), and the whole memory usage of
backup is limited at 128 MB.)

(OTOH, the minimum should probably be 1 MB.)


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