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Re: [Qemu-devel] [PATCH 2/5] qcow2: multiple clusters write compressed

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH 2/5] qcow2: multiple clusters write compressed
Date: Wed, 15 Nov 2017 17:30:44 +0100
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On 2017-11-15 17:28, Anton Nefedov wrote:
> On 15/11/2017 6:11 PM, Max Reitz wrote:
>> On 2017-11-14 11:16, Anton Nefedov wrote:
>>> From: Pavel Butsykin <address@hidden>
>>> At the moment, qcow2_co_pwritev_compressed can process the requests size
>>> less than or equal to one cluster. This patch added possibility to write
>>> compressed data in the QCOW2 more than one cluster. The implementation
>>> is simple, we just split large requests into separate clusters and write
>>> using existing functionality. For unaligned requests we use a workaround
>>> and do write data without compression.
>>> Signed-off-by: Anton Nefedov <address@hidden>
>>> ---
>>>   block/qcow2.c | 77
>>> +++++++++++++++++++++++++++++++++++++++++++----------------
>>>   1 file changed, 56 insertions(+), 21 deletions(-)
>> On one hand, it might be better to do this centrally somewhere in
>> block/io.c.  On the other, that would require more work because it would
>> probably mean having to introduce another field in BlockLimits, and it
>> wouldn't do much -- because qcow (v1) is, well, qcow v1...  And vmdk
>> seems to completely not care about this single cluster limitation.  So
>> for now we probably wouldn't even gain anything by doing this in
>> block/io.c.
>> So long story short, it's OK to do this locally in qcow2, yes.
> [..]
>>> +        qemu_iovec_reset(&hd_qiov);
>>> +        chunk_size = MIN(bytes, s->cluster_size);
>>> +        qemu_iovec_concat(&hd_qiov, qiov, curr_off, chunk_size);
>>> +
>>> +        ret = qcow2_co_pwritev_cluster_compressed(bs, offset +
>>> curr_off,
>>> +                                                  chunk_size,
>>> &hd_qiov);
>>> +        if (ret == -ENOTSUP) {
>> Why this?  I mean, I can see the appeal, but then we should probably
>> actually return -ENOTSUP somewhere (e.g. for unaligned clusters and the
>> like) -- and we should not abort if offset_into_cluster(s, cluster) is
>> true, but we should write the header uncompressed and compress the main
>> bulk.
>> Max
> Right, sorry, missed this part when porting the patch.
> I think this needs to be removed (and the commit message fixed
> accordingly).
> Returning an error, rather than silently falling back to uncompressed
> seems preferable to me. "Compressing writers" (backup, img convert and
> now stream) are aware that they have to cluster-align, and if they fail
> to do so that means there is an error somewhere.

OK for me.

> If it won't return an error anymore, then the unaligned tail shouldn't
> either. So we can end up 'letting' the callers send small unaligned
> requests which will never get compressed.

Either way is fine.  It just looks to me like vmdk falls back to
uncompressed writes, so it's not like it would be completely new behavior...

(But I won't judge whether what vmdk does is a good idea.)


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