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

From: Anton Nefedov
Subject: Re: [Qemu-devel] [PATCH 2/5] qcow2: multiple clusters write compressed
Date: Wed, 15 Nov 2017 19:28:05 +0300
User-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

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


Right, sorry, missed this part when porting the patch.

I think this needs to be removed (and the commit message fixed
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.
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.


+            ret = qcow2_co_pwritev(bs, offset + curr_off, chunk_size,
+                                   &hd_qiov, 0);

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