Re: [Qemu-devel] Assertion failure on qcow2 disk with cluster_size != 64k

[Eric Blake <eblake@redhat.com>]

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On 10/20/2016 07:24 PM, Ed Swierk wrote:
> Shortly after I start qemu 2.7.0 with a qcow2 disk image created with
> -o cluster_size=1048576, it prints the following and dies:
> 
> block/qcow2.c:2451: qcow2_co_pwrite_zeroes: Assertion `head + count <=
> s->cluster_size' failed.
> 
> I narrowed the problem to bdrv_co_do_pwrite_zeroes(), called by
> bdrv_aligned_pwritev() with flags & BDRV_REQ_ZERO_WRITE set.
> 
> On the first loop iteration, offset=8003584, count=2093056,
> head=663552, tail=659456 and num=2093056. qcow2_co_pwrite_zeroes() is
> called with offset=8003584 and count=385024 and finds that the head
> portion is not already zero, so it returns -ENOTSUP.
> bdrv_co_do_pwrite_zeroes() falls back to a normal write, with
> max_transfer=65536.

How are you getting max_transfer == 65536?  I can't reproduce it with
the following setup:

$ qemu-img create -f qcow2 -o cluster_size=1M file 10M
$ qemu-io -f qcow2 -c 'w 7m 1k' file
$ qemu-io -f qcow2 -c 'w -z 8003584 2093056' file

although I did confirm that the above sequence was enough to get the
-ENOTSUP failure and fall into the code calculating max_transfer.

I'm guessing that you are using something other than a file system as
the backing protocol for your qcow2 image.  But do you really have a
protocol that takes AT MOST 64k per transaction, while still trying to a
cluster size of 1M in the qcow2 format?  That's rather awkward, as it
means that you are required to do 16 transactions per cluster (the whole
point of using larger clusters is usually to get fewer transactions).  I
think we need to get to a root cause of why you are seeing such a small
max_transfer, before I can propose the right patch, since I haven't been
able to reproduce it locally yet (although I admit I haven't tried to
see if blkdebug could reliably introduce artificial limits to simulate
your setup).  And it may turn out that I just have to fix the
bdrv_co_do_pwrite_zeroes() code to loop multiple times if the size of
the unaligned head really does exceed the max_transfer size that the
underlying protocol is able to support, rather than assuming that the
unaligned head/tail always fit in a single fallback write.

Can you also try this patch? If I'm right, you'll still fail, but the
assertion will be slightly different.  (Again, I'm passing locally, but
that's because I'm using the file protocol, and my file system does not
impose a puny 64k max transfer).

diff --git i/block/io.c w/block/io.c
index b136c89..8757063 100644
--- i/block/io.c
+++ w/block/io.c
@@ -1179,6 +1179,8 @@ static int coroutine_fn
bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
     int max_write_zeroes = MIN_NON_ZERO(bs->bl.max_pwrite_zeroes, INT_MAX);
     int alignment = MAX(bs->bl.pwrite_zeroes_alignment,
                         bs->bl.request_alignment);
+    int max_transfer = MIN_NON_ZERO(bs->bl.max_transfer,
+                                    MAX_WRITE_ZEROES_BOUNCE_BUFFER);

     assert(alignment % bs->bl.request_alignment == 0);
     head = offset % alignment;
@@ -1197,6 +1199,8 @@ static int coroutine_fn
bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,
             /* Make a small request up to the first aligned sector.  */
             num = MIN(count, alignment - head);
             head = 0;
+            assert(num < max_write_zeroes);
+            assert(num < max_transfer);
         } else if (tail && num > alignment) {
             /* Shorten the request to the last aligned sector.  */
             num -= tail;
@@ -1222,8 +1226,6 @@ static int coroutine_fn
bdrv_co_do_pwrite_zeroes(BlockDriverState *bs,

         if (ret == -ENOTSUP) {
             /* Fall back to bounce buffer if write zeroes is unsupported */
-            int max_transfer = MIN_NON_ZERO(bs->bl.max_transfer,
-
MAX_WRITE_ZEROES_BOUNCE_BUFFER);
             BdrvRequestFlags write_flags = flags & ~BDRV_REQ_ZERO_WRITE;

             if ((flags & BDRV_REQ_FUA) &&


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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