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[Qemu-block] Assertion failure on qcow2 disk with cluster_size != 64k

From: Ed Swierk
Subject: [Qemu-block] Assertion failure on qcow2 disk with cluster_size != 64k
Date: Thu, 20 Oct 2016 17:24:24 -0700

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

The next iteration starts with offset incremented by 65536, count and
num decremented by 65536, and head=0, violating the assumption that
the entire 385024 bytes of the head remainder had been zeroed the
first time around. Then it calls qcow2_co_pwrite_zeroes() with an
unaligned offset=8069120 and a count=1368064 greater than the cluster
size, triggering the assertion failure.

Changing max_transfer in the normal write case to
the problem, but I don't pretend to understand all the subtleties


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