Re: [Qemu-devel] [Qemu-block] [PATCH for-2.8] qcow2: Don't strand clusters near 2G intervals during commit

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Sat, Dec 03, 2016 at 11:34:02AM -0600, Eric Blake wrote:
> The qcow2_make_empty() function is reached during 'qemu-img commit',
> in order to clear out ALL clusters of an image.  However, if the
> image cannot use the fast code path (true if the image is format
> 0.10, or if the image contains a snapshot), the cluster size is
> larger than 512, and the image is larger than 2G in size, then our
> choice of sector_step causes problems.  Since it is not cluster
> aligned, but qcow2_discard_clusters() silently ignores an unaligned
> head or tail, we are leaving clusters allocated.
> 
> Enhance the testsuite to expose the flaw, and patch the problem by
> ensuring our step size is aligned.
> 
> [qcow2_discard_clusters() is a GROSS interface: it takes a mix of
> byte offset and sector count to perform cluster operations. But
> fixing it to use a saner byte/byte rather than byte/sector interface,
> and/or asserting that the counts are now aligned thanks to both
> this patch and commit 3482b9b, is material for another day.]
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  block/qcow2.c              |   3 +-
>  tests/qemu-iotests/097     |  41 +++++---
>  tests/qemu-iotests/097.out | 249 +++++++++++++++++++++++++++++++++------------
>  3 files changed, 210 insertions(+), 83 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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