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qcow2: add discard-no-unref option

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When we for example have a sparse qcow2 image and discard: unmap is enabled,
there can be a lot of fragmentation in the image after some time. Especially on VM's
that do a lot of writes/deletes.
This causes the qcow2 image to grow even over 110% of its virtual size,
because the free gaps in the image get too small to allocate new
continuous clusters. So it allocates new space at the end of the image.

Disabling discard is not an option, as discard is needed to keep the
incremental backup size as low as possible. Without discard, the
incremental backups would become large, as qemu thinks it's just dirty
blocks but it doesn't know the blocks are unneeded.
So we need to avoid fragmentation but also 'empty' the unneeded blocks in
the image to have a small incremental backup.

In addition, we also want to send the discards further down the stack, so
the underlying blocks are still discarded.

Therefor we introduce a new qcow2 option "discard-no-unref".
When setting this option to true, discards will no longer have the qcow2
driver relinquish cluster allocations. Other than that, the request is
handled as normal: All clusters in range are marked as zero, and, if
pass-discard-request is true, it is passed further down the stack.
The only difference is that the now-zero clusters are preallocated
instead of being unallocated.
This will avoid fragmentation on the qcow2 image.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1621
Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
Message-Id: <20230605084523.34134-2-jean-louis@dupond.be>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
This commit is contained in:
Jean-Louis Dupond 2023-06-05 10:45:24 +02:00 • committed by Hanna Czenczek
parent 029136f261
commit 42a2890a76
5 changed files with 73 additions and 4 deletions
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12
qemu-options.hx
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@ -1431,6 +1431,18 @@ SRST
issued on other occasions where a cluster gets freed
(on/off; default: off)
``discard-no-unref``
When enabled, discards from the guest will not cause cluster
allocations to be relinquished. This prevents qcow2 fragmentation
that would be caused by such discards. Besides potential
performance degradation, such fragmentation can lead to increased
allocation of clusters past the end of the image file,
resulting in image files whose file length can grow much larger
than their guest disk size would suggest.
If image file length is of concern (e.g. when storing qcow2
images directly on block devices), you should consider enabling
this option.
``overlap-check``
Which overlap checks to perform for writes to the image
(none/constant/cached/all; default: cached). For details or