qcow2: add discard-no-unref option

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>

1 files changed, 12 insertions, 0 deletions

diff --git a/qemu-options.hx b/qemu-options.hx
index b37eb9662b..b57489d7ca 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -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