diff options
| author | Jean-Louis Dupond <jean-louis@dupond.be> | 2023-10-03 14:52:37 +0200 |
|---|---|---|
| committer | Hanna Czenczek <hreitz@redhat.com> | 2023-11-06 16:15:07 +0100 |
| commit | b2b109041ecd1095384f5be5bb9badd13c1cf286 (patch) | |
| tree | 3c6cc355936a8c74e356a86bc3b3526d0def0bc4 /qemu-options.hx | |
| parent | 3e01f1147a16ca566694b97eafc941d62fa1e8d8 (diff) | |
qcow2: keep reference on zeroize with discard-no-unref enabled
When the discard-no-unref flag is enabled, we keep the reference for
normal discard requests.
But when a discard is executed on a snapshot/qcow2 image with backing,
the discards are saved as zero clusters in the snapshot image.
When committing the snapshot to the backing file, not
discard_in_l2_slice is called but zero_in_l2_slice. Which did not had
any logic to keep the reference when discard-no-unref is enabled.
Therefor we add logic in the zero_in_l2_slice call to keep the reference
on commit.
Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1621
Signed-off-by: Jean-Louis Dupond <jean-louis@dupond.be>
Message-Id: <20231003125236.216473-2-jean-louis@dupond.be>
[hreitz: Made the documentation change more verbose, as discussed
on-list]
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Diffstat (limited to 'qemu-options.hx')
| -rw-r--r-- | qemu-options.hx | 10 |
1 files changed, 7 insertions, 3 deletions
diff --git a/qemu-options.hx b/qemu-options.hx index e26230bac5..7809036d8c 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -1457,9 +1457,13 @@ SRST (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 + When enabled, data clusters will remain preallocated when they are + no longer used, e.g. because they are discarded or converted to + zero clusters. As usual, whether the old data is discarded or kept + on the protocol level (i.e. in the image file) depends on the + setting of the pass-discard-request option. Keeping the clusters + preallocated prevents qcow2 fragmentation that would otherwise be + caused by freeing and re-allocating them later. 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 |