Re: [PATCH v3] qcow2: add discard-no-unref option

[Hanna Czenczek <hreitz@redhat.com>]

On 05.06.23 10:45, Jean-Louis Dupond wrote:
> 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>
> ---
>   block/qcow2-cluster.c | 32 ++++++++++++++++++++++++++++----
>   block/qcow2.c         | 18 ++++++++++++++++++
>   block/qcow2.h         |  3 +++
>   qapi/block-core.json  | 12 ++++++++++++
>   qemu-options.hx       | 12 ++++++++++++
>   5 files changed, 73 insertions(+), 4 deletions(-)

Thanks!

Reviewed-by: Hanna Czenczek <hreitz@redhat.com>