[Devel] [PATCH VZ10 v2 00/10] dm-qcow2: improve discard and read-only merge handling

[Pavel Tikhomirov]

Looks good.

It's hard to understand it completely, but in general it ok. No obvious
code problems, no obvious logical problems.

Note that AI trips on some patches not passing check-patch, some minor
reformatting needed.

Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

On 8/12/26 22:19, Andrey Zhadchenko wrote:
> This series makes discard update qcow2 metadata and permits backward
> merge from read-only source images.
> 
> First patch improves revert_cluster_alloc: this function didn't expect
> L2 entries to contain subcluster descriptions (when ext2_l2 is set).
> 
> The second patch renames COW machinery to a more general 'replace entry'.
> 
> Next five patches improve discard support in general: now it clears
> metadata and have a subcluster granularity.
> 
> The next two patches support backward merge when the image we merge
> from is read-only. Such merges now leave the source mappings and
> metadata unchanged. Previously the module ignored file mode and
> submitted writes anyway.
> 
> Last patch makes merge respect all metadata-zeroed blocks. For now it is
> writing zeroes as data, but we will improve this to metadata when we
> support REQ_OP_WRITE_ZEROES.
> 
> 
> v2:
>  - patch#6: expand qio_discard_updates_metadata() condition to return
> true when backing file is present and fully covered sublcu zero bit isn't
> already set (prepare_cluster_discard() already handled this case but
> if forgot about it here)
> 
> Andrey Zhadchenko (10):
>   drivers/md/dm-qcow2: fix revert_cluster_alloc() for ext_l2 case
>   drivers/md/dm-qcow2: generalize COW index update machinery
>   drivers/md/dm-qcow2: update metadata on whole cluster discard
>   drivers/md/dm-qcow2: never trigger COW or allocation on discard
>   drivers/md/dm-qcow2: set L2_READS_ALL_ZEROES after some discards
>   drivers/md/dm-qcow2: update metadata on subclusters discard
>   drivers/md/dm-qcow2: unmap cluster when discard clears last allocated
>     subclusters
>   drivers/md/dm-qcow2: do discards during backward merge only for
>     writable image
>   drivers/md/dm-qcow2: allow shared L1 entries during merge from RO
>     image
>   drivers/md/dm-qcow2: respect zeroes during merge
> 
>  drivers/md/dm-qcow2-cmd.c    |  21 +-
>  drivers/md/dm-qcow2-map.c    | 390 ++++++++++++++++++++++++++++-------
>  drivers/md/dm-qcow2-target.c |   2 +-
>  drivers/md/dm-qcow2.h        |  19 +-
>  4 files changed, 337 insertions(+), 95 deletions(-)
> 

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.