From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vasileios Almpanis Date: Wed, 19 Aug 2026 11:05:02 +0000 Subject: Re: [Devel] [PATCH VZ10 v2 00/10] dm-qcow2: improve discard and read-only merge handling In-Reply-To: <20260812202005.288994-1-andrey.zhadchenko@virtuozzo.com> References: <20260812202005.288994-1-andrey.zhadchenko@virtuozzo.com> Message-ID: <178713750239.404937.17287395566759655742.b4-review@b4> List-Id: > 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. Generally with my limited knowledge is looks good. > > 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(-) > > -- > 2.43.5 Reviewed-by: Vasileios Almpanis -- Vasileios Almpanis