Since we have fd16fe965ae45 ("md: mark dm-ploop and dm-qcow2 targets non-fua"),
the upper layer will consult with dm that REQ_FUA is not available and won't
send them. So we should not have any REQ_FUA in dm-qcow2/dm-ploop. (AI
confirms).
Though for dm in general this can be useful of course.
On 7/13/26 02:36, Andrey Zhadchenko wrote:
> A REQ_FUA write forces the data to non-volatile media before
> completion, so its latency is relevant for both the write and the
> flush histograms. Classify such writes into both classes: they keep
> being accounted as ordinary writes by "histogram:" and
> "wr_histogram:", and additionally show up in "flush_histogram:".
>
> https://virtuozzo.atlassian.net/browse/VSTOR-103846
> Signed-off-by: Andrey Zhadchenko <[email protected]>
> ---
> Documentation/admin-guide/device-mapper/statistics.rst | 10 ++++++----
> drivers/md/dm-stats.c | 5 +++--
> 2 files changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/admin-guide/device-mapper/statistics.rst
> b/Documentation/admin-guide/device-mapper/statistics.rst
> index ce02fd8a2873d..ea7dbfd0b30e6 100644
> --- a/Documentation/admin-guide/device-mapper/statistics.rst
> +++ b/Documentation/admin-guide/device-mapper/statistics.rst
> @@ -103,10 +103,12 @@ Messages
> account write requests.
> flush_histogram:n1,n2,n3,n4,...
> collect a latency histogram like "histogram", but only
> - account data-less cache flushes. Since a data-less
> - flush is not directed at any particular sector, it is
> - accounted in the first step-sized area of the region
> - and does not modify any other counters.
> + account cache flushes: data-less flush requests as well
> + as FUA writes (which are also accounted by "histogram"
> + and "wr_histogram" as ordinary writes). Since a
> + data-less flush is not directed at any particular
> + sector, it is accounted in the first step-sized area of
> + the region and does not modify any other counters.
> Multiple histogram arguments may be specified for one region;
> each histogram is reported as an additional counter set in the
> @stats_print output, in the order the histograms were specified.
> diff --git a/drivers/md/dm-stats.c b/drivers/md/dm-stats.c
> index 3a0d80bbf78da..c26a0a2358309 100644
> --- a/drivers/md/dm-stats.c
> +++ b/drivers/md/dm-stats.c
> @@ -41,7 +41,8 @@ struct dm_stat_shared {
>
> /*
> * Histograms account I/O classes independently. An I/O is classified once
> - * per dm_stats_account_io() and may belong to several classes at once.
> + * per dm_stats_account_io() and may belong to several classes at once:
> + * a write with REQ_FUA is both a write and a flush
> */
> enum {
> DM_STAT_CLASS_READ,
> @@ -641,7 +642,7 @@ static unsigned int dm_stat_io_classes(blk_opf_t bi_opf,
> unsigned int bi_sectors
> if (bi_sectors)
> classes |= 1 << (op_is_write(bi_opf) ? DM_STAT_CLASS_WRITE :
> DM_STAT_CLASS_READ);
> - if ((bi_opf & REQ_PREFLUSH) ||
> + if ((bi_opf & (REQ_PREFLUSH | REQ_FUA)) ||
> (bi_opf & REQ_OP_MASK) == REQ_OP_FLUSH)
> classes |= 1 << DM_STAT_CLASS_FLUSH;
>
--
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.
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