On Wed, Aug 16, 2023 at 11:14 AM David Jeffery <djeffery@xxxxxxxxxx> wrote:
>
> When a bio is split by md raid0, the newly created bio will not be tracked
> by md for I/O accounting. Only the portion of I/O still assigned to the
> original bio which was reduced by the split will be accounted for. This
> results in md iostat data sometimes showing I/O values far below the actual
> amount of data being sent through md.
>
> md_account_bio() needs to be called for all bio generated by the bio split.
>
> A simple example of the issue was generated using a raid0 device on partitions
> to the same device. Since all raid0 I/O then goes to one device, it makes it
> easy to see a gap between the md device and its sd storage. Reading an lvm
> device on top of the md device, the iostat output (some 0 columns and extra
> devices removed to make the data more compact) was:
>
> Device tps kB_read/s kB_wrtn/s kB_dscd/s kB_read
> md2 0.00 0.00 0.00 0.00 0
> sde 0.00 0.00 0.00 0.00 0
> md2 1364.00 411496.00 0.00 0.00 411496
> sde 1734.00 646144.00 0.00 0.00 646144
> md2 1699.00 510680.00 0.00 0.00 510680
> sde 2155.00 802784.00 0.00 0.00 802784
> md2 803.00 241480.00 0.00 0.00 241480
> sde 1016.00 377888.00 0.00 0.00 377888
> md2 0.00 0.00 0.00 0.00 0
> sde 0.00 0.00 0.00 0.00 0
>
> I/O was generated doing large direct I/O reads (12M) with dd to a linear
> lvm volume on top of the 4 leg raid0 device.
>
> The md2 reads were showing as roughly 2/3 of the reads to the sde device
> containing all of md2's raid partitions. The sum of reads to sde was
> 1826816 kB, which was the expected amount as it was the amount read by
> dd. With the patch, the total reads from md will match the reads from
> sde and be consistent with the amount of I/O generated.
>
> Fixes: 10764815ff47 ("md: add io accounting for raid0 and raid5")
> Signed-off-by: David Jeffery <djeffery@xxxxxxxxxx>
> Tested-by: Laurence Oberman <loberman@xxxxxxxxxx>
> Reviewed-by: Laurence Oberman <loberman@xxxxxxxxxx>
> Reviewed-by: Yu Kuai <yukuai3@xxxxxxxxxx>
Applied to md-next. Thanks!
Song
> ---
>
> Patch v2: ported to apply on top of md-next
>
> drivers/md/raid0.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
> index abbd77977f98..c50a7abda744 100644
> --- a/drivers/md/raid0.c
> +++ b/drivers/md/raid0.c
> @@ -553,8 +553,7 @@ static void raid0_map_submit_bio(struct mddev *mddev, struct bio *bio)
> sector_t bio_sector = bio->bi_iter.bi_sector;
> sector_t sector = bio_sector;
>
> - if (bio->bi_pool != &mddev->bio_set)
> - md_account_bio(mddev, &bio);
> + md_account_bio(mddev, &bio);
>
> zone = find_zone(mddev->private, §or);
> switch (conf->layout) {
> --
> 2.41.0
>
