5.15-stable review patch. If anyone has any objections, please let me know.
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From: David Jeffery djeffery@redhat.com
[ Upstream commit cc22b5407e9ca76adb7efeed843146510b1b72a5 ]
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@redhat.com Tested-by: Laurence Oberman loberman@redhat.com Reviewed-by: Laurence Oberman loberman@redhat.com Reviewed-by: Yu Kuai yukuai3@huawei.com Signed-off-by: Song Liu song@kernel.org Link: https://lore.kernel.org/r/20230816181433.13289-1-djeffery@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 b1df1877c864b..2290307e0910f 100644 --- a/drivers/md/raid0.c +++ b/drivers/md/raid0.c @@ -578,8 +578,7 @@ 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) {