[Devel] [PATCH hci-8.1 0/2] dm-qcow2, dm-ploop: do not merge discards with gaps

[Devel] [PATCH hci-8.1 0/2] dm-qcow2, dm-ploop: do not merge discards with gaps

[Andrey Zhadchenko]

The block layer may merge non-contiguous discard bios into a single
request when the queue allows more than one discard segment, and the
stacking default for dm is max_discard_segments = USHRT_MAX.

Both dm-qcow2 and dm-ploop handle a discard request as one contiguous
range taken from blk_rq_pos() and blk_rq_bytes(). For a request built
from bios [0, 512) and [1024, 1536) they discard [0, 1024), which hits
sectors that were never asked to be discarded and misses part of the
range that was.

Set max_discard_segments to 1 in both targets so each discard request
always describes a single contiguous range.

https://virtuozzo.atlassian.net/browse/VSTOR-154159

Andrey Zhadchenko (2):
  dm-qcow2: set max discard segments to one
  dm-ploop: set max discard segments to one

 drivers/md/dm-ploop-target.c | 1 +
 drivers/md/dm-qcow2-target.c | 1 +
 2 files changed, 2 insertions(+)

-- 
2.43.5

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[Devel] [PATCH hci-8.1 1/2] dm-qcow2: set max discard segments to one

[Andrey Zhadchenko]

Discard is a special type of request that can have non-contiguous
bios.
Imagine we have a request with starting sector 0 and length 1024,
so read or write means [0, 1024) range in the file. For discard this
request can have 2 bios [0, 512) and [1024, 1536), but we always
discarded [0, 1024).
Set max_discard_segments to 1 to avoid such merges.

https://virtuozzo.atlassian.net/browse/VSTOR-154159
Fixes: 222b6f1291c1 ("dm-qcow2: Introduce driver to create block devices over QCOW2 files")
Feature: dm-qcow2: block device over QCOW2 files driver
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 drivers/md/dm-qcow2-target.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/md/dm-qcow2-target.c b/drivers/md/dm-qcow2-target.c
index 7c572bd66e1ca..8337efc5c6238 100644
--- a/drivers/md/dm-qcow2-target.c
+++ b/drivers/md/dm-qcow2-target.c
@@ -1053,6 +1053,7 @@ static void qcow2_io_hints(struct dm_target *ti, struct queue_limits *limits)
 	 */
 	limits->discard_granularity = sb->s_blocksize;
 	limits->max_hw_discard_sectors = to_sector(qcow2->clu_size);
+	limits->max_discard_segments = 1;
 
 	limits->logical_block_size = block_size;
 	limits->physical_block_size = block_size;
-- 
2.43.5

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[Devel] [PATCH hci-8.1 2/2] dm-ploop: set max discard segments to one

[Andrey Zhadchenko]

Discard is a special type of request that can have non-contiguous
bios.
Imagine we have a request with starting sector 0 and length 1024,
so read or write means [0, 1024) range in the file. For discard this
request can have 2 bios [0, 512) and [1024, 1536), but we always
discarded [0, 1024).
Set max_discard_segments to 1 to avoid such merges.

https://virtuozzo.atlassian.net/browse/VSTOR-154159
Fixes: 6ddad5a4d226 ("dm-ploop: Add ploop target driver")
Feature: dm-ploop: ploop target driver
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>
---
 drivers/md/dm-ploop-target.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/md/dm-ploop-target.c b/drivers/md/dm-ploop-target.c
index d8279c79ef56d..b1f502d664447 100644
--- a/drivers/md/dm-ploop-target.c
+++ b/drivers/md/dm-ploop-target.c
@@ -661,6 +661,7 @@ static void ploop_io_hints(struct dm_target *ti, struct queue_limits *limits)
 	limits->max_hw_discard_sectors = CLU_TO_SEC(ploop, 1);
 	limits->discard_granularity = CLU_SIZE(ploop);
 	limits->discard_alignment = 0;
+	limits->max_discard_segments = 1;
 
 	limits->fua = BLK_NO_FUA;
 }
-- 
2.43.5

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Re: [Devel] [PATCH hci-8.1 1/2] dm-qcow2: set max discard segments to one

[no-reply]

> Discard is a special type of request that can have non-contiguous
> bios.
> Imagine we have a request with starting sector 0 and length 1024,
> so read or write means [0, 1024) range in the file. For discard this
> request can have 2 bios [0, 512) and [1024, 1536), but we always
> discarded [0, 1024).
> Set max_discard_segments to 1 to avoid such merges.
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-154159
> Fixes: 222b6f1291c1 ("dm-qcow2: Introduce driver to create block devices over QCOW2 files")
> Feature: dm-qcow2: block device over QCOW2 files driver
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261109153619.277888-1-andrey.zhadchenko@virtuozzo.com?part=1


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Re: [Devel] [PATCH hci-8.1 2/2] dm-ploop: set max discard segments to one

[no-reply]

> Discard is a special type of request that can have non-contiguous
> bios.
> Imagine we have a request with starting sector 0 and length 1024,
> so read or write means [0, 1024) range in the file. For discard this
> request can have 2 bios [0, 512) and [1024, 1536), but we always
> discarded [0, 1024).
> Set max_discard_segments to 1 to avoid such merges.
> 
> https://virtuozzo.atlassian.net/browse/VSTOR-154159
> Fixes: 6ddad5a4d226 ("dm-ploop: Add ploop target driver")
> Feature: dm-ploop: ploop target driver
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.vzint.dev/#/patchset/20261109153619.277888-1-andrey.zhadchenko@virtuozzo.com?part=2


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