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[dm-devel] [PATCH v3] dm stripe: implement merge method

Hello Mustafa,

On Mon, Mar 07 2011 at  5:10am -0500,
Mustafa Mesanovic <mume linux vnet ibm com> wrote:

> On 12/27/2010 01:23 PM, Mustafa Mesanovic wrote:
> >On Mon December 27 2010 12:54:59 Neil Brown wrote:
> >>On Mon, 27 Dec 2010 12:19:55 +0100 Mustafa Mesanovic
> >>
> >><mume linux vnet ibm com>  wrote:
> >>>From: Mustafa Mesanovic<mume linux vnet ibm com>
> >>>
> >>>A short explanation in prior: in this case we have "stacked" dm devices.
> >>>Two multipathed luns combined together to one striped logical volume.
> >>>
> >>>I/O throughput degradation happens at __bio_add_page when bio's get
> >>>checked upon max_sectors. In this setup max_sectors is always set to 8
> >>>->  what is 4KiB.
> >>>A standalone striped logical volume on luns which are not multipathed do
> >>>not have the problem: the logical volume will take over the max_sectors
> >>>from luns below.
> [...]
> >>>Using the patch improves read I/O up to 3x. In this specific case from
> >>>600MiB/s up to 1800MiB/s.
> >>and using this patch will cause IO to fail sometimes.
> >>If an IO request which is larger than a page crosses a device boundary in
> >>the underlying e.g. RAID0, the RAID0 will return an error as such things
> >>should not happen - they are prevented by merge_bvec_fn.
> >>
> >>If merge_bvec_fn is not being honoured, then you MUST limit requests to a
> >>single entry iovec of at most one page.
> >>
> >>NeilBrown
> >>
> >Thank you for that hint, I will try to write a merge_bvec_fn for dm-stripe.c
> >which solves the problem, if that is ok?
> >
> >Mustafa Mesanovic
> >
> Now here my new suggestion to fix this issue, what is your opinion?
> I tested this with different setups, and it worked fine and I had
> very good performance improvements.
> [RFC][PATCH] dm: improve read performance - v2
> This patch adds a merge_fn for the dm stripe target. This merge_fn
> prevents dm_set_device_limits() setting the max_sectors to 4KiB
> (PAGE_SIZE). (As in a prior patch already mentioned.)
> Now the read performance improved up to 3x higher compared to before.
> What happened before:
> I/O throughput degradation happened at __bio_add_page() when bio's got checked
> at the very beginning upon max_sectors. In this setup max_sectors is always
> set to 8. So bio's entered the dm target with a max of 4KiB.

So to recap:
- you are stacking dm-stripe on DM multipath devices
- dm_set_device_limits() will set max_hw_sectors to PAGE_SIZE if:
  1) q->merge_bvec_fn is defined (as is the case for all DM devices)
  2) target does not define a merge_fn (as was the case for dm-stripe)
- dm_merge_bvec (aka DM's q->merge_bvec_fn) will only allow a single
  page for the situation described in the previous point.

> Now dm-stripe target will have its own merge_fn so max_sectors will not
> pushed down to 8 (4KiB), and bio's can get bigger than 4KiB.
> Signed-off-by: Mustafa Mesanovic<mume linux vnet ibm com>

Here is a revised version of your patch that uses the relatively new
stripe_map_sector(), removes an unnecessary cast, and a few other

This v3 should work as well as your v2 but should be suitable for
upstream inclusion -- authorship of the change is still attributed to
you as I only updated the code slightly.  I'd appreciate it if you could
verify your config still performs as expected.


From: Mustafa Mesanovic <mume linux vnet ibm com>

When the stripe target's underlying devices provide a merge_bvec_fn
(e.g. DM) it is important to call down to them when building a biovec
that doesn't span a stripe boundary.

Without the merge method, a striped DM device stacked on DM devices
would cause bios with a single page to be submitted.  The small bios
that were built resulted in unnecessary overhead that hurt performance.

In one setup, striped DM multipath devices, read performance was
improved from 600MB/s to 1800MB/s.

Signed-off-by: Mustafa Mesanovic <mume linux vnet ibm com>
Signed-off-by: Mike Snitzer <snitzer redhat com>
 drivers/md/dm-stripe.c |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
index dddfa14..fdfc1c6 100644
--- a/drivers/md/dm-stripe.c
+++ b/drivers/md/dm-stripe.c
@@ -396,9 +396,29 @@ static void stripe_io_hints(struct dm_target *ti,
 	blk_limits_io_opt(limits, chunk_size * sc->stripes);
+static int stripe_merge(struct dm_target *ti, struct bvec_merge_data *bvm,
+			struct bio_vec *biovec, int max_size)
+	struct stripe_c *sc = ti->private;
+	sector_t bvm_sector = bvm->bi_sector;
+	uint32_t stripe;
+	struct request_queue *q;
+	stripe_map_sector(sc, bvm_sector, &stripe, &bvm_sector);
+	q = bdev_get_queue(sc->stripe[stripe].dev->bdev);
+	if (!q->merge_bvec_fn)
+		return max_size;
+	bvm->bi_bdev = sc->stripe[stripe].dev->bdev;
+	bvm->bi_sector = sc->stripe[stripe].physical_start + bvm_sector;
+	return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
 static struct target_type stripe_target = {
 	.name   = "striped",
-	.version = {1, 3, 1},
+	.version = {1, 3, 2},
 	.module = THIS_MODULE,
 	.ctr    = stripe_ctr,
 	.dtr    = stripe_dtr,
@@ -407,6 +427,7 @@ static struct target_type stripe_target = {
 	.status = stripe_status,
 	.iterate_devices = stripe_iterate_devices,
 	.io_hints = stripe_io_hints,
+	.merge  = stripe_merge,
 int __init dm_stripe_init(void)

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