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@xxxxxxxxxxxxxxxxxx> wrote:
From: Mustafa Mesanovic<mume@xxxxxxxxxxxxxxxxxx>
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.
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@xxxxxxxxxxxxxxxxxx>
---
dm-stripe.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
Index: linux-2.6/drivers/md/dm-stripe.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-stripe.c 2011-02-28 10:23:37.000000000 +0100
+++ linux-2.6/drivers/md/dm-stripe.c 2011-02-28 10:24:29.000000000 +0100
@@ -396,6 +396,29 @@
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 = (struct stripe_c *) ti->private;
+ sector_t offset, chunk;
+ uint32_t stripe;
+ struct request_queue *q;
+
+ offset = bvm->bi_sector - ti->begin;
+ chunk = offset>> sc->chunk_shift;
+ stripe = sector_div(chunk, sc->stripes);
+
+ if (!bdev_get_queue(sc->stripe[stripe].dev->bdev)->merge_bvec_fn)
+ return max_size;
+
+ bvm->bi_bdev = sc->stripe[stripe].dev->bdev;
+ q = bdev_get_queue(bvm->bi_bdev);
+ bvm->bi_sector = sc->stripe[stripe].physical_start +
+ (chunk<< sc->chunk_shift) + (offset& sc->chunk_mask);
+
+ return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
+}
+
static struct target_type stripe_target = {
.name = "striped",
.version = {1, 3, 1},
@@ -403,6 +426,7 @@
.ctr = stripe_ctr,
.dtr = stripe_dtr,
.map = stripe_map,
+ .merge = stripe_merge,
.end_io = stripe_end_io,
.status = stripe_status,
.iterate_devices = stripe_iterate_devices,
--
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