[RFC][PATCH] dm: improve read performance

[Mustafa Mesanovic]

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. 

Same happens with luns which are multipathed -> the multipathed targets have 
the same max_sectors as the luns below.

So "magic" happens only when target has no own merge_fn and below lying 
devices
have a merge function -> we got then max_sectors=PAGE_SIZE >> 9.
This patch prevents that max_sectors will be set to PAGE_SIZE >> 9.
Instead it will use the minimum max_sectors value from below devices.

Using the patch improves read I/O up to 3x. In this specific case from 600MiB/s
up to 1800MiB/s.

Signed-off-by: Mustafa Mesanovic <mume@xxxxxxxxxxxxxxxxxx>
---

 dm-table.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/drivers/md/dm-table.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-table.c	2010-12-23 13:49:18.000000000 +0100
+++ linux-2.6/drivers/md/dm-table.c	2010-12-23 13:50:22.000000000 +0100
@@ -518,7 +518,7 @@
 
 	if (q->merge_bvec_fn && !ti->type->merge)
 		blk_limits_max_hw_sectors(limits,
-					  (unsigned int) (PAGE_SIZE >> 9));
+					  q->limits.max_sectors);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(dm_set_device_limits);
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