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. > > 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. 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 > > 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); -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
