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. > > > > 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 > 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 > > 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); > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
