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    SubjectRe: [RFC][PATCH] dm: improve read performance
    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.
    >
    > 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@linux.vnet.ibm.com>
    > ---
    >
    > 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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