On Wed, 23 Nov 2011 14:22:16 +0100 "Ramon Schönborn" [off-list ref]
wrote:

quoted

RAID1 will only limit requests to 4K if the device beneath it is
non-contiguous - e.g. a striped array or LVM arrangement were consecutive
blocks might be on different devices.

how does it know if a device is non-contiguous? Is there a way to have a dm device "marked" like that or force md to use bigger requests?
Lets assume a host with about 20 raid1 devices consisting of dm-devices with the mentioned overhead - do you think that not splitting the requests could lead to a noticeable performance improvement?

Thanx for your help,
Ramon Schönborn

If the device provide a "merge_bvec_fn", then it is assumed to not be
contiguous.
dm always sets this on its devices.

I really have no idea what sort of overhead this creates.  You would need to
test it.
I assume you are using dm simply as a partitioning tool with a single linear
mapping per device.
If this is the case it should be safe for testing to remove the line

	blk_queue_merge_bvec(md->queue, dm_merge_bvec);

from drivers/md/dm.c and see how that change performance.  If you have any dm
targets more simple than a single linear mapping with will almost certainly
cause IO failure at some point so this should only be used for testing.

NeilBrown