From: NeilBrown <neilb@suse.de>
Date: Tue, 11 Oct 2011 05:50:01 +0000 (+1100)
Subject: md: add proper write-congestion reporting to RAID1 and RAID10.
X-Git-Tag: v3.2-rc1~154^2~6
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=34db0cd60f8a1f4ab73d118a8be3797c20388223;p=pandora-kernel.git

md: add proper write-congestion reporting to RAID1 and RAID10.

RAID1 and RAID10 handle write requests by queuing them for handling by
a separate thread.  This is because when a write-intent-bitmap is
active we might need to update the bitmap first, so it is good to
queue a lot of writes, then do one big bitmap update for them all.

However writeback request devices to appear to be congested after a
while so it can make some guesstimate of throughput.  The infinite
queue defeats that (note that RAID5 has already has a finite queue so
it doesn't suffer from this problem).

So impose a limit on the number of pending write requests.  By default
it is 1024 which seems to be generally suitable.  Make it configurable
via module option just in case someone finds a regression.

Signed-off-by: NeilBrown <neilb@suse.de>
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