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authorShaohua Li <shli@kernel.org>2012-07-31 10:03:53 +1000
committerNeilBrown <neilb@suse.de>2012-07-31 10:03:53 +1000
commit12cee5a8a29e7263e39953f1d941f723c617ca5f (patch)
tree8aaf2fe512cf82e0640656640335ea2d7f0b2ec0 /drivers/md/raid1.h
parent9dedf60313fa4dddfd5b9b226a0ef12a512bf9dc (diff)
md/raid1: prevent merging too large request
For SSD, if request size exceeds specific value (optimal io size), request size isn't important for bandwidth. In such condition, if making request size bigger will cause some disks idle, the total throughput will actually drop. A good example is doing a readahead in a two-disk raid1 setup. So when should we split big requests? We absolutly don't want to split big request to very small requests. Even in SSD, big request transfer is more efficient. This patch only considers request with size above optimal io size. If all disks are busy, is it worth doing a split? Say optimal io size is 16k, two requests 32k and two disks. We can let each disk run one 32k request, or split the requests to 4 16k requests and each disk runs two. It's hard to say which case is better, depending on hardware. So only consider case where there are idle disks. For readahead, split is always better in this case. And in my test, below patch can improve > 30% thoughput. Hmm, not 100%, because disk isn't 100% busy. Such case can happen not just in readahead, for example, in directio. But I suppose directio usually will have bigger IO depth and make all disks busy, so I ignored it. Note: if the raid uses any hard disk, we don't prevent merging. That will make performace worse. Signed-off-by: Shaohua Li <shli@fusionio.com> Signed-off-by: NeilBrown <neilb@suse.de>
Diffstat (limited to 'drivers/md/raid1.h')
-rw-r--r--drivers/md/raid1.h1
1 files changed, 1 insertions, 0 deletions
diff --git a/drivers/md/raid1.h b/drivers/md/raid1.h
index 3770b4a2766..0ff3715fb7e 100644
--- a/drivers/md/raid1.h
+++ b/drivers/md/raid1.h
@@ -9,6 +9,7 @@ struct raid1_info {
* we try to keep sequential reads one the same device
*/
sector_t next_seq_sect;
+ sector_t seq_start;
};
/*