block: Added in stricter no merge semantics for block I/O
Updated 'nomerges' tunable to accept a value of '2' - indicating that _no_
merges at all are to be attempted (not even the simple one-hit cache).
The following table illustrates the additional benefit - 5 minute runs of
a random I/O load were applied to a dozen devices on a 16-way x86_64 system.
nomerges Throughput %System Improvement (tput / %sys)
-------- ------------ ----------- -------------------------
0 12.45 MB/sec 0.669365609
1 12.50 MB/sec 0.641519199 0.40% / 2.71%
2 12.52 MB/sec 0.639849750 0.56% / 2.96%
Signed-off-by: Alan D. Brunelle <alan.brunelle@hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index d2f9033..4873c75 100644
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -128,3 +128,17 @@
preferred request size for workloads where sustained
throughput is desired. If no optimal I/O size is
reported this file contains 0.
+
+What: /sys/block/<disk>/queue/nomerges
+Date: January 2010
+Contact:
+Description:
+ Standard I/O elevator operations include attempts to
+ merge contiguous I/Os. For known random I/O loads these
+ attempts will always fail and result in extra cycles
+ being spent in the kernel. This allows one to turn off
+ this behavior on one of two ways: When set to 1, complex
+ merge checks are disabled, but the simple one-shot merges
+ with the previous I/O request are enabled. When set to 2,
+ all merge tries are disabled. The default value is 0 -
+ which enables all types of merge tries.