|author||Jerome Marchand <firstname.lastname@example.org>||2008-02-08 11:10:56 +0100|
|committer||Jens Axboe <email@example.com>||2008-02-08 12:42:11 +0100|
Enhanced partition statistics: documentation update
Update the documentation to reflect the change in userspace interface. Signed-off-by: Jerome Marchand <firstname.lastname@example.org> Signed-off-by: Jens Axboe <email@example.com>
Diffstat (limited to 'Documentation/iostats.txt')
1 files changed, 14 insertions, 1 deletions
diff --git a/Documentation/iostats.txt b/Documentation/iostats.txt
index b963c3b4afa5..5925c3cd030d 100644
@@ -58,7 +58,7 @@ they should not wrap twice before you notice them.
Each set of stats only applies to the indicated device; if you want
system-wide stats you'll have to find all the devices and sum them all up.
-Field 1 -- # of reads issued
+Field 1 -- # of reads completed
This is the total number of reads completed successfully.
Field 2 -- # of reads merged, field 6 -- # of writes merged
Reads and writes which are adjacent to each other may be merged for
@@ -132,6 +132,19 @@ words, the number of reads for partitions is counted slightly before time
of queuing for partitions, and at completion for whole disks. This is
a subtle distinction that is probably uninteresting for most cases.
+More significant is the error induced by counting the numbers of
+reads/writes before merges for partitions and after for disks. Since a
+typical workload usually contains a lot of successive and adjacent requests,
+the number of reads/writes issued can be several times higher than the
+number of reads/writes completed.
+In 2.6.25, the full statistic set is again available for partitions and
+disk and partition statistics are consistent again. Since we still don't
+keep record of the partition-relative address, an operation is attributed to
+the partition which contains the first sector of the request after the
+eventual merges. As requests can be merged across partition, this could lead
+to some (probably insignificant) innacuracy.