From 0e53c2be0495afa97c6b0d06397adcbff9c65347 Mon Sep 17 00:00:00 2001 From: Jerome Marchand Date: Fri, 8 Feb 2008 11:10:56 +0100 Subject: Enhanced partition statistics: documentation update Update the documentation to reflect the change in userspace interface. Signed-off-by: Jerome Marchand Signed-off-by: Jens Axboe --- Documentation/iostats.txt | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) (limited to 'Documentation/iostats.txt') diff --git a/Documentation/iostats.txt b/Documentation/iostats.txt index b963c3b4afa..5925c3cd030 100644 --- a/Documentation/iostats.txt +++ b/Documentation/iostats.txt @@ -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. + Additional notes ---------------- -- cgit v1.2.3