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-rw-r--r--Documentation/iostats.txt15
1 files changed, 14 insertions, 1 deletions
diff --git a/Documentation/iostats.txt b/Documentation/iostats.txt
index b963c3b4afa5..5925c3cd030d 100644
--- a/Documentation/iostats.txt
+++ b/Documentation/iostats.txt
@@ -58,7 +58,7 @@ they should not wrap twice before you notice them.
58Each set of stats only applies to the indicated device; if you want 58Each set of stats only applies to the indicated device; if you want
59system-wide stats you'll have to find all the devices and sum them all up. 59system-wide stats you'll have to find all the devices and sum them all up.
60 60
61Field 1 -- # of reads issued 61Field 1 -- # of reads completed
62 This is the total number of reads completed successfully. 62 This is the total number of reads completed successfully.
63Field 2 -- # of reads merged, field 6 -- # of writes merged 63Field 2 -- # of reads merged, field 6 -- # of writes merged
64 Reads and writes which are adjacent to each other may be merged for 64 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
132of queuing for partitions, and at completion for whole disks. This is 132of queuing for partitions, and at completion for whole disks. This is
133a subtle distinction that is probably uninteresting for most cases. 133a subtle distinction that is probably uninteresting for most cases.
134 134
135More significant is the error induced by counting the numbers of
136reads/writes before merges for partitions and after for disks. Since a
137typical workload usually contains a lot of successive and adjacent requests,
138the number of reads/writes issued can be several times higher than the
139number of reads/writes completed.
140
141In 2.6.25, the full statistic set is again available for partitions and
142disk and partition statistics are consistent again. Since we still don't
143keep record of the partition-relative address, an operation is attributed to
144the partition which contains the first sector of the request after the
145eventual merges. As requests can be merged across partition, this could lead
146to some (probably insignificant) innacuracy.
147
135Additional notes 148Additional notes
136---------------- 149----------------
137 150