X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=Documentation%2Fiostats.txt;h=5925c3cd030d55884b3f962ed40ced5a20709cf4;hb=0d63081d418c73cc187c893069e0f24c4c6eecd3;hp=09a1bafe25289d370e9ecb9aaae7892d1a1add91;hpb=1da177e4c3f41524e886b7f1b8a0c1fc7321cac2;p=linux-2.6

diff --git a/Documentation/iostats.txt b/Documentation/iostats.txt
index 09a1bafe25..5925c3cd03 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
@@ -79,7 +79,7 @@ Field  8 -- # of milliseconds spent writing
     measured from __make_request() to end_that_request_last()).
 Field  9 -- # of I/Os currently in progress
     The only field that should go to zero. Incremented as requests are
-    given to appropriate request_queue_t and decremented as they finish.
+    given to appropriate struct request_queue and decremented as they finish.
 Field 10 -- # of milliseconds spent doing I/Os
     This field is increases so long as field 9 is nonzero.
 Field 11 -- weighted # of milliseconds spent doing I/Os
@@ -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
 ----------------