[PATCH] Documentation for disk iostats

From: Rick Lindsley <ricklind@us.ibm.com>

Here is a file to add to the Documentation/ directory which describes the
disk statistics fields.

Documentation/iostats.txt

+I/O statistics fields
+---------------
+
+Last modified 5/15/03
+
+In 2.4.20 (and some versions before, with patches), and 2.5.45,
+more extensive disk statistics were introduced to help measure disk
+activity. Tools such as sar and iostat typically interpret these and do
+the work for you, but in case you are interested in creating your own
+tools, the fields are explained here.
+
+In most versions of the 2.4 patch, the information is found as additional
+fields in /proc/partitions.  In 2.5, the same information is found in
+two places: one is in the file /proc/diskstats (appears in 2.5.69 and
+beyond), and the other is within the sysfs file system, which must be
+mounted in order to obtain the information. Throughout this document
+we'll assume that sysfs is mounted on /sys, although of course it may
+be mounted anywhere.  In 2.5, both /proc/diskstats and sysfs use the
+same source for the information and so should not differ.
+
+Here are examples of these different formats:
+
+2.4:
+   3     0   39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
+   3     1    9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
+
+
+2.5 sysfs:
+   446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
+   35486    38030    38030    38030
+
+2.5 diskstats:
+   3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
+   3    1   hda1 35486 38030 38030 38030
+
+On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.5, you have
+a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
+The advantage of one over the other is that the sysfs choice works well
+if you are watching a known, small set of disks.  /proc/diskstats may
+be a better choice if you are watching a large number of disks because
+you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
+each snapshot of your disk statistics.
+
+In 2.4, the statistics fields are those after the device name. In
+the above example, the first field of statistics would be 446216.
+By contrast, in 2.5 if you look at /sys/block/hda/stat, you'll
+find just the eleven fields, beginning with 446216.  If you look at
+/proc/diskstats, the eleven fields will be preceded by the major and
+minor device numbers, and device name.  Each of these formats provide
+eleven fields of statistics, each meaning exactly the same things.
+All fields except field 9 are cumulative since boot.  Field 9 should
+go to zero as I/Os complete; all others only increase.  Yes, these are
+32 bit unsigned numbers, and on a very busy or long-lived system they
+may wrap. Applications should be prepared to deal with that; unless
+your observations are measured in large numbers of minutes or hours,
+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
+    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
+    efficiency.  Thus two 4K reads may become one 8K read before it is
+    ultimately handed to the disk, and so it will be counted (and queued)
+    as only one I/O.  This field lets you know how often this was done.
+Field  3 -- # of sectors read
+    This is the total number of sectors read successfully.
+Field  4 -- # of milliseconds spent reading
+    This is the total number of milliseconds spent by all reads (as
+    measured from __make_request() to end_that_request_last()).
+Field  5 -- # of writes completed
+    This is the total number of writes completed successfully.
+Field  7 -- # of sectors written
+    This is the total number of sectors written successfully.
+Field  8 -- # of milliseconds spent writing
+    This is the total number of milliseconds spent by all writes (as
+    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.
+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
+    This field is incremented at each I/O start, I/O completion, I/O
+    merge, or read of these stats by the number of I/Os in progress
+    (field 9) times the number of milliseconds spent doing I/O since the
+    last update of this field.  This can provide an easy measure of both
+    I/O completion time and the backlog that may be accumulating.
+
+
+To avoid introducing performance bottlenecks, no locks are held while
+modifying these counters.  This implies that minor inaccuracies may be
+introduced when changes collide, so (for instance) adding up all the
+read I/Os issued per partition should equal those made to the disks
+... but due to the lack of locking it may only be very close.
+
+In release 2.5.65 the 2.5 counters were made per-cpu, which made the lack
+of locking almost a non-issue.  When the statistics are read, the per-cpu
+counters are summed (possibly overflowing the unsigned 32-bit variable
+they are summed to) and the result given to the user.  There is no
+convenient user interface for accessing the per-cpu counters themselves.
+
+Disks vs Partitions
+-------------------
+
+There were significant changes between 2.4 and 2.5 in the I/O subsystem.
+As a result, some statistic information disappeared. The translation from
+a disk address relative to a partition to the disk address relative to
+the host disk happens much earlier.  All merges and timings now happen
+at the disk level rather than at both the disk and partition level as
+in 2.4.  Consequently, you'll see a different statistics output on 2.5 for
+partitions from that for disks.  There are only *four* fields available
+for partitions on 2.5 machines.  This is reflected in the examples above.
+
+Field  1 -- # of reads issued
+    This is the total number of reads issued to this partition.
+Field  2 -- # of sectors read
+    This is the total number of sectors requested to be read from this
+    partition.
+Field  3 -- # of reads issued
+    This is the total number of writes issued to this partition.
+Field  4 -- # of sectors read
+    This is the total number of sectors requested to be written to
+    this partition.
+
+Note that since the address is translated to a disk-relative one, and no
+record of the partition-relative address is kept, the subsequent success
+or failure of the read cannot be attributed to the partition.  In other
+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.
+
+Additional notes
+----------------
+
+In 2.5, sysfs is not mounted by default.  Here's the line you'll want
+to add to your /etc/fstab:
+
+none /sys sysfs defaults 0 0
+
+
+In 2.5, at the same time that disk statistics appeared in sysfs, they were
+removed from /proc/stat.  In 2.4, they appear in both /proc/partitions
+and /proc/stat.
+
+-- ricklind@us.ibm.com