The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/iostats.txt

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    1 I/O statistics fields
    2 ---------------
    3 
    4 Since 2.4.20 (and some versions before, with patches), and 2.5.45,
    5 more extensive disk statistics have been introduced to help measure disk
    6 activity. Tools such as sar and iostat typically interpret these and do
    7 the work for you, but in case you are interested in creating your own
    8 tools, the fields are explained here.
    9 
   10 In 2.4 now, the information is found as additional fields in
   11 /proc/partitions.  In 2.6, the same information is found in two
   12 places: one is in the file /proc/diskstats, and the other is within
   13 the sysfs file system, which must be mounted in order to obtain
   14 the information. Throughout this document we'll assume that sysfs
   15 is mounted on /sys, although of course it may be mounted anywhere.
   16 Both /proc/diskstats and sysfs use the same source for the information
   17 and so should not differ.
   18 
   19 Here are examples of these different formats:
   20 
   21 2.4:
   22    3     0   39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
   23    3     1    9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
   24 
   25 
   26 2.6 sysfs:
   27    446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
   28    35486    38030    38030    38030
   29 
   30 2.6 diskstats:
   31    3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
   32    3    1   hda1 35486 38030 38030 38030
   33 
   34 On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have
   35 a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
   36 The advantage of one over the other is that the sysfs choice works well
   37 if you are watching a known, small set of disks.  /proc/diskstats may
   38 be a better choice if you are watching a large number of disks because
   39 you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
   40 each snapshot of your disk statistics.
   41 
   42 In 2.4, the statistics fields are those after the device name. In
   43 the above example, the first field of statistics would be 446216.
   44 By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
   45 find just the eleven fields, beginning with 446216.  If you look at
   46 /proc/diskstats, the eleven fields will be preceded by the major and
   47 minor device numbers, and device name.  Each of these formats provides
   48 eleven fields of statistics, each meaning exactly the same things.
   49 All fields except field 9 are cumulative since boot.  Field 9 should
   50 go to zero as I/Os complete; all others only increase (unless they
   51 overflow and wrap).  Yes, these are (32-bit or 64-bit) unsigned long
   52 (native word size) numbers, and on a very busy or long-lived system they
   53 may wrap. Applications should be prepared to deal with that; unless
   54 your observations are measured in large numbers of minutes or hours,
   55 they should not wrap twice before you notice them.
   56 
   57 Each set of stats only applies to the indicated device; if you want
   58 system-wide stats you'll have to find all the devices and sum them all up.
   59 
   60 Field  1 -- # of reads completed
   61     This is the total number of reads completed successfully.
   62 Field  2 -- # of reads merged, field 6 -- # of writes merged
   63     Reads and writes which are adjacent to each other may be merged for
   64     efficiency.  Thus two 4K reads may become one 8K read before it is
   65     ultimately handed to the disk, and so it will be counted (and queued)
   66     as only one I/O.  This field lets you know how often this was done.
   67 Field  3 -- # of sectors read
   68     This is the total number of sectors read successfully.
   69 Field  4 -- # of milliseconds spent reading
   70     This is the total number of milliseconds spent by all reads (as
   71     measured from __make_request() to end_that_request_last()).
   72 Field  5 -- # of writes completed
   73     This is the total number of writes completed successfully.
   74 Field  7 -- # of sectors written
   75     This is the total number of sectors written successfully.
   76 Field  8 -- # of milliseconds spent writing
   77     This is the total number of milliseconds spent by all writes (as
   78     measured from __make_request() to end_that_request_last()).
   79 Field  9 -- # of I/Os currently in progress
   80     The only field that should go to zero. Incremented as requests are
   81     given to appropriate struct request_queue and decremented as they finish.
   82 Field 10 -- # of milliseconds spent doing I/Os
   83     This field increases so long as field 9 is nonzero.
   84 Field 11 -- weighted # of milliseconds spent doing I/Os
   85     This field is incremented at each I/O start, I/O completion, I/O
   86     merge, or read of these stats by the number of I/Os in progress
   87     (field 9) times the number of milliseconds spent doing I/O since the
   88     last update of this field.  This can provide an easy measure of both
   89     I/O completion time and the backlog that may be accumulating.
   90 
   91 
   92 To avoid introducing performance bottlenecks, no locks are held while
   93 modifying these counters.  This implies that minor inaccuracies may be
   94 introduced when changes collide, so (for instance) adding up all the
   95 read I/Os issued per partition should equal those made to the disks ...
   96 but due to the lack of locking it may only be very close.
   97 
   98 In 2.6, there are counters for each CPU, which make the lack of locking
   99 almost a non-issue.  When the statistics are read, the per-CPU counters
  100 are summed (possibly overflowing the unsigned long variable they are
  101 summed to) and the result given to the user.  There is no convenient
  102 user interface for accessing the per-CPU counters themselves.
  103 
  104 Disks vs Partitions
  105 -------------------
  106 
  107 There were significant changes between 2.4 and 2.6 in the I/O subsystem.
  108 As a result, some statistic information disappeared. The translation from
  109 a disk address relative to a partition to the disk address relative to
  110 the host disk happens much earlier.  All merges and timings now happen
  111 at the disk level rather than at both the disk and partition level as
  112 in 2.4.  Consequently, you'll see a different statistics output on 2.6 for
  113 partitions from that for disks.  There are only *four* fields available
  114 for partitions on 2.6 machines.  This is reflected in the examples above.
  115 
  116 Field  1 -- # of reads issued
  117     This is the total number of reads issued to this partition.
  118 Field  2 -- # of sectors read
  119     This is the total number of sectors requested to be read from this
  120     partition.
  121 Field  3 -- # of writes issued
  122     This is the total number of writes issued to this partition.
  123 Field  4 -- # of sectors written
  124     This is the total number of sectors requested to be written to
  125     this partition.
  126 
  127 Note that since the address is translated to a disk-relative one, and no
  128 record of the partition-relative address is kept, the subsequent success
  129 or failure of the read cannot be attributed to the partition.  In other
  130 words, the number of reads for partitions is counted slightly before time
  131 of queuing for partitions, and at completion for whole disks.  This is
  132 a subtle distinction that is probably uninteresting for most cases.
  133 
  134 More significant is the error induced by counting the numbers of
  135 reads/writes before merges for partitions and after for disks. Since a
  136 typical workload usually contains a lot of successive and adjacent requests,
  137 the number of reads/writes issued can be several times higher than the
  138 number of reads/writes completed.
  139 
  140 In 2.6.25, the full statistic set is again available for partitions and
  141 disk and partition statistics are consistent again. Since we still don't
  142 keep record of the partition-relative address, an operation is attributed to
  143 the partition which contains the first sector of the request after the
  144 eventual merges. As requests can be merged across partition, this could lead
  145 to some (probably insignificant) inaccuracy.
  146 
  147 Additional notes
  148 ----------------
  149 
  150 In 2.6, sysfs is not mounted by default.  If your distribution of
  151 Linux hasn't added it already, here's the line you'll want to add to
  152 your /etc/fstab:
  153 
  154 none /sys sysfs defaults 0 0
  155 
  156 
  157 In 2.6, all disk statistics were removed from /proc/stat.  In 2.4, they
  158 appear in both /proc/partitions and /proc/stat, although the ones in
  159 /proc/stat take a very different format from those in /proc/partitions
  160 (see proc(5), if your system has it.)
  161 
  162 -- ricklind@us.ibm.com

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