docs: iostats: Rewrite intro, remove outdated formats

I/O statistics fields

=====================

Since 2.4.20 (and some versions before, with patches), and 2.5.45,

more extensive disk statistics have been 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 2.4 now, the information is found as additional fields in

``/proc/partitions``.  In 2.6 and upper, the same information is found in two

places: one is in the file ``/proc/diskstats``, 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.

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

   2.6+ sysfs:

      446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160

      35486    38030    38030    38030

   2.6+ diskstats:

      3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160

      3    1   hda1 35486 38030 38030 38030

   4.18+ diskstats:

      3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0

On 2.4 you might execute ``grep 'hda ' /proc/partitions``. On 2.6+, 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 2.6+ if you look at ``/sys/block/hda/stat``, you'll

find just the 15 fields, beginning with 446216.  If you look at

``/proc/diskstats``, the 15 fields will be preceded by the major and

minor device numbers, and device name.  Each of these formats provides

15 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 (unless they

overflow and wrap). Wrapping might eventually occur on a very busy

or long-lived system; so applications should be prepared to deal with

it. Regarding wrapping, the types of the fields are either unsigned

int (32 bit) or unsigned long (32-bit or 64-bit, depending on your

machine) as noted per-field below. Unless your observations are very

spread in time, these fields should not wrap twice before you notice it.

The kernel exposes disk statistics via ``/proc/diskstats`` and

``/sys/block/<device>/stat``. These stats are usually accessed via tools

such as ``sar`` and ``iostat``.

Here are examples using a disk with two partitions::

   /proc/diskstats:

     259       0 nvme0n1 255999 814 12369153 47919 996852 81 36123024 425995 0 301795 580470 0 0 0 0 60602 106555

     259       1 nvme0n1p1 492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0

     259       2 nvme0n1p2 255401 1 12343477 47799 996004 0 36014736 425784 0 344336 473584 0 0 0 0 0 0

   /sys/block/nvme0n1/stat:

     255999 814 12369153 47919 996858 81 36123056 426009 0 301809 580491 0 0 0 0 60605 106562

   /sys/block/nvme0n1/nvme0n1p1/stat:

     492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0

Both files contain the same 17 statistics. ``/sys/block/<device>/stat``

contains the fields for ``<device>``. In ``/proc/diskstats`` the fields

are prefixed with the major and minor device numbers and the device

name. In the example above, the first stat value for ``nvme0n1`` is

255999 in both files.

The sysfs ``stat`` file is efficient for monitoring a small, known set

of disks. If you're tracking a large number of devices,

``/proc/diskstats`` is often the better choice since it avoids the

overhead of opening and closing multiple files for each snapshot.

All fields are cumulative, monotonic counters, except for field 9, which

resets to zero as I/Os complete. The remaining fields reset at boot, on

device reattachment or reinitialization, or when the underlying counter

overflows. Applications reading these counters should detect and handle

resets when comparing stat snapshots.

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.