IOSTAT(8) | System Manager's Manual | IOSTAT(8)

# NAME

`iostat` — report I/O statistics

# SYNOPSIS

`iostat` | [`-CDdITXxyz`] [`-c` count] [`-H` height] [`-W` width] [`-w` wait] [drives]

# DESCRIPTION

`iostat` displays kernel I/O statistics on terminal, disk and CPU operations. By default, `iostat` displays one line of statistics averaged over the machine's run time. The use of `-c` or `-w` presents successive lines averaged over the wait period. The `-I` option causes `iostat` to print raw, unaveraged values (totals).

Only the last disk option specified (`-d`, `-D`, `-X`, `-x`, or `-y`) is used.

The options are as follows:

`-C`
  Show CPU statistics. This is enabled by default unless any of the `-D`, `-d`, `-T`, `-X`, `-x`, or `-y` flags are used.
`-c` count
  Repeat the display count times. Unless the `-I` flag is in effect, the first display is for the time since a reboot and each subsequent report is for the time period since the last display. If no wait interval is specified, the default is 1 second.
`-D`
  Show alternative disk statistics. Displays number of transfers, kilobytes transferred, and time spent in transfers, during the wait period (or since boot with `-I`). Use of this flag disables the default display.
`-d`
  Show disk statistics. This is the default. Displays number of transfers per second, kilobytes per transfer, and megabytes transferred per second. Use of this flag disables the default display of CPU and tty statistics.
`-H` height
  Set the page size (length, or height) explicitly, as the number of lines, height. If not set, the page length is taken from the environment variable `LINES` if set, or from the terminal (window) size, if output is to a terminal and its size is set, and otherwise defaults to 20. If explicitly set to zero, pages are considered to be infinitely long. This parameter determines the frequency at which repeated headers are output. If the value is greater than zero, but too small for the header, along with one output set, then a new header will be produced for each set of output.
`-I`
  Show the running total values, rather than an average.
`-T`
  Show tty statistics. This is enabled by default unless one, or more, of the `-C`, `-D`, `-d`, `-X`, `-x`, or `-y` flags are used.
`-W` width
  Set the page width explicitly, as the number of columns of characters, width. If not set, the page width is taken from the environment variable `COLUMNS` if set, or from the terminal (window) size, if output is to a terminal and its size is set, and otherwise defaults to 80. If explicitly set to 0, lines are considered infinitely long. This width is used only to determine the number of drives to display by default when no drive list is given. In other cases output will be as wide as needed to display the data requested.
`-w` wait
  Pause wait seconds between each display. If no repeat count is specified, the default is infinity.
`-X`
  Show limited alternative disk statistics. Displays megabytes transferred, and time spent in transfers, during the wait period (or since boot with `-I`). Use of this flag disables the default display.
`-x`
  Show extended disk statistics. Each disk is displayed on a line of its own with all available statistics. This option overrides all other display options, and all disks are displayed unless specific disk names are provided as arguments. Additionally, separate read and write statistics are displayed. The `-C` and `-T` options are ignored with this output format.
`-y`
  Shows the extended statistics (as with `-x`) and additional queuing statistics. Output does not fit in an 80 column window. The `-C` and `-T` options are ignored with this output format.
`-z`
  Replaces drive and CPU statistic outputs that are exactly zero with spaces. Note that zero values can still appear – this indicates a value that was not zero, but was rounded down so appears as zero. Drive output is replaced by spaces only when the drive did no input or output at all in the interval, or with `-I`, has never done any I/O.

`iostat` displays its information in the following format:

tty
  tin
    characters read from terminals
  tout
    characters written to terminals
disks
  Disk operations. The header of the field is the disk name and unit number. If more drives are configured in the system that fit across the current display, `iostat` displays only those drives that fit on the display. To force `iostat` to display specific drives, they may be supplied on the command line, either as names or `fnmatch`() patterns.

  t/s
    transfers per second
  KB/t
    Kilobytes transferred per disk transfer
  MB/s
    Megabytes transferred per second

  The alternative display format, (selected with `-D`), presents the following values:

  xfr
    Disk transfers
  KB
    Kilobytes transferred
  time
    Seconds spent in disk activity

  With the `-y` flag, the following queuing measurements are added:

  wait
    Number of I/O requests queued up
  actv
    Number of currently active I/O requests
  wsvc_t
    Average waiting time of an I/O request in milliseconds
  asvc_t
    Average duration of an I/O request in milliseconds
  wtime
    Seconds spent in the waiting queue. Queuing data might not be available from all drivers and is then shown as zeros.

  With the `-X` flag, the following queuing measurements are added:

  MB/s
    Megabytes transferred per second
  time
    Seconds spent in disk activity
cpu
  us
    % of CPU time in user mode
  ni
    % of CPU time in user mode running niced processes
  sy
    % of CPU time in system mode
  in
    % of CPU time in interrupt mode
  id
    % of CPU time in idle mode

  Note that because of rounding, these percentages may appear to total more or less than 100.

# SEE ALSO

fstat(1), netstat(1), nfsstat(1), ps(1), systat(1), vmstat(1), fnmatch(3), pstat(8)

The sections starting with ``Interpreting system activity'' in Installing and Operating 4.3BSD.

# HISTORY

`iostat` appeared in Version 6 AT&T UNIX. The `-x` option was added in NetBSD 1.4. Collection of queuing values and the `-y` option were added in NetBSD 8.0. The `-X` option was added in NetBSD 11.0. The archaic option format:
`iostat` [drives ...] [wait [count]]
remains supported (the first drive whose name starts with a digit is taken to be the wait period) but is deprecated, and may be removed in a future version, so should not be used.

July 28, 2023 | NetBSD 11.0