--- name: "iostat" section: "8" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["freebsd", "openbsd", "macos"] url: "https://new.carta.tech/iostat/8" --- # iostat(8) report I/O statistics ## NAME **iostat** – report I/O statistics ## SYNOPSIS **iostat** \[**-CdhIKoTxz**\] \[**-c** *count*\] \[**-M** *core*\] \[**-n** *devs*\] \[**-N** *system*\]\[ **-t** *type*,*if*,*pass*\] \[**-w** *wait*\] \[*drives*\] ## DESCRIPTION The **iostat** utility displays kernel I/O statistics on terminal, device and cpu operations. The first statistics that are printed are averaged over the system uptime (unless **-I** is specified). Any references to repetitions in this man page implicitly include the first statistics output. To get information about the current activity, a suitable wait time should be specified, so that the subsequent sets of printed statistics will be averaged over that time. The options are as follows: - **-C** Display CPU statistics. This is on by default, unless **-d** or **-x** is specified. - **-c** Repeat the display *count* times. If **-c** is not specified and a wait interval is specified, the default repeat count is infinity. If no wait interval is specified, the default repeat count is 1. - **-d** Display only device statistics. If this flag is turned on, only device statistics will be displayed, unless **-C** or **-T** is also specified to enable the display of CPU or TTY statistics. - **-h** Put **iostat** in ‘top’ mode. In this mode, **iostat** will show devices in order from highest to lowest bytes per measurement cycle. - **-I** Display total statistics for a given time period, rather than average statistics for each second during that time period. - **-K** In the old-style blocks transferred display (**-I** and **-o**), display block count in kilobytes rather than the device native block size. - **-M** Extract values associated with the name list from the specified core instead of the default “*/dev/kmem*”. - **-N** Extract the name list from the specified system instead of the default “*/boot/kernel/kernel*”. - **-n** Display up to *devs* number of devices. The **iostat** utility will display fewer devices if there are less than *devs* devices present. - **-o** Display old-style **iostat** device statistics. Sectors per second, transfers per second, and milliseconds per seek are displayed. If **-I** is specified, total blocks/sectors, total transfers, and milliseconds per seek are displayed. - **-T** Display TTY statistics. This is on by default, unless **-d** or **-x** is specified. - **-t** Specify which types of devices to display. There are three different categories of devices: - device type: - da Direct Access devices - sa Sequential Access devices - printer Printers - proc Processor devices - worm Write Once Read Multiple devices - cd CD devices - scanner Scanner devices - optical Optical Memory devices - changer Medium Changer devices - comm Communication devices - array Storage Array devices - enclosure Enclosure Services devices - floppy Floppy devices - interface: - IDE Integrated Drive Electronics devices - SCSI Small Computer System Interface devices - NVME NVM Express Interface devices - other Any other device interface - passthrough: - pass Passthrough devices The user must specify at least one device type, and may specify at most one device type from each category. Multiple device types in a single device type statement must be separated by commas and are ANDed together. Any number of **-t** arguments may be specified on the command line. All **-t** arguments are ORed together to form a matching expression against which all devices in the system are compared. Any device that fully matches any **-t** argument will be included in the **iostat** output, up to the number of devices that can be displayed in 80 columns, or the maximum number of devices (**-n**) specified by the user. - **-w** Pause *wait* seconds between each display. If a repeat count (**-c**) is not specified and a wait interval is specified, the default repeat count is infinity. If **-w** is not specified and a repeat count is specified, the default interval is 1 second. The **iostat** command will accept and honor a non-integer number of seconds. Note that the interval only has millisecond granularity. Finer values will be truncated. E.g., “-w1.0001” is the same as “-w1.000”. The interval will also suffer from modifications to *kern.hz* so your mileage may vary. - **-x** Show extended disk statistics. Each disk is displayed on a line of its own with all available statistics. If this flag is turned on, only disk statistics will be displayed, unless **-C** or **-T** is also specified to enable the display of CPU or TTY statistics. - **-z** If **-x** is specified, omit lines for devices with no activity. The **iostat** utility displays its information in the following format: - tty - tin characters read from terminals - tout characters written to terminals - devices Device operations. The header of the field is the device name and unit number. The **iostat** utility will display as many devices as will fit in a standard 80 column screen, or the maximum number of devices in the system, whichever is smaller. If **-n** is specified on the command line, **iostat** will display the smaller of the requested number of devices, and the maximum number of devices in the system. To force **iostat** to display specific drives, their names may be supplied on the command line. The **iostat** utility will not display more devices than will fit in an 80 column screen, unless the **-n** argument is given on the command line to specify a maximum number of devices to display. If fewer devices are specified on the command line than will fit in an 80 column screen, **iostat** will show only the specified devices. The standard **iostat** device display shows the following statistics: - KB/t kilobytes per transfer - tps transfers per second - MB/s megabytes per second The standard **iostat** device display, with the **-I** flag specified, shows the following statistics: - KB/t kilobytes per transfer - xfrs total number of transfers - MB total number of megabytes transferred The extended **iostat** device display, with the **-x** flag specified, shows the following statistics: - r/s read operations per second - w/s write operations per second - kr/s kilobytes read per second - kw/s kilobytes write per second - qlen transactions queue length - ms/r average duration of read transactions, in milliseconds - ms/w average duration of write transactions, in milliseconds - ms/o average duration of all other transactions, in milliseconds - ms/t average duration of all transactions, in milliseconds - %b % of time the device had one or more outstanding transactions The extended **iostat** device display, with the **-x** and **-I** flags specified, shows the following statistics: - r/i read operations per time period - w/i write operations per time period - kr/i kilobytes read per time period - kw/i kilobytes write per time period - qlen transactions queue length - tsvc\_t/i total duration of transactions per time period, in seconds - sb/i total time the device had one or more outstanding transactions per time period, in seconds The old-style **iostat** display (using **-o**) shows the following statistics: - sps sectors transferred per second - tps transfers per second - msps average milliseconds per transaction The old-style **iostat** display, with the **-I** flag specified, shows the following statistics: - blk total blocks/sectors transferred - xfr total transfers - msps average milliseconds per transaction - 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 ## FILES - */boot/kernel/kernel* Default kernel namelist. - */dev/kmem* Default memory file. ## EXAMPLES ``` iostat -w 1 da0 da1 cd0 ``` Display statistics for the first two Direct Access devices and the first CDROM device every second ad infinitum. ``` iostat -c 2 ``` Display the statistics for the first four devices in the system twice, with a one second display interval. ``` iostat -t da -t cd -w 1 ``` Display statistics for all CDROM and Direct Access devices every second ad infinitum. ``` iostat -t da,scsi,pass -t cd,scsi,pass ``` Display statistics once for all SCSI passthrough devices that provide access to either Direct Access or CDROM devices. ``` iostat -h -n 8 -w 1 ``` Display up to 8 devices with the most I/O every second ad infinitum. ``` iostat -dh -t da -w 1 ``` Omit the TTY and CPU displays, show devices in order of performance and show only Direct Access devices every second ad infinitum. ``` iostat -Iw 3 ``` Display total statistics every three seconds ad infinitum. ``` iostat -odICTw 2 -c 9 ``` Display total statistics using the old-style output format 9 times, with a two second interval between each measurement/display. The **-d** flag generally disables the TTY and CPU displays, but since the **-T** and **-C** flags are given, the TTY and CPU displays will be displayed. ## SEE ALSO [fstat(1)](/fstat/1.md), [netstat(1)](/netstat/1.md), [nfsstat(1)](/nfsstat/1.md), [ps(1)](/ps/1.md), [systat(1)](/systat/1.md), [devstat(3)](/devstat/3.md), [ctlstat(8)](/ctlstat/8.md), [gstat(8)](/gstat/8.md), [pstat(8)](/pstat/8.md), [vmstat(8)](/vmstat/8.md) The sections starting with \`\`Interpreting system activity'' in ## HISTORY This version of **iostat** first appeared in FreeBSD 3.0. ## AUTHORS Kenneth Merry ⟨*ken@FreeBSD.org*⟩ ## BUGS The use of **iostat** as a debugging tool for crash dumps is probably limited because there is currently no way to get statistics that only cover the time immediately before the crash. Each *drives* argument is passed to **isdigit**(). If **isdigit**() returns true, **iostat** switches into an undocumented traditional syntax and subsequent arguments, if any, will not be interpreted as drive names. This behavior collides with the one documented in this man page and prevents specifying drive names starting with a number. E.g., the following ``` iostat ada0 ada1 2 3 cd0 ``` This document's use of “device type” is misleading.