Identity

Property	Value
Binary	`iostat`
Config	No persistent config — invoked directly
Logs	No persistent logs — output to terminal
Type	CLI tool (part of sysstat)
Install	`apt install sysstat` / `dnf install sysstat`

Key Operations

Task	Command
Single snapshot (averages since boot)	`iostat`
Continuous output every 1 second	`iostat 1`
Extended stats (latency, queue depth, utilization)	`iostat -x 1`
Extended stats for a specific device	`iostat -x sda 1`
Extended stats for NVMe device	`iostat -x nvme0n1 1`
Human-readable sizes (KB/MB/GB)	`iostat -h 1`
Output in megabytes per second	`iostat -m 1`
Include timestamps on each line	`iostat -t 1`
Per-partition statistics	`iostat -p sda 1`
JSON output (machine-parseable)	`iostat -o JSON 1`
Extended + human-readable + timestamps	`iostat -xht 1`
10 samples at 2-second intervals	`iostat -x 2 10`

Column	Meaning	Concern threshold
`r/s`	Read operations per second	Depends on workload
`w/s`	Write operations per second	Depends on workload
`rkB/s`	Read throughput KB/s	Near device max = saturated
`wkB/s`	Write throughput KB/s	Near device max = saturated
`await`	Average I/O wait time (ms)	>20ms HDD, >1ms NVMe is concerning
`r_await`	Average read wait time (ms)	As above, separated by direction
`w_await`	Average write wait time (ms)	As above, separated by direction
`aqu-sz`	Average I/O queue depth	>1 sustained means device is busy
`%util`	% time device was busy	Near 100% = saturated (but see pain points)

Symptom	Cause	Fix
First interval shows unexpectedly low/high values	First row averages since boot, not current activity	Discard first line; use second interval onward
`iostat: command not found`	sysstat package not installed	`apt install sysstat` / `dnf install sysstat`
`await` looks fine but system is slow	High queue depth (`aqu-sz`) with moderate await means individual I/Os queue	Check `aqu-sz` alongside `await`; I/O scheduler may need tuning
NVMe shows as `nvme0n1` not `sdX`	NVMe uses its own namespace naming	Use `nvme0n1`, `nvme0n1p1` etc. in device arguments
`%util` near 100% but latency is low	SSDs and NVMe can saturate `%util` while serving queued I/Os efficiently	Rely on `await` and `aqu-sz` for SSDs — `%util` is less meaningful for parallel-IO devices
No output for a device	Device path wrong or not yet active	`lsblk` to list block devices; match name exactly in `-x <device>`

First interval is since-boot averages: like vmstat, the first iostat output row covers the entire uptime. For current disk activity, always specify an interval and read from the second line onward.
await is the key latency metric: average I/O wait in milliseconds is the most actionable single number. Over 20ms sustained on an HDD indicates queue buildup; over 1ms on NVMe is worth investigating. Cross-reference with aqu-sz to determine if latency is from queuing or from the device itself.
%util is misleading for NVMe and RAID: %util measures the fraction of time the device had at least one I/O in flight. For devices with native command queuing (NVMe) or multiple spindles (RAID), 100% util does not mean saturated — they can still process more I/Os in parallel. Use await and aqu-sz instead.
sysstat must be installed separately: unlike procps, sysstat is not part of most default installs. If iostat is missing, install sysstat explicitly.
NVMe device naming: NVMe drives appear as nvme0n1 (device), nvme0n1p1 (partition), not as sdX. Pass the correct name to -x or -p; specifying sda on an NVMe-only system returns no output.

See references/ for: