.TH biolatency-nd 8  "2015-01-30" "USER COMMANDS"
.SH NAME
biolatency-nd \- Measure block I/O latency distribution. Uses Linux SystemTap (non-debuginfo).
.SH SYNOPSIS
.B biolatency-nd.stp
[interval [count]]
.SH DESCRIPTION
This measures block I/O latency (storage I/O, ie, disk I/O), and shows the
distribution as a histogram. This can be useful to identify the characteristics
of I/O latency, beyond the averages shown by iostat(1). For example, to study
I/O latency outliers, or multi-modal distributions.

This script uses the kernel tracepoints block_rq_issue and block_rq_complete,
to trace the time from storage request to completion. To include extra time
spent queued in the kernel, change block_rq_issue to block_rq_insert.
.SH REQUIREMENTS
SystemTap.
.SH OPTIONS
.TP
interval
Interval for reports, in seconds.
.TP
count
Number of reports to output.
.SH EXAMPLES
.TP
Measure block I/O latency, and show a distribution histogram on Ctrl-C:
#
.B biolatency-nd.stp
.TP
Measure block I/O latency, and show a report every second, five times:
#
.B biolatency-nd.stp 1 5
.SH FIELDS
.TP
value
The minimum value, in nanoseconds, for this histogram bucket. Read this value
as "I/O latency was at least xxx".
.TP
count
How many I/O fell into this latency range.
.SH OVERHEAD
Should be low, relative to the rate of disk I/O. Test first.
.SH SOURCE
This is from the SystemTap lwtools collection.
.IP
https://github.com/brendangregg/systemtap-lwtools
.PP
Also look under the examples directory for a text file containing example
usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Brendan Gregg
.SH SEE ALSO
iostat(1)
