.TH iolatency 8  "2014-07-12" "USER COMMANDS"
.SH NAME
iolatency \- summarize block device I/O latency as a histogram. Uses Linux ftrace.
.SH SYNOPSIS
.B iolatency
[\-hT] [\-d device] [\-i iotype] [interval [count]]
.SH DESCRIPTION
This shows the distribution of latency, allowing modes and latency outliers
to be identified and studied. For more details of block device I/O, use
iosnoop(8).

This is a proof of concept tool using ftrace, and involves user space
processing and related overheads. See the OVERHEAD section.

NOTE: Due to the way trace buffers are switched per interval, there is the
possibility of losing a small number of I/O (usually less than 1%). The
summary therefore shows the general distribution, but may be slightly
incomplete. If 100% of I/O must be studied, use iosnoop(8) and post-process.
Also note that I/O may be missed when the trace buffer is full: see the
interval section in OPTIONS.

Since this uses ftrace, only the root user can use this tool.
.SH REQUIREMENTS
FTRACE CONFIG, and the tracepoints block:block_rq_issue and
block:block_rq_complete, which you may already have enabled and available on
recent Linux kernels. And awk.
.SH OPTIONS
.TP
\-d device
Only show I/O issued by this device. (eg, "202,1"). This matches the DEV
column in the iolatency output, and is filtered in-kernel.
.TP
\-i iotype
Only show I/O issued that matches this I/O type. This matches the TYPE column
in the iolatency output, and wildcards ("*") can be used at the beginning or
end (only). Eg, "*R*" matches all reads. This is filtered in-kernel.
.TP
\-h
Print usage message.
.TP
\-T
Include timestamps with each summary output.
.TP
interval
Interval between summary histograms, in seconds.

During the interval, trace output will be buffered in-kernel, which is then
read and processed for the summary. This buffer has a fixed size per-CPU (see
/sys/kernel/debug/tracing/buffer_size_kb). If you think events are missing,
try increasing that size (the bufsize_kb setting in iolatency). With the
default setting (4 Mbytes), I'd expect this to happen around 50k I/O per
summary.
.TP
count
Number of summaries to print.
.SH EXAMPLES
.TP
Default output, print a summary of block I/O latency every 1 second:
# 
.B iolatency
.TP
Print 5 x 1 second summaries:
#
.B iolatency 1 5
.TP
Trace reads only:
#
.B iolatency \-i '*R*'
.TP
Trace I/O issued to device 202,1 only:
#
.B iolatency \-d 202,1
.SH FIELDS
.TP
>=(ms)
Latency was greater than or equal-to this value, in milliseconds.
.TP
<(ms)
Latency was less than this value, in milliseconds.
.TP
I/O
Number of block device I/O in this latency range, during the interval.
.TP
Distribution
ASCII histogram representation of the I/O column.
.SH OVERHEAD
Block device I/O issue and completion events are traced and buffered
in-kernel, then processed and summarized in user space. There may be
measurable overhead with this approach, relative to the block device IOPS.

The overhead may be acceptable in many situations. If it isn't, this tool
can be reimplemented in C, or using a different tracer (eg, perf_events,
SystemTap, ktap.)
.SH SOURCE
This is from the perf-tools collection.
.IP
https://github.com/brendangregg/perf-tools
.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
iosnoop(8), iostat(1)