iolatency - summarize block device I/O latency as a histogram. Uses Linux
[-hQT] [-d device] [-i iotype] [interval [count]]
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
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.
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.
- -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.
- -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.
- Print usage message.
- Include block I/O queueing time. This uses block I/O queue
insertion as the start tracepoint (block:block_rq_insert), instead of
block I/O issue (block:block_rq_issue).
- Include timestamps with each summary output.
- 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.
- Number of summaries to print.
- Default output, print a summary of block I/O latency every
- # iolatency
- Include block I/O queue time:
- iolatency -Q
- Print 5 x 1 second summaries:
- # iolatency 1 5
- Trace reads only:
- # iolatency -i '*R*'
- Trace I/O issued to device 202,1 only:
- # iolatency -d 202,1
- Latency was greater than or equal-to this value, in
- Latency was less than this value, in milliseconds.
- Number of block device I/O in this latency range, during
- ASCII histogram representation of the I/O column.
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,
This is from the perf-tools collection.
Also look under the examples directory for a text file containing example usage,
output, and commentary for this tool.
Unstable - in development.