bcc-biolatency man page

biolatency ā€” Summarize block device I/O latency as a histogram.

Synopsis

biolatency [-h] [-F] [-T] [-Q] [-m] [-D] [interval [count]]

Description

biolatency traces block device I/O (disk I/O), and records the distribution of I/O latency (time). This is printed as a histogram either on Ctrl-C, or after a given interval in seconds.

The latency of the disk I/O is measured from the issue to the device to its completion. A -Q option can be used to include time queued in the kernel.

This tool uses in-kernel eBPF maps for storing timestamps and the histogram, for efficiency.

This works by tracing various kernel blk_*() functions using dynamic tracing, and will need updating to match any changes to these functions.

Since this uses BPF, only the root user can use this tool.

Requirements

CONFIG_BPF and bcc.

Options

-h Print usage message.

-T

Include timestamps on output.

-m

Output histogram in milliseconds.

-D

Print a histogram per disk device.

-F

Print a histogram per set of I/O flags.

interval

Output interval, in seconds.

count

Number of outputs.

Examples

Summarize block device I/O latency as a histogram:

# biolatency

Print 1 second summaries, 10 times:

# biolatency 1 10

Print 1 second summaries, using milliseconds as units for the histogram, and

include timestamps on output: # biolatency -mT 1

Include OS queued time in I/O time:

# biolatency -Q

Show a latency histogram for each disk device separately:

# biolatency -D

Fields

usecs

Microsecond range

msecs

Millisecond range

count

How many I/O fell into this range

distribution

An ASCII bar chart to visualize the distribution (count column)

Overhead

This traces kernel functions and maintains in-kernel timestamps and a histogram, which are asynchronously copied to user-space. This method is very efficient, and the overhead for most storage I/O rates (< 10k IOPS) should be negligible. If you have a higher IOPS storage environment, test and quantify the overhead before use.

Source

This is from bcc.

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

OS

Linux

Stability

Unstable - in development.

Author

Brendan Gregg

See Also

biosnoop(8)

Info

2015-08-20 USER COMMANDS