NAME

biolatency - 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 disk I/O operations is measured from when requests are issued to the device up to 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.
-j: Print a histogram dictionary
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
Show a latency histogram in a dictionary format:: # biolatency -j

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