blktrace - generate traces of the i/o traffic on block devices
blktrace -d dev [ -r debugfs_path ] [ -o output ] [-k ] [ -w time ] [
-a action ] [ -A action_mask ] [ -v ]
blktrace is a block layer IO tracing mechanism which provides detailed
information about request queue operations up to user space. There are
three major components: a kernel component, a utility to record the i/o
trace information for the kernel to user space, and utilities to
analyse and view the trace information. This man page describes
blktrace, which records the i/o event trace information for a specific
block device to a file.
The blktrace utility extracts event traces from the kernel (via the
relaying through the debug file system). Some background details
concerning the run-time behaviour of blktrace will help to understand
some of the more arcane command line options:
- blktrace receives data from the kernel in buffers passed up through
the debug file system (relay). Each device being traced has a file
created in the mounted directory for the debugfs, which defaults to
/sys/kernel/debug -- this can be overridden with the -r command line
- blktrace defaults to collecting all events that can be traced. To
limit the events being captured, you can specify one or more filter
masks via the -a option.
Alternatively, one may specify the entire mask utilising a
hexadecimal value that is version-specific. (Requires understanding
of the internal representation of the filter mask.)
- As noted above, the events are passed up via a series of buffers
stored into debugfs files. The size and number of buffers can be
specified via the -b and -n arguments respectively.
- blktrace stores the extracted data into files stored in the local
directory. The format of the file names is (by default)
device.blktrace.cpu, where device is the base device name (e.g, if we
are tracing /dev/sda, the base device name would be sda); and cpu
identifies a CPU for the event stream.
The device portion of the event file name can be changed via the -o
- blktrace may also be run concurrently with blkparse to produce live
output -- to do this specify -o - for blktrace.
- The default behaviour for blktrace is to run forever until explicitly
killed by the user (via a control-C, or kill utility invocation).
There are two ways to modify this:
1. You may utilise the blktrace utility itself to kill a running
trace -- via the -k option.
2. You can specify a run-time duration for blktrace via the -w option
-- then blktrace will run for the specified number of seconds, and
Set filter mask to hex-mask (see below for masks)
Add mask to current filter (see below for masks)
Specifies buffer size for event extraction (scaled by 1024). The
default buffer size is 512KiB.
Adds dev as a device to trace
Adds the devices found in file as devices to trace
Kill on-going trace
Specifies number of buffers to use. blktrace defaults to 4 sub
Prepend file to output file name(s)
Specifies debugfs mount point
--version Outputs version
Sets run time to the number of seconds specified
The following masks may be passed with the -a command line option,
multiple filters may be combined via multiple -a command line options.
barrier: barrier attribute
complete: completed by driver
issue: issued to driver
pc: packet command events
queue: queue operations
read: read traces
requeue: requeue operations
sync: synchronous attribute
write: write traces
notify: trace messages
blktrace distinguishes between two types of block layer requests, file
system and SCSI commands. The former are dubbed fs requests, the latter
pc requests. File system requests are normal read/write operations,
i.e. any type of read or write from a specific disk location at a
given size. These requests typically originate from a user process, but
they may also be initiated by the vm flushing dirty data to disk or the
file system syncing a super or journal block to disk. pc requests are
SCSI commands. blktrace sends the command data block as a payload so
that blkparse can decode it.
To trace the i/o on the device /dev/hda and parse the output to human
readable form, use the following command:
% blktrace -d /dev/sda -o - | blkparse -i -
This same behaviour can be achieve with the convenience script btrace.
% btrace /dev/sda
has exactly the same effect as the previous command. See btrace (8) for
To trace the i/o on a device and save the output for later processing
with blkparse, use blktrace like this:
% blktrace /dev/sda /dev/sdb
This will trace i/o on the devices /dev/sda and /dev/sdb and save the
recorded information in the files sda and sdb in the current directory,
for the two different devices, respectively. This trace information
can later be parsed by the blkparse utility:
% blkparse sda sdb
which will output the previously recorded tracing information in human
readable form to stdout. See blkparse (1) for more information.
blktrace was written by Jens Axboe, Alan D. Brunelle and Nathan Scott.
This man page was created from the blktrace documentation by Bas
Report bugs to <email@example.com>
Copyright © 2006 Jens Axboe, Alan D. Brunelle and Nathan Scott.
This is free software. You may redistribute copies of it under the
terms of the GNU General Public License
<http://www.gnu.org/licenses/gpl.html>. There is NO WARRANTY, to the
extent permitted by law.
This manual page was created for Debian by Bas Zoetekouw. It was
derived from the documentation provided by the authors and it may be
used, distributed and modified under the terms of the GNU General
Public License, version 2.
On Debian systems, the text of the GNU General Public License can be
found in /usr/share/common-licenses/GPL-2.
btrace (8), blkparse (1), verify_blkparse (1), blkrawverify (1), btt