BLKTRACE(8)

NAME
blktrace - generate traces of the i/o traffic on block devices

SYNOPSIS
blktrace -d dev [ -r debugfs_path ] [ -o output ] [ -w time ] [ -a action ] [ -A action_mask ] [ -v ]

DESCRIPTION
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 argument.

-   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 option.

-   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 sending SIGINT signal to the process via invocation the kill (1) utility). Also you can specify a run-time duration for blktrace via the -w option -- then blktrace will run for the specified number of seconds, and then halt.

OPTIONS

-A hex-mask
--set-mask=hex-mask
    Set filter mask to hex-mask (see below for masks)

-a mask
--act-mask=mask
    Add mask to current filter (see below for masks)

-b size
--buffer-size=size
    Specifies buffer size for event extraction (scaled by 1024). The default buffer size is 512KiB.

-d dev
--dev=dev
    Adds dev as a device to trace

-I file
--input-devs=file
    Adds the devices found in file as devices to trace

-n num-sub
--num-sub-buffers=num-sub
    Specifies number of buffers to use. blktrace defaults to 4 sub buffers.

-l
--listen
    Run in network listen mode (blktrace server)

-h hostname
--host=hostname
    Run in network client mode, connecting to the given host

-p number
--port=number
    Network port to use (default 8462)

-s
--no-sendfile
    Make the network client NOT use sendfile() to transfer data

-o basename
--output=basename
    Specifies base name for input files. Default is device.blktrace.cpu. Specifying -o - runs in live mode with blkparse (writing data to standard out).

-D dir
--output-dir=dir
    Prepend file to output file name(s)

    This only works when supplying a single device, or when piping the output via "-o -" with multiple devices.

-r rel-path
--relay=rel-path
    Specifies debugfs mount point

-v
--version
    Outputs version

-V
--version
    Outputs version

-w seconds
--stopwatch=seconds
    Sets run time to the number of seconds specified

FILTER MASKS
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
    discard: discard / trim traces
    fs: requests
    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
    drv_data: additional driver specific trace

REQUEST TYPES
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.

EXAMPLES
To trace the i/o on the device /dev/sda 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. The command

    % btrace /dev/sda

has exactly the same effect as the previous command. See btrace (8) for more information.

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.

AUTHORS
blktrace was written by Jens Axboe, Alan D. Brunelle and Nathan Scott. This man page was created from the blktrace documentation by Bas Zoetekouw.

REPORTING BUGS
Report bugs to <linux-btrace@vger.kernel.org>

COPYRIGHT
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.

SEE ALSO
btrace (8), blkparse (1), verify_blkparse (1), blkrawverify (1), btt (1)