blktrace man page

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 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.


-A hex-mask

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

-a mask      

Add mask to current filter (see below for masks)

-b size    

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

-d dev

Adds dev as a device to trace  

-I file

Adds the devices found in file as devices to trace

-n num-sub

Specifies number of buffers to use. blktrace defaults to 4 sub buffers.


Run in network listen mode (blktrace server)

-h hostname

Run in network client mode, connecting to the given host

-p number

Network port to use (default 8462)


Make the network client NOT use sendfile() to transfer data

-o 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

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

Specifies debugfs mount point  


Outputs version


Outputs version  

-w 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
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.


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. 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.


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 <>

See Also

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

Referenced By

blkiomon(8), blkparse(1), blkrawverify(1), bno_plot(1), btrace(8), btrecord(8), btreplay(8), btt(1), iowatcher(1), verify_blkparse(1).

March 6, 2007 blktrace git-20070306202522