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
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 kill utility invocation).
There are two ways to modify this:
You may utilise the blktrace utility itself to kill
a running trace -- via the -k option.
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.
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 buffers.
Prepend file to output file name(s)
Specifies debugfs mount point
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
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 Zoetekouw.