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BLKTRACE(8)                                                        BLKTRACE(8)

       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 anal-
       yse 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 con-
       cerning 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 hexadeci-
         mal 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.blk-
         trace.cpu, where device is the base device name (e.g, if we are trac-
         ing /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
            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

              Kill on-going trace

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

       -o file
              Prepend file to output file name(s)

       -r rel-path
              Specifies debugfs mount point

       --version Outputs version

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

       The following masks may be passed with the -a command line option, mul-
       tiple 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

       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

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

       Copyright (C) 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  Pub-
       lic 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

blktrace git-20070306202522     March  6, 2007                     BLKTRACE(8)

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