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% Copyright (C) 2005, 2006 Alan D. Brunelle <>
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\title{blktrace User Guide}
\author{blktrace: Jens Axboe (\\
User Guide: Alan D. Brunelle (}
\date{27 May 2008}
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 that are provided:
\item[Kernel patch] A patch to the Linux kernel which includes the
kernel event logging interfaces, and patches to areas within the block
layer to emit event traces. If you run a 2.6.17-rc1 or newer kernel,
you don't need to patch blktrace support as it is already included.
\item[blktrace] A utility which transfers event traces from the kernel
into either long-term on-disk storage, or provides direct formatted
output (via blkparse).
\item[blkparse] A utility which formats events stored in files, or when
run in \emph{live} mode directly outputs data collected by blktrace.
\subsection{blktrace Download Area}
The blktrace and blkparse utilities and associated kernel patch are provided
as part of the following git repository:
git:// bt
\newpage\section{\label{sec:quick-start}Quick Start Guide}
The following sections outline some quick steps towards utilizing
blktrace. Some of the specific instructions below may need to be tailored
to your environment.
\subsection{\label{sec:get-blktrace}Retrieving blktrace}
As noted above, the kernel patch along with the blktrace and blkparse utilities are stored in a git repository. One simple way to get going would be:
% git clone git:// bt
% cd bt
% git checkout
\subsection{\label{sec:patching}Patching and configuring the Linux kernel}
A patch for a \emph{specific Linux kernel} is provided in bt/kernel (where
\emph{bt} is the name of the directory from the above git sequence). The
detailed actual patching instructions for a Linux kernel is outside the
scope of this document, but the following may be used as a sample template.
Note that you may skip this step, if you kernel is at least 2.6.17-rc1.
As an example, bt/kernel contains blk-trace-2.6.14-rc1-git-G2, download
linux-2.6.13.tar.bz2 and patch-2.6.14-rc1.bz2
% tar xjf linux-2.6.13.tar.bz2
% mv linux-2.6.13 linux-2.6.14-rc1
% cd linux-2.6.14-rc1
% bunzip2 -c ../patch-2.6.14-rc1.bz2 | patch -p1
At this point you may (optionally) remove linux-2.6.13.tar.bz2 and
At this point you should configure the Linux kernel for your specific
system -- again, outside the scope of this document -- and then enable
\emph{Support for tracing block io actions.} To do this, run
% make menuconfig or make xconfig, or edit .config, or ...
and navigate through \emph{Device Drivers} and \emph{Block devices}
and then down to \emph{Support for tracing block io actions} and hit Y.
Install the new kernel (and modules\ldots) and reboot.
\subsection{\label{sec:mount}Mounting the debugfs file system}
blktrace utilizes files under the debug file system, and thus must have
the mount point set up -- mounted on the directory /sys/kernel/debug.
To do this one may do either of the following:
\item Manually mount after each boot:
% mount -t debugfs debugfs /sys/kernel/debug
\item Add an entry into /etc/fstab, and have it done automatically at
each boot\footnote{Note: after adding the entry to /etc/fstab, you
could then mount the directory this time only by doing: \% mount debug}:
debug /sys/kernel/debug debugfs default 0 0
\subsection{\label{sec:build}Build the tools}
To build and install the tools, execute the following sequence (as root):
% cd bt
% make && make install
\subsection{\label{sec:live-blktrace}blktrace -- live}
Now to simply watch what is going on for a specific disk (to stop the
trace, hit control-C):
% blktrace -d /dev/sda -o - | blkparse -i -
8,0 3 1 0.000000000 697 G W 223490 + 8 [kjournald]
8,0 3 2 0.000001829 697 P R [kjournald]
8,0 3 3 0.000002197 697 Q W 223490 + 8 [kjournald]
8,0 3 4 0.000005533 697 M W 223498 + 8 [kjournald]
8,0 3 5 0.000008607 697 M W 223506 + 8 [kjournald]
8,0 3 6 0.000011569 697 M W 223514 + 8 [kjournald]
8,0 3 7 0.000014407 697 M W 223522 + 8 [kjournald]
8,0 3 8 0.000017367 697 M W 223530 + 8 [kjournald]
8,0 3 9 0.000020161 697 M W 223538 + 8 [kjournald]
8,0 3 10 0.000024062 697 D W 223490 + 56 [kjournald]
8,0 1 11 0.009507758 0 C W 223490 + 56 [0]
8,0 1 12 0.009538995 697 G W 223546 + 8 [kjournald]
8,0 1 13 0.009540033 697 P R [kjournald]
8,0 1 14 0.009540313 697 Q W 223546 + 8 [kjournald]
8,0 1 15 0.009542980 697 D W 223546 + 8 [kjournald]
8,0 1 16 0.013542170 0 C W 223546 + 8 [0]
CPU1 (8,0):
Reads Queued: 0, 0KiB Writes Queued: 7, 128KiB
Read Dispatches: 0, 0KiB Write Dispatches: 7, 128KiB
Reads Completed: 0, 0KiB Writes Completed: 11, 168KiB
Read Merges: 0 Write Merges: 25
IO unplugs: 0 Timer unplugs: 0
CPU3 (8,0):
Reads Queued: 0, 0KiB Writes Queued: 1, 28KiB
Read Dispatches: 0, 0KiB Write Dispatches: 1, 28KiB
Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
Read Merges: 0 Write Merges: 6
IO unplugs: 0 Timer unplugs: 0
Total (8,0):
Reads Queued: 0, 0KiB Writes Queued: 11, 168KiB
Read Dispatches: 0, 0KiB Write Dispatches: 11, 168KiB
Reads Completed: 0, 0KiB Writes Completed: 11, 168KiB
Read Merges: 0 Write Merges: 31
IO unplugs: 0 Timer unplugs: 3
Events (8,0): 89 entries, 0 skips
A \emph{btrace} script is included in the distribution to ease live
tracing of devices. The above could also be accomplished by issuing:
% btrace /dev/sda
By default, \emph{btrace} runs the trace in quiet mode so it will not
include statistics when you break the run. Add the \emph{-S} option to
get that dumped as well.
\subsection{\label{sec:pc-blktrace}blktrace -- SCSI commands}
The previous section showed typical file system io actions, but blktrace
can also show SCSI commands going in and out of the queue as submitted
by applications using the SCSI Generic (\emph{sg}) interface.
% btrace /dev/cdrom
3,0 0 25 0.004884107 13528 G R 0 + 0 [inquiry]
3,0 0 26 0.004890361 13528 I R 56 (12 00 00 00 38 ..) [inquiry]
3,0 0 27 0.004891223 13528 P R [inquiry]
3,0 0 28 0.004893250 13528 D R 56 (12 00 00 00 38 ..) [inquiry]
3,0 0 29 0.005344910 0 C R (12 00 00 00 38 ..) [0]
Here we see a program issuing an INQUIRY command to the CDROM device.
The program requested a read of 56 bytes of data, the CDB is included
in parenthesis after the data length. The completion event shows shows
that the command completed successfully. Tracing SCSI commands can be
very useful for debugging problems with programs talking directly to the
device. An example of that would be \emph{cdrecord} burning.
\subsection{\label{sec:blktrace-post}blktrace -- post-processing}
Another way to run blktrace is to have blktrace save data away for later
formatting by blkparse. This would be useful if you want to get
measurements while running specific loads.
To do this, one would specify the device (or devices) to be watched. Then
go run you test cases. Stop the trace, and at your leisure utilize
blkparse to see the results.
In this example, devices /dev/sdaa, /dev/sdc and /dev/sdo are used in an
LVM volume called adb3/vol.
% blktrace /dev/sdaa /dev/sdc /dev/sdo &
[1] 9713
% mkfs -t ext3 /dev/adb3/vol
mke2fs 1.35 (28-Feb-2004)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
16793600 inodes, 33555456 blocks
1677772 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
1025 block groups
32768 blocks per group, 32768 fragments per group
16384 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000, 7962624, 11239424, 20480000, 23887872
Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 27 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
% kill -15 9713
Then you could process the events later:
% blkparse sdaa sdc sdo > events
% less events
8,32 1 1 0.000000000 9728 G R 384 + 32 [mkfs.ext3]
8,32 1 2 0.000001959 9728 P R [mkfs.ext3]
8,32 1 3 0.000002446 9728 Q R 384 + 32 [mkfs.ext3]
8,32 1 4 0.000005110 9728 D R 384 + 32 [mkfs.ext3]
8,32 3 5 0.000200570 0 C R 384 + 32 [0]
8,224 3 1 0.021658989 9728 G R 384 + 32 [mkfs.ext3]
65,160 3 163392 41.117070504 0 C W 87469088 + 1376 [0]
8,32 3 163374 41.122683668 0 C W 88168160 + 1376 [0]
65,160 3 163393 41.129952433 0 C W 87905984 + 1376 [0]
65,160 3 163394 41.130049431 0 D W 89129344 + 1376 [swapper]
65,160 3 163395 41.130067135 0 D W 89216704 + 1376 [swapper]
65,160 3 163396 41.130083785 0 D W 89304096 + 1376 [swapper]
65,160 3 163397 41.130099455 0 D W 89391488 + 1376 [swapper]
65,160 3 163398 41.130114732 0 D W 89478848 + 1376 [swapper]
65,160 3 163399 41.130128885 0 D W 89481536 + 64 [swapper]
8,32 3 163375 41.134758196 0 C W 86333152 + 1376 [0]
65,160 3 163400 41.142229726 0 C W 89129344 + 1376 [0]
65,160 3 163401 41.144952314 0 C W 89481536 + 64 [0]
8,32 3 163376 41.147441930 0 C W 88342912 + 1376 [0]
65,160 3 163402 41.155869604 0 C W 89478848 + 1376 [0]
8,32 3 163377 41.159466082 0 C W 86245760 + 1376 [0]
65,160 3 163403 41.166944976 0 C W 89216704 + 1376 [0]
65,160 3 163404 41.178968252 0 C W 89304096 + 1376 [0]
65,160 3 163405 41.191860173 0 C W 89391488 + 1376 [0]
Events (sdo): 0 entries, 0 skips
CPU0 (65,160):
Reads Queued: 0, 0KiB Writes Queued: 9, 5,520KiB
Read Dispatches: 0, 0KiB Write Dispatches: 0, 0KiB
Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
Read Merges: 0 Write Merges: 336
IO unplugs: 0 Timer unplugs: 0
CPU1 (65,160):
Reads Queued: 2,411, 38,576KiB Writes Queued: 769, 425,408KiB
Read Dispatches: 2,407, 38,512KiB Write Dispatches: 118, 61,680KiB
Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
Read Merges: 0 Write Merges: 25,819
IO unplugs: 0 Timer unplugs: 4
CPU2 (65,160):
Reads Queued: 2, 32KiB Writes Queued: 18, 10,528KiB
Read Dispatches: 2, 32KiB Write Dispatches: 3, 1,344KiB
Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
Read Merges: 0 Write Merges: 640
IO unplugs: 0 Timer unplugs: 0
CPU3 (65,160):
Reads Queued: 20,572, 329,152KiB Writes Queued: 594, 279,712KiB
Read Dispatches: 20,576, 329,216KiB Write Dispatches: 1,474, 740,720KiB
Reads Completed: 22,985, 367,760KiB Writes Completed: 1,390, 721,168KiB
Read Merges: 0 Write Merges: 16,888
IO unplugs: 0 Timer unplugs: 0
Total (65,160):
Reads Queued: 22,985, 367,760KiB Writes Queued: 1,390, 721,168KiB
Read Dispatches: 22,985, 367,760KiB Write Dispatches: 1,595, 803,744KiB
Reads Completed: 22,985, 367,760KiB Writes Completed: 1,390, 721,168KiB
Read Merges: 0 Write Merges: 43,683
IO unplugs: 0 Timer unplugs: 4
\newpage\section{\label{sec:blktrace-ug}blktrace User Guide}
The \emph{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:
\item 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 \emph{/sys/kernel/debug} -- this can be overridden with the \emph{-r}
command line argument.
\item blktrace defaults to collecting \emph{all} events that can be
traced. To limit the events being captured, you can specify one or
more filter masks via the \emph{-a} option.
Alternatively, one may specify the entire mask utilizing a hexadecimal
value that is version-specific. (Requires understanding of the internal
representation of the filter mask.)
\item 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 \emph{-b} and \emph{-n} arguments respectively.
\item blktrace stores the extracted data into files stored in the
\emph{local} directory. The format of the file names is (by default)
\emph{device}.blktrace.\emph{cpu}, where \emph{device} is the base
device name (e.g, if we are tracing /dev/sda, the base device name would
be \emph{sda}); and \emph{cpu} identifies a CPU for the event stream.
The \emph{device} portion of the event file name can be changed via
the \emph{-o} option.
\item blktrace may also be run concurrently with blkparse to produce
\emph{live} output -- to do this specify \emph{-o -} for blktrace.
\item The default behaviour for blktrace is to run forever until explicitly killed by the user (via a control-C, or \emph{kill} utility invocation). There are two ways to modify this:
\item You may utilize the blktrace utility itself to \emph{kill}
a running trace -- via the \emph{-k} option.
\item You can specify a run-time duration for blktrace via the
\emph{-w} option -- then blktrace will run for the specified number
of seconds, and then halt.
\subsection{\label{sec:blktrace-args}Command line arguments}
Short & Long & Description \\ \hline\hline
-A \emph{hex-mask} & --set-mask=\emph{hex-mask} & Set filter mask to \emph{hex-mask} \\ \hline
-a \emph{mask} & --act-mask=\emph{mask} & Add \emph{mask} to current filter (see below for masks) \\ \hline
-b \emph{size} & --buffer-size=\emph{size} & Specifies buffer size for event extraction (scaled by $2^{10}$) \\ \hline
-d \emph{dev} & --dev=\emph{dev} & Adds \emph{dev} as a device to trace \\ \hline
-k & --kill & Kill on-going trace \\ \hline
-n \emph{num-sub} & --num-sub=\emph{num-sub} & Specifies number of buffers to use \\ \hline
-o \emph{file} & --output=\emph{file} & Prepend \emph{file} to output file name(s) \\
& & \textbf{This only works when using a single device} \\
& & \textbf{or when piping the output via \texttt{-o -}} \\
& & \textbf{with multiple devices.} \\ \hline
-r \emph{rel-path} & --relay=\emph{rel-path} & Specifies debugfs mount point \\ \hline
-V & --version & Outputs version \\ \hline
-w \emph{seconds} & --stopwatch=\emph{seconds} & Sets run time to the number of seconds specified \\ \hline
-I \emph{devs file}& --input-devs=\emph{devs file}& Adds devices found in \emph{devs file} to list of devices to trace. \\
& & (One device per line.) \\ \hline
\subsubsection{\label{sec:filter-mask}Filter Masks}
The following masks may be passed with the \emph{-a} command line
option, multiple filters may be combined via multiple \emph{-a} command
line options.\smallskip
barrier & \emph{barrier} attribute \\ \hline
complete & \emph{completed} by driver \\ \hline
fs & \emph{FS} requests \\ \hline
issue & \emph{issued} to driver \\ \hline
pc & \emph{packet command} events \\ \hline
queue & \emph{queue} operations \\ \hline
read & \emph{read} traces \\ \hline
requeue & \emph{requeue} operations \\ \hline
sync & \emph{synchronous} attribute \\ \hline
write & \emph{write} traces \\ \hline
notify & \emph{notify} trace messages \\ \hline
\subsubsection{\label{sec:request-types}Request types}
blktrace disguingishes between two types of block layer requests,
file system and scsi commands. The former are dubbed \emph{fs}
requests, the latter \emph{pc} requests. File system requests are
normal read/write operations, ie 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. \emph{pc} requests are SCSI
commands. blktrace sends the command data block as a payload
so that blkparse can decode it.
\newpage\section{\label{sec:blkparse-ug}blkparse User Guide}
The \emph{blkparse} utility will attempt to combine streams of events
for various devices on various CPUs, and produce a formatted output of
the event information. As with blktrace, some details concerning blkparse
will help in understanding the command line options presented below.
\item By default, blkparse expects to run in a post-processing mode
-- one where the trace events have been saved by a previous run
of blktrace, and blkparse is combining event streams and dumping
formatted data.
blkparse \emph{may} be run in a \emph{live} manner concurrently with
blktrace by specifying \emph{-i -} to blkparse, and combining it with
the live option for blktrace. An example would be:
% blktrace -d /dev/sda -o - | blkparse -i -
\item You can set how many blkparse batches event reads via the
\emph{-b} option, the default is to handle events in batches of 512.
\item If you have saved event traces in blktrace with different output
names (via the \emph{-o} option to blktrace), you must specify the
same \emph{input} name via the \emph{-i} option.
\item The format of the output data can be controlled via the \emph{-f}
or \emph{-F} options -- see section~\ref{sec:blkparse-format} for details.
By default, blkparse sends formatted data to standard output. This may
be changed via the \emph{-o} option, or text output can be disabled
via the\emph{-O} option. A merged binary stream can be produced using
the \emph{-d} option.
\newpage\subsection{\label{sec:blkparse-args}Command line arguments}
Short & Long & Description \\ \hline\hline
-b \emph{batch} & --batch={batch} & Standard input read batching \\ \hline
-i \emph{file} & --input=\emph{file} & Specifies base name for input files -- default is \emph{device}.blktrace.\emph{cpu}. \\
& & As noted above, specifying \emph{-i -} runs in \emph{live} mode with blktrace \\
& & (reading data from standard in). \\ \hline
-F \emph{typ,fmt} & --format=\emph{typ,fmt} & Sets output format \\
-f \emph{fmt} & --format-spec=\emph{fmt} & (See section~\ref{sec:blkparse-format} for details.) \\
& & \\
& & The -f form specifies a format for all events \\
& & \\
& & The -F form allows one to specify a format for a specific \\
& & event type. The single-character \emph{typ} field is one of the \\
& & action specifiers in section~\ref{sec:act-table} \\ \hline
-m & --missing & Print missing entries\\ \hline
-h & --hash-by-name & Hash processes by name, not by PID\\ \hline
-o \emph{file} & --output=\emph{file} & Output file \\ \hline
-O & --no-text-output & Do \emph{not} produce text output, used for binary (-d) only \\ \hline
-d \emph{file} & --dump-binary=\emph{file} & Binary output file \\ \hline
-q & --quiet & Quite mode \\ \hline
-s & --per-program-stats & Displays data sorted by program \\ \hline
-t & --track-ios & Display time deltas per IO \\ \hline
-w \emph{span} & --stopwatch=\emph{span} & Display traces for the \emph{span} specified -- where span can be: \\
& & \emph{end-time} -- Display traces from time 0 through \emph{end-time} (in ns) \\
& & or \\
& & \emph{start:end-time} -- Display traces from time \emph{start} \\
& & through {end-time} (in ns). \\ \hline
-M & --no-msgs & Do not add messages to binary output file \\\hline
-v & --verbose & More verbose marginal on marginal errors \\ \hline
-V & --version & Display version \\ \hline
\subsection{\label{sec:blkparse-actions}Trace actions}
\item[C -- complete] A previously issued request has been completed.
The output will detail the sector and size of that request, as well
as the success or failure of it.
\item[D -- issued] A request that previously resided on the block layer
queue or in the io scheduler has been sent to the driver.
\item[I -- inserted] A request is being sent to the io scheduler for
addition to the internal queue and later service by the driver. The
request is fully formed at this time.
\item[Q -- queued] This notes intent to queue io at the given location.
No real requests exists yet.
\item[B -- bounced] The data pages attached to this \emph{bio} are
not reachable by the hardware and must be bounced to a lower memory
location. This causes a big slowdown in io performance, since the data
must be copied to/from kernel buffers. Usually this can be fixed with
using better hardware - either a better io controller, or a platform
with an IOMMU.
\item[m -- message] Text message generated via kernel call to
\item[M -- back merge] A previously inserted request exists that ends
on the boundary of where this io begins, so the io scheduler can merge
them together.
\item[F -- front merge] Same as the back merge, except this io ends
where a previously inserted requests starts.
\item[G -- get request] To send any type of request to a block device,
a \emph{struct request} container must be allocated first.
\item[S -- sleep] No available request structures were available, so
the issuer has to wait for one to be freed.
\item[P -- plug] When io is queued to a previously empty block device
queue, Linux will plug the queue in anticipation of future ios being
added before this data is needed.
\item[U -- unplug] Some request data already queued in the device,
start sending requests to the driver. This may happen automatically
if a timeout period has passed (see next entry) or if a number of
requests have been added to the queue.
\item[T -- unplug due to timer] If nobody requests the io that was queued
after plugging the queue, Linux will automatically unplug it after a
defined period has passed.
\item[X -- split] On raid or device mapper setups, an incoming io may
straddle a device or internal zone and needs to be chopped up into
smaller pieces for service. This may indicate a performance problem due
to a bad setup of that raid/dm device, but may also just be part of
normal boundary conditions. dm is notably bad at this and will clone
lots of io.
\item[A -- remap] For stacked devices, incoming io is remapped to device
below it in the io stack. The remap action details what exactly is
being remapped to what.
\subsection{\label{sec:blkparse-format}Output Description and Formatting}
The output from blkparse can be tailored for specific use - in particular,
to ease parsing of output, and/or limit output fields to those the user
wants to see. The data for fields which can be output include:
Field & Description \\
Specifier & \\ \hline\hline
\emph{a} & Action, a (small) string (1 or 2 characters) -- see table below for more details \\ \hline
\emph{c} & CPU id \\ \hline
\emph{C} & Command \\ \hline
\emph{d} & RWBS field, a (small) string (1-3 characters) -- see section below for more details \\ \hline
\emph{D} & 7-character string containing the major and minor numbers of
the event's device \\
& (separated by a comma). \\ \hline
\emph{e} & Error value \\ \hline
\emph{m} & Minor number of event's device. \\ \hline
\emph{M} & Major number of event's device. \\ \hline
\emph{n} & Number of blocks \\ \hline
\emph{N} & Number of bytes \\ \hline
\emph{p} & Process ID \\ \hline
\emph{P} & Display packet data -- series of hexadecimal values\\ \hline
\emph{s} & Sequence numbers \\ \hline
\emph{S} & Sector number \\ \hline
\emph{t} & Time stamp (nanoseconds) \\ \hline
\emph{T} & Time stamp (seconds) \\ \hline
\emph{u} & Elapsed value in microseconds (\emph{-t} command line option) \\ \hline
\emph{U} & Payload unsigned integer \\ \hline
Note that the user can optionally specify field display width, and
optionally a left-aligned specifier. These precede field specifiers,
with a '\%' character, followed by the optional left-alignment specifer
(-) followed by the width (a decimal number) and then the field.
Thus, to specify the command in a 12-character field that is left aligned:
-f "%-12C"
\subsubsection{\label{sec:act-table}Action Table}
The following table shows the various actions which may be output.
Act & Description \\ \hline\hline
A & IO was remapped to a different device \\ \hline
B & IO bounced \\ \hline
C & IO completion \\ \hline
D & IO issued to driver \\ \hline
F & IO front merged with request on queue \\ \hline
G & Get request \\ \hline
I & IO inserted onto request queue \\ \hline
M & IO back merged with request on queue \\ \hline
P & Plug request \\ \hline
Q & IO handled by request queue code \\ \hline
S & Sleep request \\ \hline
T & Unplug due to timeout \\ \hline
U & Unplug request \\ \hline
X & Split \\ \hline
\subsubsection{\label{sec:act-table}RWBS Description}
This is a small string containing at least one character ('R' for read,
'W' for write, or 'D' for block discard operation), and optionally either
a 'B' (for barrier operations) or 'S' (for synchronous operations).
\subsubsection{\label{sec:default-output}Default output}
The standard \emph{header} (or initial fields displayed) include:
"%D %2c %8s %5T.%9t %5p %2a %3d "
Breaking this down:
\item[\%D] Displays the event's device major/minor as: \%3d,\%-3d.
\item[\%2c] CPU ID (2-character field).
\item[\%8s] Sequence number
\item[\%5T.\%9t] 5-charcter field for the seconds portion of the
time stamp and a 9-character field for the nanoseconds in the time stamp.
\item[\%5p] 5-character field for the process ID.
\item[\%2a] 2-character field for one of the actions.
\item[\%3d] 3-character field for the RWBS data.
Seeing this in action:
8,0 3 1 0.000000000 697 G W 223490 + 8 [kjournald]
The header is the data in this line up to the 223490 (starting block).
The default output for all event types includes this header.
\paragraph{Default output per action}
\item[C -- complete] If a payload is present, this is presented between
parenthesis following the header, followed by the error value.
If no payload is present, the sector and number of blocks are presented
(with an intervening plus (+) character). If the \emph{-t} option
was specified, then the elapsed time is presented. In either case,
it is followed by the error value for the completion.
\item[D -- issued]
\item[I -- inserted]
\item[Q -- queued]
\item[B -- bounced] If a payload is present, the number of payload bytes
is output, followed by the payload in hexadecimal between parenthesis.
If no payload is present, the sector and number of blocks are presented
(with an intervening plus (+) character). If the \emph{-t} option was
specified, then the elapsed time is presented (in parenthesis). In
either case, it is followed by the command associated with the event
(surrounded by square brackets).
\item[M -- back merge]
\item[F -- front merge]
\item[G -- get request]
\item[S -- sleep] The starting sector and number of blocks is output
(with an intervening plus (+) character), followed by the command
associated with the event (surrounded by square brackets).
\item[P -- plug] The command associated with the event (surrounded by
square brackets) is output.
\item[U -- unplug]
\item[T -- unplug due to timer] The command associated with the event
(surrounded by square brackets) is output, followed by the number of
requests outstanding.
\item[X -- split] The original starting sector followed by the new
sector (separated by a slash (/) is output, followed by the command
associated with the event (surrounded by square brackets).
\item[A -- remap] Sector and length is output, along with the original
device and sector offset.
\item[m -- message] The supplied message is appended to the end of
the standard header.
\newpage\section*{\label{sec:blktrace-kg}Appendix: blktrace Kernel Guide}
The blktrace facility provides an efficient event transfer mechanism which
supplies block IO layer state transition data via the relay
filesystem. This section provides some details as to the interfaces
blktrace utilizes in the kernel to effect this. It is good background data
to help understand some of the outputs and command-line options above.
\subsection{blktrace.h Definitions}
Files which include $<linux/blktrace.h>$ are supplied with the following
\subsubsection{Trace Action Specifiers}
BLK\_TA\_QUEUE & (RQ) Command queued to request\_queue. \\
& (BIO) Command queued by elevator. \\ \hline
BLK\_TA\_BACKMERGE & Back merging elevator operation \\ \hline
BLK\_TA\_FRONTMERGE & Front merging elevator operation \\ \hline
BLK\_TA\_GETRQ & Free request retrieved. \\ \hline
BLK\_TA\_SLEEPRQ & No requests available, device unplugged. \\ \hline
BLK\_TA\_REQUEUE & Request requeued. \\ \hline
BLK\_TA\_ISSUE & Command set to driver for request\_queue. \\ \hline
BLK\_TA\_COMPLETE & Command completed by driver. \\ \hline
BLK\_TA\_PLUG & Device is plugged \\ \hline
BLK\_TA\_UNPLUG\_IO & Unplug device as IO is made available. \\ \hline
BLK\_TA\_UNPLUG\_TIMER & Unplug device after timer expired. \\ \hline
BLK\_TA\_INSERT & Insert request into queue. \\ \hline
BLK\_TA\_SPLIT & BIO split into 2 or more requests. \\ \hline
BLK\_TA\_BOUNCE & BIO was bounced \\ \hline
BLK\_TA\_REMAP & BIO was remapped \\ \hline
\subsection{blktrace.h Routines}
Files which include $<linux/blktrace.h>$ are supplied with the following
kernel routine invocable interfaces:
\item[blk\_add\_trace\_rq(struct request\_queue *q, struct request\_queue
*rq, u32 what)]
Adds a trace event describing the state change of the passed in
request\_queue. The \emph{what} parameter describes the change in
the request\_queue state, and is one of the request queue action
\item[blk\_add\_trace\_bio(struct request\_queue *q, struct bio *bio,
u32 what)]
Adds a trace event for the BIO passed in. The \emph{what} parameter
describes the action being performed on the BIO, and is one of
\item[blk\_add\_trace\_generic(struct request\_queue *q, struct bio *bio,
int rw, u32 what)]
Adds a \emph{generic} trace event -- not one of the request queue
or BIO traces. The \emph{what} parameter describes the action being
performed on the BIO (if bio is non-NULL), and is one of
\item[blk\_add\_trace\_pdu\_int(struct request\_queue *q, u32 what,
u32 pdu)]
Adds a trace with some payload data -- in this case, an unsigned
32-bit entity (the \emph{pdu} parameter). The \emph{what} parameter
describes the nature of the payload, and is one of
\item[blk\_add\_trace\_remap(struct request\_queue *q, struct bio *bio,
dev\_t dev, sector\_t sector)]
Adds a trace with a remap event. \emph{dev} and \emph{sector} denote
the original device this \emph{bio} was mapped from.
\item[blk\_add\_trace\_msg(struct request\_queue *q, char *fmt, ...)]
Adds a formatted message to the output stream. The total message
size can not exceed BLK\_TN\_MSG\_MSG characters (currently
1024). Standard format conversions are supported (as supplied
by \texttt{vscnprintf}.