1 \documentclass{article}
4 % Copyright (C) 2005 Alan D. Brunelle <Alan.Brunelle@hp.com>
6 % This program is free software; you can redistribute it and/or modify
7 % it under the terms of the GNU General Public License as published by
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13 % MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 % GNU General Public License for more details.
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21 \title{blktrace User Guide}
22 \author{blktrace: Jens Axboe (axboe@suse.de)\\
23 User Guide: Alan D. Brunelle (Alan.Brunelle@hp.com)}
28 %---------------------
29 \section{\label{sec:intro}Introduction}
31 blktrace is a block layer IO tracing mechanism which provides detailed
32 information about request queue operations up to user space. There are
33 three major components that are provided:
36 \item[Kernel patch] A patch to the Linux kernel which includes the
37 kernel event logging interfaces, and patches to areas within the block
38 layer to emit event traces.
40 \item[blktrace] A utility which transfers event traces from the kernel
41 into either long-term on-disk storage, or provides direct formatted
42 output (via blkparse).
44 \item[blkparse] A utility which formats events stored in files, or when
45 run in \emph{live} mode directly outputs data collected by blktrace.
48 \subsection{blktrace Download Area}
50 The blktrace and blkparse utilities and associated kernel patch are provided
51 as part of the following git repository:
53 rsync://rsync.kernel.org/pub/scm/linux/kernel/git/axboe/blktrace.git
55 %--------------------------
56 \newpage\section{\label{sec:quick-start}Quick Start Guide}
58 The following sections outline some quick steps towards utilizing
59 blktrace. Some of the specific instructions below may need to be tailored
62 \subsection{\label{sec:get-blktrace}Retrieving blktrace}
64 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:
67 % git clone rsync://rsync.kernel.org/pub/scm/linux/kernel/git/axboe/blktrace.git bt
72 \subsection{\label{sec:patching}Patching and configuring the Linux kernel}
74 A patch for a \emph{specific Linux kernel} is provided in bt/kernel (where
75 \emph{bt} is the name of the directory from the above git sequence). The
76 detailed actual patching instructions for a Linux kernel is outside the
77 scope of this document, but the following may be used as a sample template.
79 As an example, bt/kernel contains blk-trace-2.6.14-rc1-git-G2, download
80 linux-2.6.13.tar.bz2 and patch-2.6.14-rc1.bz2
83 % tar xjf linux-2.6.13.tar.bz2
84 % mv linux-2.6.13 linux-2.6.14-rc1
86 % bunzip2 -c ../patch-2.6.14-rc1.bz2 | patch -p1
89 At this point you may (optionally) remove linux-2.6.13.tar.bz2 and
92 At this point you should configure the Linux kernel for your specific
93 system -- again, outside the scope of this document -- and then enable
94 \emph{Support for tracing block io actions.} To do this, run
97 % make menuconfig or make xconfig, or edit .config, or ...
100 and navigate through \emph{Device Drivers} and \emph{Block devices}
101 and then down to \emph{Support for tracing block io actions} and hit Y.
103 Install the new kernel (and modules\ldots) and reboot.
105 \subsection{\label{sec:mount}Mounting the RelayFS file system}
107 blktrace utilizes files under the Relay file system, and thus must have
108 the mount point set up -- mounted on the directory /relay. To do this
109 one may do either of the following:
112 \item Manually mount after each boot:
114 % mount -t relayfs relayfs /relay
117 \item Add an entry into /etc/fstab, and have it done automatically at
118 each boot\footnote{Note: after adding the entry to /etc/fstab, you
119 could then mount the directory this time only by doing: \% mount /relay}:
121 relay /relay relayfs default 0 0
125 \subsection{\label{sec:build}Build the tools}
127 To build and install the tools, execute the following sequence (as root):
131 % make && make install
134 \subsection{\label{sec:live-blktrace}blktrace -- live}
136 Now to simply watch what is going on for a specific disk (to stop the
137 trace, hit control-C):
140 % blktrace -d /dev/sda -o - | blkparse -i -
141 8,0 3 1 0.000000000 697 G W 223490 + 8 [kjournald]
142 8,0 3 2 0.000001829 697 P R [kjournald]
143 8,0 3 3 0.000002197 697 Q W 223490 + 8 [kjournald]
144 8,0 3 4 0.000005533 697 M W 223498 + 8 [kjournald]
145 8,0 3 5 0.000008607 697 M W 223506 + 8 [kjournald]
146 8,0 3 6 0.000011569 697 M W 223514 + 8 [kjournald]
147 8,0 3 7 0.000014407 697 M W 223522 + 8 [kjournald]
148 8,0 3 8 0.000017367 697 M W 223530 + 8 [kjournald]
149 8,0 3 9 0.000020161 697 M W 223538 + 8 [kjournald]
150 8,0 3 10 0.000024062 697 D W 223490 + 56 [kjournald]
151 8,0 1 11 0.009507758 0 C W 223490 + 56 [0]
152 8,0 1 12 0.009538995 697 G W 223546 + 8 [kjournald]
153 8,0 1 13 0.009540033 697 P R [kjournald]
154 8,0 1 14 0.009540313 697 Q W 223546 + 8 [kjournald]
155 8,0 1 15 0.009542980 697 D W 223546 + 8 [kjournald]
156 8,0 1 16 0.013542170 0 C W 223546 + 8 [0]
161 Reads Queued: 0, 0KiB Writes Queued: 7, 128KiB
162 Read Dispatches: 0, 0KiB Write Dispatches: 7, 128KiB
163 Reads Completed: 0, 0KiB Writes Completed: 11, 168KiB
164 Read Merges: 0 Write Merges: 25
165 IO unplugs: 0 Timer unplugs: 0
168 Reads Queued: 0, 0KiB Writes Queued: 1, 28KiB
169 Read Dispatches: 0, 0KiB Write Dispatches: 1, 28KiB
170 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
171 Read Merges: 0 Write Merges: 6
172 IO unplugs: 0 Timer unplugs: 0
175 Reads Queued: 0, 0KiB Writes Queued: 11, 168KiB
176 Read Dispatches: 0, 0KiB Write Dispatches: 11, 168KiB
177 Reads Completed: 0, 0KiB Writes Completed: 11, 168KiB
178 Read Merges: 0 Write Merges: 31
179 IO unplugs: 0 Timer unplugs: 3
181 Events (8,0): 89 entries, 0 skips
184 A \emph{btrace} script is included in the distribution to ease live
185 tracing of devices. The above could also be accomplished by issuing:
191 By default, \emph{btrace} runs the trace in quiet mode so it will not
192 include statistics when you break the run. Add the \emph{-S} option to
193 get that dumped as well.
195 \subsection{\label{sec:pc-blktrace}blktrace -- SCSI commands}
197 The previous section showed typical file system io actions, but blktrace
198 can also show SCSI commands going in and out of the queue as submitted
199 by applications using the SCSI Generic (\emph{sg}) interface.
204 3,0 0 25 0.004884107 13528 G R 0 + 0 [inquiry]
205 3,0 0 26 0.004890361 13528 I R 56 (12 00 00 00 38 ..) [inquiry]
206 3,0 0 27 0.004891223 13528 P R [inquiry]
207 3,0 0 28 0.004893250 13528 D R 56 (12 00 00 00 38 ..) [inquiry]
208 3,0 0 29 0.005344910 0 C R (12 00 00 00 38 ..) [0]
211 Here we see a program issuing an INQUIRY command to the CDROM device.
212 The program requested a read of 56 bytes of data, the CDB is included
213 in parenthesis after the data length. The completion event shows shows
214 that the command completed successfully. Tracing SCSI commands can be
215 very useful for debugging problems with programs talking directly to the
216 device. An example of that would be \emph{cdrecord} burning.
218 \subsection{\label{sec:blktrace-post}blktrace -- post-processing}
220 Another way to run blktrace is to have blktrace save data away for later
221 formatting by blkparse. This would be useful if you want to get
222 measurements while running specific loads.
224 To do this, one would specify the device (or devices) to be watched. Then
225 go run you test cases. Stop the trace, and at your leisure utilize
226 blkparse to see the results.
228 In this example, devices /dev/sdaa, /dev/sdc and /dev/sdo are used in an
229 LVM volume called adb3/vol.
232 % blktrace /dev/sdaa /dev/sdc /dev/sdo &
235 % mkfs -t ext3 /dev/adb3/vol
236 mke2fs 1.35 (28-Feb-2004)
239 Block size=4096 (log=2)
240 Fragment size=4096 (log=2)
241 16793600 inodes, 33555456 blocks
242 1677772 blocks (5.00%) reserved for the super user
244 Maximum filesystem blocks=4294967296
246 32768 blocks per group, 32768 fragments per group
247 16384 inodes per group
248 Superblock backups stored on blocks:
249 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
250 4096000, 7962624, 11239424, 20480000, 23887872
252 Writing inode tables: done
253 Creating journal (8192 blocks): done
254 Writing superblocks and filesystem accounting information: done
256 This filesystem will be automatically checked every 27 mounts or
257 180 days, whichever comes first. Use tune2fs -c or -i to override.
262 Then you could process the events later:
266 % blkparse sdaa sdc sdo > events
268 8,32 1 1 0.000000000 9728 G R 384 + 32 [mkfs.ext3]
269 8,32 1 2 0.000001959 9728 P R [mkfs.ext3]
270 8,32 1 3 0.000002446 9728 Q R 384 + 32 [mkfs.ext3]
271 8,32 1 4 0.000005110 9728 D R 384 + 32 [mkfs.ext3]
272 8,32 3 5 0.000200570 0 C R 384 + 32 [0]
273 8,224 3 1 0.021658989 9728 G R 384 + 32 [mkfs.ext3]
275 65,160 3 163392 41.117070504 0 C W 87469088 + 1376 [0]
276 8,32 3 163374 41.122683668 0 C W 88168160 + 1376 [0]
277 65,160 3 163393 41.129952433 0 C W 87905984 + 1376 [0]
278 65,160 3 163394 41.130049431 0 D W 89129344 + 1376 [swapper]
279 65,160 3 163395 41.130067135 0 D W 89216704 + 1376 [swapper]
280 65,160 3 163396 41.130083785 0 D W 89304096 + 1376 [swapper]
281 65,160 3 163397 41.130099455 0 D W 89391488 + 1376 [swapper]
282 65,160 3 163398 41.130114732 0 D W 89478848 + 1376 [swapper]
283 65,160 3 163399 41.130128885 0 D W 89481536 + 64 [swapper]
284 8,32 3 163375 41.134758196 0 C W 86333152 + 1376 [0]
285 65,160 3 163400 41.142229726 0 C W 89129344 + 1376 [0]
286 65,160 3 163401 41.144952314 0 C W 89481536 + 64 [0]
287 8,32 3 163376 41.147441930 0 C W 88342912 + 1376 [0]
288 65,160 3 163402 41.155869604 0 C W 89478848 + 1376 [0]
289 8,32 3 163377 41.159466082 0 C W 86245760 + 1376 [0]
290 65,160 3 163403 41.166944976 0 C W 89216704 + 1376 [0]
291 65,160 3 163404 41.178968252 0 C W 89304096 + 1376 [0]
292 65,160 3 163405 41.191860173 0 C W 89391488 + 1376 [0]
294 Events (sdo): 0 entries, 0 skips
297 Reads Queued: 0, 0KiB Writes Queued: 9, 5,520KiB
298 Read Dispatches: 0, 0KiB Write Dispatches: 0, 0KiB
299 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
300 Read Merges: 0 Write Merges: 336
301 IO unplugs: 0 Timer unplugs: 0
303 Reads Queued: 2,411, 38,576KiB Writes Queued: 769, 425,408KiB
304 Read Dispatches: 2,407, 38,512KiB Write Dispatches: 118, 61,680KiB
305 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
306 Read Merges: 0 Write Merges: 25,819
307 IO unplugs: 0 Timer unplugs: 4
309 Reads Queued: 2, 32KiB Writes Queued: 18, 10,528KiB
310 Read Dispatches: 2, 32KiB Write Dispatches: 3, 1,344KiB
311 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB
312 Read Merges: 0 Write Merges: 640
313 IO unplugs: 0 Timer unplugs: 0
315 Reads Queued: 20,572, 329,152KiB Writes Queued: 594, 279,712KiB
316 Read Dispatches: 20,576, 329,216KiB Write Dispatches: 1,474, 740,720KiB
317 Reads Completed: 22,985, 367,760KiB Writes Completed: 1,390, 721,168KiB
318 Read Merges: 0 Write Merges: 16,888
319 IO unplugs: 0 Timer unplugs: 0
322 Reads Queued: 22,985, 367,760KiB Writes Queued: 1,390, 721,168KiB
323 Read Dispatches: 22,985, 367,760KiB Write Dispatches: 1,595, 803,744KiB
324 Reads Completed: 22,985, 367,760KiB Writes Completed: 1,390, 721,168KiB
325 Read Merges: 0 Write Merges: 43,683
326 IO unplugs: 0 Timer unplugs: 4
330 %----------------------------
331 \newpage\section{\label{sec:blktrace-ug}blktrace User Guide}
333 The \emph{blktrace} utility extracts event traces from the kernel (via
334 the relay file system). Some background details concerning the run-time
335 behaviour of blktrace will help to understand some of the more arcane
336 command line options:
339 \item blktrace receives data from the kernel in buffers passed up
340 through the Relay file system (RelayFS). Each device being traced has
341 a file created in the mounted directory for the RelayFS, which defaults
342 to \emph{/relay} -- this can be overridden with the \emph{-r} command
345 \item blktrace defaults to collecting \emph{all} events that can be
346 traced. To limit the events being captured, you can specify one or
347 more filter masks via the \emph{-a} option.
349 Alternatively, one may specify the entire mask utilizing a hexadecimal
350 value that is version-specific. (Requires understanding of the internal
351 representation of the filter mask.)
353 \item As noted above, the events are passed up via a series of buffers
354 stored into RelayFS files. The size and number of buffers can be
355 specified via the \emph{-b} and \emph{-n} arguments respectively.
357 \item blktrace stores the extracted data into files stored in the
358 \emph{local} directory. The format of the file names is (by default)
359 \emph{device}.blktrace.\emph{cpu}, where \emph{device} is the base
360 device name (e.g, if we are tracing /dev/sda, the base device name would
361 be \emph{sda}); and \emph{cpu} identifies a CPU for the event stream.
363 The \emph{device} portion of the event file name can be changed via
364 the \emph{-o} option.
366 \item blktrace may also be run concurrently with blkparse to produce
367 \emph{live} output -- to do this specify \emph{-o -} for blktrace.
369 \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:
372 \item You may utilize the blktrace utility itself to \emph{kill}
373 a running trace -- via the \emph{-k} option.
375 \item You can specify a run-time duration for blktrace via the
376 \emph{-w} option -- then blktrace will run for the specified number
377 of seconds, and then halt.
381 \subsection{\label{sec:blktrace-args}Command line arguments}
382 \begin{tabular}{|l|l|l|}\hline
383 Short & Long & Description \\ \hline\hline
384 -A \emph{hex-mask} & --set-mask=\emph{hex-mask} & Set filter mask to \emph{hex-mask} \\ \hline
385 -a \emph{mask} & --act-mask=\emph{mask} & Add \emph{mask} to current filter (see below for masks) \\ \hline
386 -b \emph{size} & --buffer-size=\emph{size} & Specifies buffer size for event extraction (scaled by $2^{10}$) \\ \hline
387 -d \emph{dev} & --dev=\emph{dev} & Adds \emph{dev} as a device to trace \\ \hline
388 -k & --kill & Kill on-going trace \\ \hline
389 -n \emph{num-sub} & --num-sub=\emph{num-sub} & Specifies number of buffers to use \\ \hline
390 -o \emph{file} & --output=\emph{file} & Prepend \emph{file} to output file name(s) \\ \hline
391 -r \emph{rel-path} & --relay=\emph{rel-path} & Specifies RelayFS mount point \\ \hline
392 -v & --version & Outputs version \\ \hline
393 -w \emph{seconds} & --stopwatch=\emph{seconds} & Sets run time to the number of seconds specified \\ \hline
396 \subsubsection{\label{sec:filter-mask}Filter Masks}
397 The following masks may be passed with the \emph{-a} command line
398 option, multiple filters may be combined via multiple \emph{-a} command
399 line options.\smallskip
401 \begin{tabular}{|l|l|}\hline
402 barrier & \emph{barrier} attribute \\ \hline
403 complete & \emph{completed} by driver \\ \hline
404 fs & \emph{FS} requests \\ \hline
405 issue & \emph{issued} to driver \\ \hline
406 pc & \emph{packet command} events \\ \hline
407 queue & \emph{queue} operations \\ \hline
408 read & \emph{read} traces \\ \hline
409 requeue & \emph{requeue} operations \\ \hline
410 sync & \emph{synchronous} attribute \\ \hline
411 write & \emph{write} traces \\ \hline
414 \subsubsection{\label{sec:request-types}Request types}
415 blktrace disguingishes between two types of block layer requests,
416 file system and scsi commands. The former are dubbed \emph{fs}
417 requests, the latter \emph{pc} requests. File system requests are
418 normal read/write operations, ie any type of read or write from a
419 specific disk location at a given size. These requests typically
420 originate from a user process, but they may also be initiated by
421 the vm flushing dirty data to disk or the file system syncing
422 a super or journal block to disk. \emph{pc} requests are SCSI
423 commands. blktrace sends the command data block as a payload
424 so that blkparse can decode it.
426 %----------------------------
427 \newpage\section{\label{sec:blkparse-ug}blkparse User Guide}
429 The \emph{blkparse} utility will attempt to combine streams of events
430 for various devices on various CPUs, and produce a formatted output of
431 the event information. As with blktrace, some details concerning blkparse
432 will help in understanding the command line options presented below.
435 \item By default, blkparse expects to run in a post-processing mode
436 -- one where the trace events have been saved by a previous run
437 of blktrace, and blkparse is combining event streams and dumping
440 blkparse \emph{may} be run in a \emph{live} manner concurrently with
441 blktrace by specifying \emph{-i -} to blkparse, and combining it with
442 the live option for blktrace. An example would be:
445 % blktrace -d /dev/sda -o - | blkparse -i -
448 \item You can set how many blkparse batches event reads via the
449 \emph{-b} option, the default is to handle events in batches of 512.
451 \item If you have saved event traces in blktrace with different output
452 names (via the \emph{-o} option to blktrace), you must specify the
453 same \emph{input} name via the \emph{-i} option.
455 \item The format of the output data can be controlled via the \emph{-f}
456 or \emph{-F} options -- see section~\ref{sec:blkparse-format} for details.
458 By default, blkparse sends formatted data to standard output. This
459 may be changed via the \emph{-o} option.
463 \newpage\subsection{\label{sec:blkparse-args}Command line arguments}
464 \begin{tabular}{|l|l|l|}\hline
465 Short & Long & Description \\ \hline\hline
466 -b \emph{batch} & --batch={batch} & Standard input read batching \\ \hline
468 -i \emph{file} & --input=\emph{file} & Specifies base name for input files -- default is \emph{device}.blktrace.\emph{cpu}. \\
469 & & As noted above, specifying \emph{-i -} runs in \emph{live} mode with blktrace \\
470 & & (reading data from standard in). \\ \hline
472 -F \emph{typ,fmt} & --format=\emph{typ,fmt} & Sets output format \\
473 -f \emph{fmt} & --format-spec=\emph{fmt} & (See section~\ref{sec:blkparse-format} for details.) \\
475 & & The -f form specifies a format for all events \\
477 & & The -F form allows one to specify a format for a specific \\
478 & & event type. The single-character \emph{typ} field is one of the \\
479 & & action specifiers in section~\ref{sec:act-table} \\ \hline
482 -m & --missing & Print missing entries\\ \hline
484 -n & --hash-by-name & Hash processes by name, not by PID\\ \hline
486 -o \emph{file} & --output=\emph{file} & Output file \\ \hline
488 -q & --quiet & Quite mode \\ \hline
490 -s & --per-program-stats & Displays data sorted by program \\ \hline
492 -t & --track-ios & Display time deltas per IO \\ \hline
494 -w \emph{span} & --stopwatch=\emph{span} & Display traces for the \emph{span} specified -- where span can be: \\
495 & & \emph{end-time} -- Display traces from time 0 through \emph{end-time} (in ns) \\
497 & & \emph{start:end-time} -- Display traces from time \emph{start} \\
498 & & through {end-time} (in ns). \\ \hline
500 -v & --version & Display version \\ \hline
505 \subsection{\label{sec:blkparse-actions}Trace actions}
508 \item[C -- complete] A previously issued request has been completed.
509 The output will detail the sector and size of that request, as well
510 as the success or failure of it.
512 \item[D -- issued] A request that previously resided on the block layer
513 queue or in the io scheduler has been sent to the driver.
515 \item[I -- inserted] A request is being sent to the io scheduler for
516 addition to the internal queue and later service by the driver. The
517 request is fully formed at this time.
519 \item[Q -- queued] This notes intent to queue io at the given location.
520 No real requests exists yet.
522 \item[W -- bounced] The data pages attached to this \emph{bio} are
523 not reachable by the hardware and must be bounced to a lower memory
524 location. This causes a big slowdown in io performance, since the data
525 must be copied to/from kernel buffers. Usually this can be fixed with
526 using better hardware - either a better io controller, or a platform
529 \item[B -- back merge] A previously inserted request exists that ends
530 on the boundary of where this io begins, so the io scheduler can merge
533 \item[F -- front merge] Same as the back merge, except this io ends
534 where a previously inserted requests starts.
536 \item[M -- front or back merge] One of the above.
538 \item[G -- get request] To send any type of request to a block device,
539 a \emph{struct request} container must be allocated first.
541 \item[S -- sleep] No available request structures were available, so
542 the issuer has to wait for one to be freed.
544 \item[P -- plug] When io is queued to a previously empty block device
545 queue, Linux will plug the queue in anticipation of future ios being
546 added before this data is needed.
548 \item[U -- unplug] Some request data already queued in the device,
549 start sending requests to the driver. This may happen automatically
550 if a timeout period has passed (see next entry) or if a number of
551 requests have been added to the queue.
553 \item[T -- unplug due to timer] If nobody requests the io that was queued
554 after plugging the queue, Linux will automatically unplug it after a
555 defined period has passed.
557 \item[X -- split] On raid or device mapper setups, an incoming io may
558 straddle a device or internal zone and needs to be chopped up into
559 smaller pieces for service. This may indicate a performance problem due
560 to a bad setup of that raid/dm device, but may also just be part of
561 normal boundary conditions. dm is notably bad at this and will clone
564 \item[A -- remap] For stacked devices, incoming io is remapped to device
565 below it in the io stack. The remap action details what exactly is
566 being remapped to what.
570 \subsection{\label{sec:blkparse-format}Output Description and Formatting}
572 The output from blkparse can be tailored for specific use - in particular,
573 to ease parsing of output, and/or limit output fields to those the user
574 wants to see. The data for fields which can be output include:
577 \begin{tabular}{|l|l|}\hline
578 Field & Description \\
579 Specifier & \\ \hline\hline
580 \emph{a} & Action, a (small) string (1 or 2 characters) -- see table below for more details \\ \hline
581 \emph{c} & CPU id \\ \hline
582 \emph{C} & Command \\ \hline
583 \emph{d} & RWBS field, a (small) string (1-3 characters) -- see section below for more details \\ \hline
584 \emph{D} & 7-character string containing the major and minor numbers of
585 the event's device \\
586 & (separated by a comma). \\ \hline
587 \emph{e} & Error value \\ \hline
588 \emph{m} & Minor number of event's device. \\ \hline
589 \emph{M} & Major number of event's device. \\ \hline
590 \emph{n} & Number of blocks \\ \hline
591 \emph{N} & Number of bytes \\ \hline
592 \emph{p} & Process ID \\ \hline
593 \emph{P} & Display packet data -- series of hexadecimal values\\ \hline
594 \emph{s} & Sequence numbers \\ \hline
595 \emph{S} & Sector number \\ \hline
596 \emph{t} & Time stamp (nanoseconds) \\ \hline
597 \emph{T} & Time stamp (seconds) \\ \hline
598 \emph{u} & Elapsed value in microseconds (\emph{-t} command line option) \\ \hline
599 \emph{U} & Payload unsigned integer \\ \hline
602 Note that the user can optionally specify field display width, and
603 optionally a left-aligned specifier. These precede field specifiers,
604 with a '\%' character, followed by the optional left-alignment specifer
605 (-) followed by the width (a decimal number) and then the field.
607 Thus, to specify the command in a 12-character field that is left aligned:
614 \subsubsection{\label{sec:act-table}Action Table}
615 The following table shows the various actions which may be output.
617 \begin{tabular}{|l|l|}\hline
618 Act & Description \\ \hline\hline
619 A & IO was remapped to a different device \\ \hline
620 B & IO back merged with request on queue \\ \hline
621 C & IO completion \\ \hline
622 D & IO issued to driver \\ \hline
623 F & IO front merged with request on queue \\ \hline
624 G & Get request \\ \hline
625 I & IO inserted onto request queue \\ \hline
626 P & Plug request \\ \hline
627 Q & IO handled by request queue code \\ \hline
628 S & Sleep request \\ \hline
629 T & Unplug due to timeout \\ \hline
630 U & Unplug request \\ \hline
631 W & IO bounced \\ \hline
635 \subsubsection{\label{sec:act-table}RWBS Description}
636 This is a small string containing at least one character ('R' for read,
637 'W' for write operation), and optionally either a 'B' (for barrier
638 operations) or 'S' (for synchronous operations).
640 \subsubsection{\label{sec:default-output}Default output}
642 The standard \emph{header} (or initial fields displayed) include:
645 "%D %2c %8s %5T.%9t %5p %2a %3d "
651 \item[\%D] Displays the event's device major/minor as: \%3d,\%-3d.
652 \item[\%2c] CPU ID (2-character field).
653 \item[\%8s] Sequence number
654 \item[\%5T.\%9t] 5-charcter field for the seconds portion of the
655 time stamp and a 9-character field for the nanoseconds in the time stamp.
656 \item[\%5p] 5-character field for the process ID.
657 \item[\%2a] 2-character field for one of the actions.
658 \item[\%3d] 3-character field for the RWBS data.
661 Seeing this in action:
664 8,0 3 1 0.000000000 697 G W 223490 + 8 [kjournald]
667 The header is the data in this line up to the 223490 (starting block).
669 The default output for all event types includes this header.
671 \paragraph{Default output per action}
674 \item[C -- complete] If a payload is present, this is presented between
675 parenthesis following the header, followed by the error value.
677 If no payload is present, the sector and number of blocks are presented
678 (with an intervening plus (+) character). If the \emph{-t} option
679 was specified, then the elapsed time is presented. In either case,
680 it is followed by the error value for the completion.
685 \item[W -- bounced] If a payload is present, the number of payload bytes
686 is output, followed by the payload in hexadecimal between parenthesis.
688 If no payload is present, the sector and number of blocks are presented
689 (with an intervening plus (+) character). If the \emph{-t} option was
690 specified, then the elapsed time is presented (in parenthesis). In
691 either case, it is followed by the command associated with the event
692 (surrounded by square brackets).
694 \item[B -- back merge]
695 \item[F -- front merge]
696 \item[G -- get request]
697 \item[M -- front or back merge]
698 \item[S -- sleep] The starting sector and number of blocks is output
699 (with an intervening plus (+) character), followed by the command
700 associated with the event (surrounded by square brackets).
702 \item[P -- plug] The command associated with the event (surrounded by
703 square brackets) is output.
706 \item[T -- unplug due to timer] The command associated with the event
707 (surrounded by square brackets) is output, followed by the number of
708 requests outstanding.
710 \item[X -- split] The original starting sector followed by the new
711 sector (separated by a slash (/) is output, followed by the command
712 associated with the event (surrounded by square brackets).
714 \item[A -- remap] Sector and length is output, along with the original
715 device and sector offset.
719 %------------------------------
721 \newpage\section*{\label{sec:blktrace-kg}Appendix: blktrace Kernel Guide}
723 The blktrace facility provides an efficient event transfer mechanism which
724 supplies block IO layer state transition data via the relay
725 filesystem. This section provides some details as to the interfaces
726 blktrace utilizes in the kernel to effect this. It is good background data
727 to help understand some of the outputs and command-line options above.
729 \subsection{blktrace.h Definitions}
730 Files which include $<linux/blktrace.h>$ are supplied with the following
733 \subsubsection{Trace Action Specifiers}
734 \begin{tabular}{|l|l|}\hline
735 BLK\_TA\_QUEUE & (RQ) Command queued to request\_queue. \\
736 & (BIO) Command queued by elevator. \\ \hline
737 BLK\_TA\_BACKMERGE & Back merging elevator operation \\ \hline
738 BLK\_TA\_FRONTMERGE & Front merging elevator operation \\ \hline
739 BLK\_TA\_GETRQ & Free request retrieved. \\ \hline
740 BLK\_TA\_SLEEPRQ & No requests available, device unplugged. \\ \hline
741 BLK\_TA\_REQUEUE & Request requeued. \\ \hline
742 BLK\_TA\_ISSUE & Command set to driver for request\_queue. \\ \hline
743 BLK\_TA\_COMPLETE & Command completed by driver. \\ \hline
744 BLK\_TA\_PLUG & Device is plugged \\ \hline
745 BLK\_TA\_UNPLUG\_IO & Unplug device as IO is made available. \\ \hline
746 BLK\_TA\_UNPLUG\_TIMER & Unplug device after timer expired. \\ \hline
747 BLK\_TA\_INSERT & Insert request into queue. \\ \hline
748 BLK\_TA\_SPLIT & BIO split into 2 or more requests. \\ \hline
749 BLK\_TA\_BOUNCE & BIO was bounced \\ \hline
750 BLK\_TA\_REMAP & BIO was remapped \\ \hline
753 %..........................................
754 \subsection{blktrace.h Routines}
755 Files which include $<linux/blktrace.h>$ are supplied with the following
756 kernel routine invocable interfaces:
759 \item[blk\_add\_trace\_rq(struct request\_queue *q, struct request\_queue
761 Adds a trace event describing the state change of the passed in
762 request\_queue. The \emph{what} parameter describes the change in
763 the request\_queue state, and is one of the request queue action
764 specifiers -- BLK\_TA\_QUEUE, BLK\_TA\_REQUEUE, BLK\_TA\_ISSUE,
765 or BLK\_TA\_COMPLETE.
767 \item[blk\_add\_trace\_bio(struct request\_queue *q, struct bio *bio,
769 Adds a trace event for the BIO passed in. The \emph{what} parameter
770 describes the action being performed on the BIO, and is one of
771 BLK\_TA\_BACKMERGE, BLK\_TA\_FRONTMERGE, or BLK\_TA\_QUEUE.
773 \item[blk\_add\_trace\_generic(struct request\_queue *q, struct bio *bio,
775 Adds a \emph{generic} trace event -- not one of the request queue
776 or BIO traces. The \emph{what} parameter describes the action being
777 performed on the BIO (if bio is non-NULL), and is one of
778 BLK\_TA\_PLUG, BLK\_TA\_GETRQ or BLK\_TA\_SLEEPRQ.
780 \item[blk\_add\_trace\_pdu\_int(struct request\_queue *q, u32 what,
782 Adds a trace with some payload data -- in this case, an unsigned
783 32-bit entity (the \emph{pdu} parameter). The \emph{what} parameter
784 describes the nature of the payload, and is one of
785 BLK\_TA\_UNPLUG\_IO or BLK\_TA\_UNPLUG\_TIMER.
787 \item[blk\_add\_trace\_remap(struct request\_queue *q, struct bio *bio,
788 dev\_t dev, sector\_t sector)]
789 Adds a trace with a remap event. \emph{dev} and \emph{sector} denote
790 the original device this \emph{bio} was mapped from.