1\documentclass{article} 2 3% 4% Copyright (C) 2005, 2006 Alan D. Brunelle <[email protected]> 5% 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 8% the Free Software Foundation; either version 2 of the License, or 9% (at your option) any later version. 10% 11% This program is distributed in the hope that it will be useful, 12% but WITHOUT ANY WARRANTY; without even the implied warranty of 13% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 14% GNU General Public License for more details. 15% 16% You should have received a copy of the GNU General Public License 17% along with this program; if not, write to the Free Software 18% Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA 19% 20 21\title{blktrace User Guide} 22\author{blktrace: Jens Axboe ([email protected])\\ 23 User Guide: Alan D. Brunelle ([email protected])} 24\date{27 May 2008} 25 26\begin{document} 27\maketitle 28%--------------------- 29\section{\label{sec:intro}Introduction} 30 31blktrace is a block layer IO tracing mechanism which provides detailed 32information about request queue operations up to user space. There are 33three major components that are provided: 34 35\begin{description} 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. If you run a 2.6.17-rc1 or newer kernel, 39 you don't need to patch blktrace support as it is already included. 40 41 \item[blktrace] A utility which transfers event traces from the kernel 42 into either long-term on-disk storage, or provides direct formatted 43 output (via blkparse). 44 45 \item[blkparse] A utility which formats events stored in files, or when 46 run in \emph{live} mode directly outputs data collected by blktrace. 47\end{description} 48 49\subsection{blktrace Download Area} 50 51The blktrace and blkparse utilities and associated kernel patch are provided 52as part of the following git repository: 53 54git://git.kernel.org/pub/scm/linux/kernel/git/axboe/blktrace.git bt 55 56%-------------------------- 57\newpage\section{\label{sec:quick-start}Quick Start Guide} 58 59The following sections outline some quick steps towards utilizing 60blktrace. Some of the specific instructions below may need to be tailored 61to your environment. 62 63\subsection{\label{sec:get-blktrace}Retrieving blktrace} 64 65As 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: 66 67\begin{verbatim} 68% git clone git://git.kernel.org/pub/scm/linux/kernel/git/axboe/blktrace.git bt 69% cd bt 70% git checkout 71\end{verbatim} 72 73\subsection{\label{sec:patching}Patching and configuring the Linux kernel} 74 75A patch for a \emph{specific Linux kernel} is provided in bt/kernel (where 76\emph{bt} is the name of the directory from the above git sequence). The 77detailed actual patching instructions for a Linux kernel is outside the 78scope of this document, but the following may be used as a sample template. 79Note that you may skip this step, if you kernel is at least 2.6.17-rc1. 80 81As an example, bt/kernel contains blk-trace-2.6.14-rc1-git-G2, download 82linux-2.6.13.tar.bz2 and patch-2.6.14-rc1.bz2 83 84\begin{verbatim} 85% tar xjf linux-2.6.13.tar.bz2 86% mv linux-2.6.13 linux-2.6.14-rc1 87% cd linux-2.6.14-rc1 88% bunzip2 -c ../patch-2.6.14-rc1.bz2 | patch -p1 89\end{verbatim} 90 91At this point you may (optionally) remove linux-2.6.13.tar.bz2 and 92patch-2.6.14-rc1.bz2. 93 94At this point you should configure the Linux kernel for your specific 95system -- again, outside the scope of this document -- and then enable 96\emph{Support for tracing block io actions.} To do this, run 97 98\begin{verbatim} 99% make menuconfig or make xconfig, or edit .config, or ... 100\end{verbatim} 101 102and navigate through \emph{Device Drivers} and \emph{Block devices} 103and then down to \emph{Support for tracing block io actions} and hit Y. 104 105Install the new kernel (and modules\ldots) and reboot. 106 107\subsection{\label{sec:mount}Mounting the debugfs file system} 108 109blktrace utilizes files under the debug file system, and thus must have 110the mount point set up -- mounted on the directory /sys/kernel/debug. 111To do this one may do either of the following: 112 113\begin{enumerate} 114 \item Manually mount after each boot: 115\begin{verbatim} 116% mount -t debugfs debugfs /sys/kernel/debug 117\end{verbatim} 118 119 \item Add an entry into /etc/fstab, and have it done automatically at 120 each boot\footnote{Note: after adding the entry to /etc/fstab, you 121 could then mount the directory this time only by doing: \% mount debug}: 122\begin{verbatim} 123debug /sys/kernel/debug debugfs default 0 0 124\end{verbatim} 125\end{enumerate} 126 127\subsection{\label{sec:build}Build the tools} 128 129To build and install the tools, execute the following sequence (as root): 130 131\begin{verbatim} 132% cd bt 133% make && make install 134\end{verbatim} 135 136\subsection{\label{sec:live-blktrace}blktrace -- live} 137 138Now to simply watch what is going on for a specific disk (to stop the 139trace, hit control-C): 140 141\begin{verbatim} 142% blktrace -d /dev/sda -o - | blkparse -i - 143 8,0 3 1 0.000000000 697 G W 223490 + 8 [kjournald] 144 8,0 3 2 0.000001829 697 P R [kjournald] 145 8,0 3 3 0.000002197 697 Q W 223490 + 8 [kjournald] 146 8,0 3 4 0.000005533 697 M W 223498 + 8 [kjournald] 147 8,0 3 5 0.000008607 697 M W 223506 + 8 [kjournald] 148 8,0 3 6 0.000011569 697 M W 223514 + 8 [kjournald] 149 8,0 3 7 0.000014407 697 M W 223522 + 8 [kjournald] 150 8,0 3 8 0.000017367 697 M W 223530 + 8 [kjournald] 151 8,0 3 9 0.000020161 697 M W 223538 + 8 [kjournald] 152 8,0 3 10 0.000024062 697 D W 223490 + 56 [kjournald] 153 8,0 1 11 0.009507758 0 C W 223490 + 56 [0] 154 8,0 1 12 0.009538995 697 G W 223546 + 8 [kjournald] 155 8,0 1 13 0.009540033 697 P R [kjournald] 156 8,0 1 14 0.009540313 697 Q W 223546 + 8 [kjournald] 157 8,0 1 15 0.009542980 697 D W 223546 + 8 [kjournald] 158 8,0 1 16 0.013542170 0 C W 223546 + 8 [0] 159... 160^C 161... 162CPU1 (8,0): 163 Reads Queued: 0, 0KiB Writes Queued: 7, 128KiB 164 Read Dispatches: 0, 0KiB Write Dispatches: 7, 128KiB 165 Reads Completed: 0, 0KiB Writes Completed: 11, 168KiB 166 Read Merges: 0 Write Merges: 25 167 IO unplugs: 0 Timer unplugs: 0 168... 169CPU3 (8,0): 170 Reads Queued: 0, 0KiB Writes Queued: 1, 28KiB 171 Read Dispatches: 0, 0KiB Write Dispatches: 1, 28KiB 172 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB 173 Read Merges: 0 Write Merges: 6 174 IO unplugs: 0 Timer unplugs: 0 175 176Total (8,0): 177 Reads Queued: 0, 0KiB Writes Queued: 11, 168KiB 178 Read Dispatches: 0, 0KiB Write Dispatches: 11, 168KiB 179 Reads Completed: 0, 0KiB Writes Completed: 11, 168KiB 180 Read Merges: 0 Write Merges: 31 181 IO unplugs: 0 Timer unplugs: 3 182 183Events (8,0): 89 entries, 0 skips 184\end{verbatim} 185 186A \emph{btrace} script is included in the distribution to ease live 187tracing of devices. The above could also be accomplished by issuing: 188 189\begin{verbatim} 190% btrace /dev/sda 191\end{verbatim} 192 193By default, \emph{btrace} runs the trace in quiet mode so it will not 194include statistics when you break the run. Add the \emph{-S} option to 195get that dumped as well. 196 197\subsection{\label{sec:pc-blktrace}blktrace -- SCSI commands} 198 199The previous section showed typical file system io actions, but blktrace 200can also show SCSI commands going in and out of the queue as submitted 201by applications using the SCSI Generic (\emph{sg}) interface. 202 203\begin{verbatim} 204% btrace /dev/cdrom 205[...] 206 3,0 0 25 0.004884107 13528 G R 0 + 0 [inquiry] 207 3,0 0 26 0.004890361 13528 I R 56 (12 00 00 00 38 ..) [inquiry] 208 3,0 0 27 0.004891223 13528 P R [inquiry] 209 3,0 0 28 0.004893250 13528 D R 56 (12 00 00 00 38 ..) [inquiry] 210 3,0 0 29 0.005344910 0 C R (12 00 00 00 38 ..) [0] 211\end{verbatim} 212 213Here we see a program issuing an INQUIRY command to the CDROM device. 214The program requested a read of 56 bytes of data, the CDB is included 215in parenthesis after the data length. The completion event shows shows 216that the command completed successfully. Tracing SCSI commands can be 217very useful for debugging problems with programs talking directly to the 218device. An example of that would be \emph{cdrecord} burning. 219 220\subsection{\label{sec:blktrace-post}blktrace -- post-processing} 221 222Another way to run blktrace is to have blktrace save data away for later 223formatting by blkparse. This would be useful if you want to get 224measurements while running specific loads. 225 226To do this, one would specify the device (or devices) to be watched. Then 227go run you test cases. Stop the trace, and at your leisure utilize 228blkparse to see the results. 229 230In this example, devices /dev/sdaa, /dev/sdc and /dev/sdo are used in an 231LVM volume called adb3/vol. 232 233\begin{verbatim} 234% blktrace /dev/sdaa /dev/sdc /dev/sdo & 235[1] 9713 236% 237% mkfs -t ext3 /dev/adb3/vol 238mke2fs 1.35 (28-Feb-2004) 239Filesystem label= 240OS type: Linux 241Block size=4096 (log=2) 242Fragment size=4096 (log=2) 24316793600 inodes, 33555456 blocks 2441677772 blocks (5.00%) reserved for the super user 245First data block=0 246Maximum filesystem blocks=4294967296 2471025 block groups 24832768 blocks per group, 32768 fragments per group 24916384 inodes per group 250Superblock backups stored on blocks: 251 32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 252 4096000, 7962624, 11239424, 20480000, 23887872 253 254Writing inode tables: done 255Creating journal (8192 blocks): done 256Writing superblocks and filesystem accounting information: done 257 258This filesystem will be automatically checked every 27 mounts or 259180 days, whichever comes first. Use tune2fs -c or -i to override. 260% 261% kill -15 9713 262\end{verbatim} 263 264Then you could process the events later: 265 266\begin{verbatim} 267% 268% blkparse sdaa sdc sdo > events 269% less events 270 8,32 1 1 0.000000000 9728 G R 384 + 32 [mkfs.ext3] 271 8,32 1 2 0.000001959 9728 P R [mkfs.ext3] 272 8,32 1 3 0.000002446 9728 Q R 384 + 32 [mkfs.ext3] 273 8,32 1 4 0.000005110 9728 D R 384 + 32 [mkfs.ext3] 274 8,32 3 5 0.000200570 0 C R 384 + 32 [0] 275 8,224 3 1 0.021658989 9728 G R 384 + 32 [mkfs.ext3] 276... 277 65,160 3 163392 41.117070504 0 C W 87469088 + 1376 [0] 278 8,32 3 163374 41.122683668 0 C W 88168160 + 1376 [0] 279 65,160 3 163393 41.129952433 0 C W 87905984 + 1376 [0] 280 65,160 3 163394 41.130049431 0 D W 89129344 + 1376 [swapper] 281 65,160 3 163395 41.130067135 0 D W 89216704 + 1376 [swapper] 282 65,160 3 163396 41.130083785 0 D W 89304096 + 1376 [swapper] 283 65,160 3 163397 41.130099455 0 D W 89391488 + 1376 [swapper] 284 65,160 3 163398 41.130114732 0 D W 89478848 + 1376 [swapper] 285 65,160 3 163399 41.130128885 0 D W 89481536 + 64 [swapper] 286 8,32 3 163375 41.134758196 0 C W 86333152 + 1376 [0] 287 65,160 3 163400 41.142229726 0 C W 89129344 + 1376 [0] 288 65,160 3 163401 41.144952314 0 C W 89481536 + 64 [0] 289 8,32 3 163376 41.147441930 0 C W 88342912 + 1376 [0] 290 65,160 3 163402 41.155869604 0 C W 89478848 + 1376 [0] 291 8,32 3 163377 41.159466082 0 C W 86245760 + 1376 [0] 292 65,160 3 163403 41.166944976 0 C W 89216704 + 1376 [0] 293 65,160 3 163404 41.178968252 0 C W 89304096 + 1376 [0] 294 65,160 3 163405 41.191860173 0 C W 89391488 + 1376 [0] 295... 296Events (sdo): 0 entries, 0 skips 297 298CPU0 (65,160): 299 Reads Queued: 0, 0KiB Writes Queued: 9, 5,520KiB 300 Read Dispatches: 0, 0KiB Write Dispatches: 0, 0KiB 301 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB 302 Read Merges: 0 Write Merges: 336 303 IO unplugs: 0 Timer unplugs: 0 304CPU1 (65,160): 305 Reads Queued: 2,411, 38,576KiB Writes Queued: 769, 425,408KiB 306 Read Dispatches: 2,407, 38,512KiB Write Dispatches: 118, 61,680KiB 307 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB 308 Read Merges: 0 Write Merges: 25,819 309 IO unplugs: 0 Timer unplugs: 4 310CPU2 (65,160): 311 Reads Queued: 2, 32KiB Writes Queued: 18, 10,528KiB 312 Read Dispatches: 2, 32KiB Write Dispatches: 3, 1,344KiB 313 Reads Completed: 0, 0KiB Writes Completed: 0, 0KiB 314 Read Merges: 0 Write Merges: 640 315 IO unplugs: 0 Timer unplugs: 0 316CPU3 (65,160): 317 Reads Queued: 20,572, 329,152KiB Writes Queued: 594, 279,712KiB 318 Read Dispatches: 20,576, 329,216KiB Write Dispatches: 1,474, 740,720KiB 319 Reads Completed: 22,985, 367,760KiB Writes Completed: 1,390, 721,168KiB 320 Read Merges: 0 Write Merges: 16,888 321 IO unplugs: 0 Timer unplugs: 0 322 323Total (65,160): 324 Reads Queued: 22,985, 367,760KiB Writes Queued: 1,390, 721,168KiB 325 Read Dispatches: 22,985, 367,760KiB Write Dispatches: 1,595, 803,744KiB 326 Reads Completed: 22,985, 367,760KiB Writes Completed: 1,390, 721,168KiB 327 Read Merges: 0 Write Merges: 43,683 328 IO unplugs: 0 Timer unplugs: 4 329... 330\end{verbatim} 331 332%---------------------------- 333\newpage\section{\label{sec:blktrace-ug}blktrace User Guide} 334 335The \emph{blktrace} utility extracts event traces from the kernel (via 336the relaying through the debug file system). Some background details 337concerning the run-time behaviour of blktrace will help to understand some 338of the more arcane command line options: 339 340\begin{itemize} 341 \item blktrace receives data from the kernel in buffers passed up 342 through the debug file system (relay). Each device being traced has 343 a file created in the mounted directory for the debugfs, which defaults 344 to \emph{/sys/kernel/debug} -- this can be overridden with the \emph{-r} 345 command line argument. 346 347 \item blktrace defaults to collecting \emph{all} events that can be 348 traced. To limit the events being captured, you can specify one or 349 more filter masks via the \emph{-a} option. 350 351 Alternatively, one may specify the entire mask utilizing a hexadecimal 352 value that is version-specific. (Requires understanding of the internal 353 representation of the filter mask.) 354 355 \item As noted above, the events are passed up via a series of buffers 356 stored into debugfs files. The size and number of buffers can be 357 specified via the \emph{-b} and \emph{-n} arguments respectively. 358 359 \item blktrace stores the extracted data into files stored in the 360 \emph{local} directory. The format of the file names is (by default) 361 \emph{device}.blktrace.\emph{cpu}, where \emph{device} is the base 362 device name (e.g, if we are tracing /dev/sda, the base device name would 363 be \emph{sda}); and \emph{cpu} identifies a CPU for the event stream. 364 365 The \emph{device} portion of the event file name can be changed via 366 the \emph{-o} option. 367 368 \item blktrace may also be run concurrently with blkparse to produce 369 \emph{live} output -- to do this specify \emph{-o -} for blktrace. 370 371 \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: 372 373 \begin{enumerate} 374 \item You may utilize the blktrace utility itself to \emph{kill} 375 a running trace -- via the \emph{-k} option. 376 377 \item You can specify a run-time duration for blktrace via the 378 \emph{-w} option -- then blktrace will run for the specified number 379 of seconds, and then halt. 380 \end{enumerate} 381\end{itemize} 382 383\subsection{\label{sec:blktrace-args}Command line arguments} 384\begin{tabular}{|l|l|l|}\hline 385Short & Long & Description \\ \hline\hline 386-A \emph{hex-mask} & --set-mask=\emph{hex-mask} & Set filter mask to \emph{hex-mask} \\ \hline 387-a \emph{mask} & --act-mask=\emph{mask} & Add \emph{mask} to current filter (see below for masks) \\ \hline 388-b \emph{size} & --buffer-size=\emph{size} & Specifies buffer size for event extraction (scaled by $2^{10}$) \\ \hline 389-d \emph{dev} & --dev=\emph{dev} & Adds \emph{dev} as a device to trace \\ \hline 390-k & --kill & Kill on-going trace \\ \hline 391-n \emph{num-sub} & --num-sub=\emph{num-sub} & Specifies number of buffers to use \\ \hline 392-o \emph{file} & --output=\emph{file} & Prepend \emph{file} to output file name(s) \\ 393 & & \textbf{This only works when using a single device} \\ 394 & & \textbf{or when piping the output via \texttt{-o -}} \\ 395 & & \textbf{with multiple devices.} \\ \hline 396-r \emph{rel-path} & --relay=\emph{rel-path} & Specifies debugfs mount point \\ \hline 397-V & --version & Outputs version \\ \hline 398-w \emph{seconds} & --stopwatch=\emph{seconds} & Sets run time to the number of seconds specified \\ \hline 399-I \emph{devs file}& --input-devs=\emph{devs file}& Adds devices found in \emph{devs file} to list of devices to trace. \\ 400 & & (One device per line.) \\ \hline 401\end{tabular} 402 403\subsubsection{\label{sec:filter-mask}Filter Masks} 404The following masks may be passed with the \emph{-a} command line 405option, multiple filters may be combined via multiple \emph{-a} command 406line options.\smallskip 407 408\begin{tabular}{|l|l|}\hline 409barrier & \emph{barrier} attribute \\ \hline 410complete & \emph{completed} by driver \\ \hline 411fs & \emph{FS} requests \\ \hline 412issue & \emph{issued} to driver \\ \hline 413pc & \emph{packet command} events \\ \hline 414queue & \emph{queue} operations \\ \hline 415read & \emph{read} traces \\ \hline 416requeue & \emph{requeue} operations \\ \hline 417sync & \emph{synchronous} attribute \\ \hline 418write & \emph{write} traces \\ \hline 419notify & \emph{notify} trace messages \\ \hline 420\end{tabular} 421 422\subsubsection{\label{sec:request-types}Request types} 423blktrace disguingishes between two types of block layer requests, 424file system and scsi commands. The former are dubbed \emph{fs} 425requests, the latter \emph{pc} requests. File system requests are 426normal read/write operations, ie any type of read or write from a 427specific disk location at a given size. These requests typically 428originate from a user process, but they may also be initiated by 429the vm flushing dirty data to disk or the file system syncing 430a super or journal block to disk. \emph{pc} requests are SCSI 431commands. blktrace sends the command data block as a payload 432so that blkparse can decode it. 433 434%---------------------------- 435\newpage\section{\label{sec:blkparse-ug}blkparse User Guide} 436 437The \emph{blkparse} utility will attempt to combine streams of events 438for various devices on various CPUs, and produce a formatted output of 439the event information. As with blktrace, some details concerning blkparse 440will help in understanding the command line options presented below. 441 442\begin{itemize} 443 \item By default, blkparse expects to run in a post-processing mode 444 -- one where the trace events have been saved by a previous run 445 of blktrace, and blkparse is combining event streams and dumping 446 formatted data. 447 448 blkparse \emph{may} be run in a \emph{live} manner concurrently with 449 blktrace by specifying \emph{-i -} to blkparse, and combining it with 450 the live option for blktrace. An example would be: 451 452 \begin{verbatim} 453 % blktrace -d /dev/sda -o - | blkparse -i - 454 \end{verbatim} 455 456 \item You can set how many blkparse batches event reads via the 457 \emph{-b} option, the default is to handle events in batches of 512. 458 459 \item If you have saved event traces in blktrace with different output 460 names (via the \emph{-o} option to blktrace), you must specify the 461 same \emph{input} name via the \emph{-i} option. 462 463 \item The format of the output data can be controlled via the \emph{-f} 464 or \emph{-F} options -- see section~\ref{sec:blkparse-format} for details. 465 466 By default, blkparse sends formatted data to standard output. This may 467 be changed via the \emph{-o} option, or text output can be disabled 468 via the\emph{-O} option. A merged binary stream can be produced using 469 the \emph{-d} option. 470 471\end{itemize} 472 473\newpage\subsection{\label{sec:blkparse-args}Command line arguments} 474\begin{tabular}{|l|l|l|}\hline 475Short & Long & Description \\ \hline\hline 476-b \emph{batch} & --batch={batch} & Standard input read batching \\ \hline 477 478-i \emph{file} & --input=\emph{file} & Specifies base name for input files -- default is \emph{device}.blktrace.\emph{cpu}. \\ 479 & & As noted above, specifying \emph{-i -} runs in \emph{live} mode with blktrace \\ 480 & & (reading data from standard in). \\ \hline 481 482-F \emph{typ,fmt} & --format=\emph{typ,fmt} & Sets output format \\ 483-f \emph{fmt} & --format-spec=\emph{fmt} & (See section~\ref{sec:blkparse-format} for details.) \\ 484 & & \\ 485 & & The -f form specifies a format for all events \\ 486 & & \\ 487 & & The -F form allows one to specify a format for a specific \\ 488 & & event type. The single-character \emph{typ} field is one of the \\ 489 & & action specifiers in section~\ref{sec:act-table} \\ \hline 490 491 492-m & --missing & Print missing entries\\ \hline 493 494-h & --hash-by-name & Hash processes by name, not by PID\\ \hline 495 496-o \emph{file} & --output=\emph{file} & Output file \\ \hline 497-O & --no-text-output & Do \emph{not} produce text output, used for binary (-d) only \\ \hline 498 499-d \emph{file} & --dump-binary=\emph{file} & Binary output file \\ \hline 500 501-q & --quiet & Quite mode \\ \hline 502 503-s & --per-program-stats & Displays data sorted by program \\ \hline 504 505-t & --track-ios & Display time deltas per IO \\ \hline 506 507-w \emph{span} & --stopwatch=\emph{span} & Display traces for the \emph{span} specified -- where span can be: \\ 508 & & \emph{end-time} -- Display traces from time 0 through \emph{end-time} (in ns) \\ 509 & & or \\ 510 & & \emph{start:end-time} -- Display traces from time \emph{start} \\ 511 & & through {end-time} (in ns). \\ \hline 512 513-M & --no-msgs & Do not add messages to binary output file \\\hline 514-v & --verbose & More verbose marginal on marginal errors \\ \hline 515-V & --version & Display version \\ \hline 516 517\end{tabular} 518 519\newpage 520\subsection{\label{sec:blkparse-actions}Trace actions} 521 522\begin{description} 523 \item[C -- complete] A previously issued request has been completed. 524 The output will detail the sector and size of that request, as well 525 as the success or failure of it. 526 527 \item[D -- issued] A request that previously resided on the block layer 528 queue or in the io scheduler has been sent to the driver. 529 530 \item[I -- inserted] A request is being sent to the io scheduler for 531 addition to the internal queue and later service by the driver. The 532 request is fully formed at this time. 533 534 \item[Q -- queued] This notes intent to queue io at the given location. 535 No real requests exists yet. 536 537 \item[B -- bounced] The data pages attached to this \emph{bio} are 538 not reachable by the hardware and must be bounced to a lower memory 539 location. This causes a big slowdown in io performance, since the data 540 must be copied to/from kernel buffers. Usually this can be fixed with 541 using better hardware - either a better io controller, or a platform 542 with an IOMMU. 543 544 \item[m -- message] Text message generated via kernel call to 545 \texttt{blk\_add\_trace\_msg}. 546 547 \item[M -- back merge] A previously inserted request exists that ends 548 on the boundary of where this io begins, so the io scheduler can merge 549 them together. 550 551 \item[F -- front merge] Same as the back merge, except this io ends 552 where a previously inserted requests starts. 553 554 \item[G -- get request] To send any type of request to a block device, 555 a \emph{struct request} container must be allocated first. 556 557 \item[S -- sleep] No available request structures were available, so 558 the issuer has to wait for one to be freed. 559 560 \item[P -- plug] When io is queued to a previously empty block device 561 queue, Linux will plug the queue in anticipation of future ios being 562 added before this data is needed. 563 564 \item[U -- unplug] Some request data already queued in the device, 565 start sending requests to the driver. This may happen automatically 566 if a timeout period has passed (see next entry) or if a number of 567 requests have been added to the queue. 568 569 \item[T -- unplug due to timer] If nobody requests the io that was queued 570 after plugging the queue, Linux will automatically unplug it after a 571 defined period has passed. 572 573 \item[X -- split] On raid or device mapper setups, an incoming io may 574 straddle a device or internal zone and needs to be chopped up into 575 smaller pieces for service. This may indicate a performance problem due 576 to a bad setup of that raid/dm device, but may also just be part of 577 normal boundary conditions. dm is notably bad at this and will clone 578 lots of io. 579 580 \item[A -- remap] For stacked devices, incoming io is remapped to device 581 below it in the io stack. The remap action details what exactly is 582 being remapped to what. 583 584\end{description} 585 586\subsection{\label{sec:blkparse-format}Output Description and Formatting} 587 588The output from blkparse can be tailored for specific use - in particular, 589to ease parsing of output, and/or limit output fields to those the user 590wants to see. The data for fields which can be output include: 591 592\smallskip 593\begin{tabular}{|l|l|}\hline 594Field & Description \\ 595Specifier & \\ \hline\hline 596\emph{a} & Action, a (small) string (1 or 2 characters) -- see table below for more details \\ \hline 597\emph{c} & CPU id \\ \hline 598\emph{C} & Command \\ \hline 599\emph{d} & RWBS field, a (small) string (1-3 characters) -- see section below for more details \\ \hline 600\emph{D} & 7-character string containing the major and minor numbers of 601the event's device \\ 602 & (separated by a comma). \\ \hline 603\emph{e} & Error value \\ \hline 604\emph{m} & Minor number of event's device. \\ \hline 605\emph{M} & Major number of event's device. \\ \hline 606\emph{n} & Number of blocks \\ \hline 607\emph{N} & Number of bytes \\ \hline 608\emph{p} & Process ID \\ \hline 609\emph{P} & Display packet data -- series of hexadecimal values\\ \hline 610\emph{s} & Sequence numbers \\ \hline 611\emph{S} & Sector number \\ \hline 612\emph{t} & Time stamp (nanoseconds) \\ \hline 613\emph{T} & Time stamp (seconds) \\ \hline 614\emph{u} & Elapsed value in microseconds (\emph{-t} command line option) \\ \hline 615\emph{U} & Payload unsigned integer \\ \hline 616\end{tabular} 617 618Note that the user can optionally specify field display width, and 619optionally a left-aligned specifier. These precede field specifiers, 620with a '\%' character, followed by the optional left-alignment specifer 621(-) followed by the width (a decimal number) and then the field. 622 623Thus, to specify the command in a 12-character field that is left aligned: 624 625\begin{verbatim} 626-f "%-12C" 627\end{verbatim} 628 629\newpage 630\subsubsection{\label{sec:act-table}Action Table} 631The following table shows the various actions which may be output. 632 633\begin{tabular}{|l|l|}\hline 634Act & Description \\ \hline\hline 635A & IO was remapped to a different device \\ \hline 636B & IO bounced \\ \hline 637C & IO completion \\ \hline 638D & IO issued to driver \\ \hline 639F & IO front merged with request on queue \\ \hline 640G & Get request \\ \hline 641I & IO inserted onto request queue \\ \hline 642M & IO back merged with request on queue \\ \hline 643P & Plug request \\ \hline 644Q & IO handled by request queue code \\ \hline 645S & Sleep request \\ \hline 646T & Unplug due to timeout \\ \hline 647U & Unplug request \\ \hline 648X & Split \\ \hline 649\end{tabular} 650 651\subsubsection{\label{sec:act-table}RWBS Description} 652This is a small string containing at least one character ('R' for read, 653'W' for write, or 'D' for block discard operation), and optionally either 654a 'B' (for barrier operations) or 'S' (for synchronous operations). 655 656\subsubsection{\label{sec:default-output}Default output} 657 658The standard \emph{header} (or initial fields displayed) include: 659 660\begin{verbatim} 661"%D %2c %8s %5T.%9t %5p %2a %3d " 662\end{verbatim} 663 664Breaking this down: 665 666\begin{description} 667 \item[\%D] Displays the event's device major/minor as: \%3d,\%-3d. 668 \item[\%2c] CPU ID (2-character field). 669 \item[\%8s] Sequence number 670 \item[\%5T.\%9t] 5-charcter field for the seconds portion of the 671 time stamp and a 9-character field for the nanoseconds in the time stamp. 672 \item[\%5p] 5-character field for the process ID. 673 \item[\%2a] 2-character field for one of the actions. 674 \item[\%3d] 3-character field for the RWBS data. 675\end{description} 676 677Seeing this in action: 678 679\begin{verbatim} 680 8,0 3 1 0.000000000 697 G W 223490 + 8 [kjournald] 681\end{verbatim} 682 683The header is the data in this line up to the 223490 (starting block). 684 685The default output for all event types includes this header. 686 687\paragraph{Default output per action} 688 689\begin{description} 690 \item[C -- complete] If a payload is present, this is presented between 691 parenthesis following the header, followed by the error value. 692 693 If no payload is present, the sector and number of blocks are presented 694 (with an intervening plus (+) character). If the \emph{-t} option 695 was specified, then the elapsed time is presented. In either case, 696 it is followed by the error value for the completion. 697 698 \item[D -- issued] 699 \item[I -- inserted] 700 \item[Q -- queued] 701 \item[B -- bounced] If a payload is present, the number of payload bytes 702 is output, followed by the payload in hexadecimal between parenthesis. 703 704 If no payload is present, the sector and number of blocks are presented 705 (with an intervening plus (+) character). If the \emph{-t} option was 706 specified, then the elapsed time is presented (in parenthesis). In 707 either case, it is followed by the command associated with the event 708 (surrounded by square brackets). 709 710 \item[M -- back merge] 711 \item[F -- front merge] 712 \item[G -- get request] 713 \item[S -- sleep] The starting sector and number of blocks is output 714 (with an intervening plus (+) character), followed by the command 715 associated with the event (surrounded by square brackets). 716 717 \item[P -- plug] The command associated with the event (surrounded by 718 square brackets) is output. 719 720 \item[U -- unplug] 721 \item[T -- unplug due to timer] The command associated with the event 722 (surrounded by square brackets) is output, followed by the number of 723 requests outstanding. 724 725 \item[X -- split] The original starting sector followed by the new 726 sector (separated by a slash (/) is output, followed by the command 727 associated with the event (surrounded by square brackets). 728 729 \item[A -- remap] Sector and length is output, along with the original 730 device and sector offset. 731 732 \item[m -- message] The supplied message is appended to the end of 733 the standard header. 734 735\end{description} 736 737%------------------------------ 738\newpage 739\newpage\section*{\label{sec:blktrace-kg}Appendix: blktrace Kernel Guide} 740 741The blktrace facility provides an efficient event transfer mechanism which 742supplies block IO layer state transition data via the relay 743filesystem. This section provides some details as to the interfaces 744blktrace utilizes in the kernel to effect this. It is good background data 745to help understand some of the outputs and command-line options above. 746 747\subsection{blktrace.h Definitions} 748Files which include $<linux/blktrace.h>$ are supplied with the following 749definitions: 750 751\subsubsection{Trace Action Specifiers} 752\begin{tabular}{|l|l|}\hline 753 BLK\_TA\_QUEUE & (RQ) Command queued to request\_queue. \\ 754 & (BIO) Command queued by elevator. \\ \hline 755 BLK\_TA\_BACKMERGE & Back merging elevator operation \\ \hline 756 BLK\_TA\_FRONTMERGE & Front merging elevator operation \\ \hline 757 BLK\_TA\_GETRQ & Free request retrieved. \\ \hline 758 BLK\_TA\_SLEEPRQ & No requests available, device unplugged. \\ \hline 759 BLK\_TA\_REQUEUE & Request requeued. \\ \hline 760 BLK\_TA\_ISSUE & Command set to driver for request\_queue. \\ \hline 761 BLK\_TA\_COMPLETE & Command completed by driver. \\ \hline 762 BLK\_TA\_PLUG & Device is plugged \\ \hline 763 BLK\_TA\_UNPLUG\_IO & Unplug device as IO is made available. \\ \hline 764 BLK\_TA\_UNPLUG\_TIMER & Unplug device after timer expired. \\ \hline 765 BLK\_TA\_INSERT & Insert request into queue. \\ \hline 766 BLK\_TA\_SPLIT & BIO split into 2 or more requests. \\ \hline 767 BLK\_TA\_BOUNCE & BIO was bounced \\ \hline 768 BLK\_TA\_REMAP & BIO was remapped \\ \hline 769\end{tabular} 770 771%.......................................... 772\subsection{blktrace.h Routines} 773Files which include $<linux/blktrace.h>$ are supplied with the following 774kernel routine invocable interfaces: 775 776\begin{description} 777 \item[blk\_add\_trace\_rq(struct request\_queue *q, struct request\_queue 778 *rq, u32 what)] 779 Adds a trace event describing the state change of the passed in 780 request\_queue. The \emph{what} parameter describes the change in 781 the request\_queue state, and is one of the request queue action 782 specifiers -- BLK\_TA\_QUEUE, BLK\_TA\_REQUEUE, BLK\_TA\_ISSUE, 783 or BLK\_TA\_COMPLETE. 784 785 \item[blk\_add\_trace\_bio(struct request\_queue *q, struct bio *bio, 786 u32 what)] 787 Adds a trace event for the BIO passed in. The \emph{what} parameter 788 describes the action being performed on the BIO, and is one of 789 BLK\_TA\_BACKMERGE, BLK\_TA\_FRONTMERGE, or BLK\_TA\_QUEUE. 790 791 \item[blk\_add\_trace\_generic(struct request\_queue *q, struct bio *bio, 792 int rw, u32 what)] 793 Adds a \emph{generic} trace event -- not one of the request queue 794 or BIO traces. The \emph{what} parameter describes the action being 795 performed on the BIO (if bio is non-NULL), and is one of 796 BLK\_TA\_PLUG, BLK\_TA\_GETRQ or BLK\_TA\_SLEEPRQ. 797 798 \item[blk\_add\_trace\_pdu\_int(struct request\_queue *q, u32 what, 799 u32 pdu)] 800 Adds a trace with some payload data -- in this case, an unsigned 801 32-bit entity (the \emph{pdu} parameter). The \emph{what} parameter 802 describes the nature of the payload, and is one of 803 BLK\_TA\_UNPLUG\_IO or BLK\_TA\_UNPLUG\_TIMER. 804 805 \item[blk\_add\_trace\_remap(struct request\_queue *q, struct bio *bio, 806 dev\_t dev, sector\_t sector)] 807 Adds a trace with a remap event. \emph{dev} and \emph{sector} denote 808 the original device this \emph{bio} was mapped from. 809 810 \item[blk\_add\_trace\_msg(struct request\_queue *q, char *fmt, ...)] 811 Adds a formatted message to the output stream. The total message 812 size can not exceed BLK\_TN\_MSG\_MSG characters (currently 813 1024). Standard format conversions are supported (as supplied 814 by \texttt{vscnprintf}. 815 816\end{description} 817\end{document} 818