1<?xml version="1.0" encoding="UTF-8" standalone="no"?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml"> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6 <title>Configuring the Logging Subsystem</title> 7 <link rel="stylesheet" href="gettingStarted.css" type="text/css" /> 8 <meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /> 9 <link rel="start" href="index.html" title="Getting Started with Berkeley DB Transaction Processing" /> 10 <link rel="up" href="filemanagement.html" title="Chapter 5. Managing DB Files" /> 11 <link rel="prev" href="logfileremoval.html" title="Removing Log Files" /> 12 <link rel="next" href="wrapup.html" title="Chapter 6. Summary and Examples" /> 13 </head> 14 <body> 15 <div class="navheader"> 16 <table width="100%" summary="Navigation header"> 17 <tr> 18 <th colspan="3" align="center">Configuring the Logging Subsystem</th> 19 </tr> 20 <tr> 21 <td width="20%" align="left"><a accesskey="p" href="logfileremoval.html">Prev</a> </td> 22 <th width="60%" align="center">Chapter 5. Managing DB Files</th> 23 <td width="20%" align="right"> <a accesskey="n" href="wrapup.html">Next</a></td> 24 </tr> 25 </table> 26 <hr /> 27 </div> 28 <div class="sect1" lang="en" xml:lang="en"> 29 <div class="titlepage"> 30 <div> 31 <div> 32 <h2 class="title" style="clear: both"><a id="logconfig"></a>Configuring the Logging Subsystem</h2> 33 </div> 34 </div> 35 </div> 36 <div class="toc"> 37 <dl> 38 <dt> 39 <span class="sect2"> 40 <a href="logconfig.html#logfilesize">Setting the Log File Size</a> 41 </span> 42 </dt> 43 <dt> 44 <span class="sect2"> 45 <a href="logconfig.html#logregionsize">Configuring the Logging Region Size</a> 46 </span> 47 </dt> 48 <dt> 49 <span class="sect2"> 50 <a href="logconfig.html#inmemorylogging">Configuring In-Memory Logging</a> 51 </span> 52 </dt> 53 <dt> 54 <span class="sect2"> 55 <a href="logconfig.html#logbuffer">Setting the In-Memory Log Buffer Size</a> 56 </span> 57 </dt> 58 </dl> 59 </div> 60 <p> 61 You can configure the following aspects of the logging 62 subsystem: 63 </p> 64 <div class="itemizedlist"> 65 <ul type="disc"> 66 <li> 67 <p> 68 Size of the log files. 69 </p> 70 </li> 71 <li> 72 <p> 73 Size of the logging subsystem's region. 74 See <a class="xref" href="logconfig.html#logregionsize" title="Configuring the Logging Region Size">Configuring the Logging Region Size</a>. 75 </p> 76 </li> 77 <li> 78 <p> 79 Maintain logs entirely in-memory. 80 See <a class="xref" href="logconfig.html#inmemorylogging" title="Configuring In-Memory Logging">Configuring In-Memory Logging</a> 81 for more information. 82 </p> 83 </li> 84 <li> 85 <p> 86 Size of the log buffer in memory. 87 See <a class="xref" href="logconfig.html#logbuffer" title="Setting the In-Memory Log Buffer Size">Setting the In-Memory Log Buffer Size</a>. 88 </p> 89 </li> 90 <li> 91 <p> 92 On-disk location of your log files. See 93 <a class="xref" href="enabletxn.html#splittingdata" title="Identifying Specific File Locations">Identifying Specific File Locations</a>. 94 </p> 95 </li> 96 </ul> 97 </div> 98 <div class="sect2" lang="en" xml:lang="en"> 99 <div class="titlepage"> 100 <div> 101 <div> 102 <h3 class="title"><a id="logfilesize"></a>Setting the Log File Size</h3> 103 </div> 104 </div> 105 </div> 106 <p> 107 Whenever a pre-defined amount of data is written to a 108 log file (10 MB by default), DB stops using the 109 current log file and starts writing to a new file. 110 You can change the maximum amount of data contained in each 111 log file by using the 112 <span> 113 114 <code class="methodname">DbEnv::set_lg_max()</code> 115 method. 116 </span> 117 118 Note that this method can be used at any time 119 during an application's lifetime. 120 </p> 121 <p> 122 Setting the log file size to something larger than its 123 default value is largely a matter of 124 convenience and a reflection of the application's 125 preference in backup media and frequency. However, if 126 you set the log file size too low relative to your 127 application's traffic patterns, you can cause 128 yourself trouble. 129 </p> 130 <p> 131 From a performance perspective, setting the log file 132 size to a low value can cause your active transactions to 133 pause their writing activities more frequently than would 134 occur with larger log file sizes. Whenever a 135 transaction completes the log buffer is flushed to 136 disk. Normally other transactions can continue to 137 write to the log buffer while this flush is in 138 progress. However, when one log file is being closed 139 and another created, all transactions must cease 140 writing to the log buffer until the switch over is 141 completed. 142 </p> 143 <p> 144 Beyond performance concerns, using smaller log files 145 can cause you to use more physical files on disk. 146 As a result, your application could run out 147 of log sequence numbers, depending on how busy your 148 application is. 149 </p> 150 <p> 151 Every log file is identified with a 10 digit number. 152 Moreover, the maximum number of log files that your application 153 is allowed to create in its lifetime is 2,000,000,000. 154 </p> 155 <p> 156 For example, if your application performs 6,000 transactions per 157 second for 24 hours a day, and you are logging 500 bytes of 158 data per transaction into 10 MB log files, then you 159 will run out of log files in around 221 years: 160 </p> 161 <pre class="programlisting"> (10 * 2^20 * 2000000000) / (6000 * 500 * 365 * 60 *60 * 24) = 221 </pre> 162 <p> 163 However, if you were writing 2000 bytes of data per 164 transaction, and using 1 MB log files, then the same 165 formula shows you running out of log files in 5 years time. 166 </p> 167 <p> 168 All of these time frames are quite long, to be sure, 169 but if you do run out of log files after, say, 5 years 170 of continuous operations, then you must reset your log 171 sequence numbers. To do so: 172 </p> 173 <div class="orderedlist"> 174 <ol type="1"> 175 <li> 176 <p> 177 Backup your databases as if to prepare for 178 catastrophic failure. See 179 <a class="xref" href="backuprestore.html" title="Backup Procedures">Backup Procedures</a> 180 for more information. 181 </p> 182 </li> 183 <li> 184 <p> 185 Reset the log file's sequence number using the 186 <span class="command"><strong>db_load</strong></span> utility's 187 <code class="literal">-r</code> option. 188 </p> 189 </li> 190 <li> 191 <p> 192 Remove all of the log files from your 193 environment. Note that this is the only 194 situation in which all of the log files are 195 removed from an environment; in all other 196 cases, at least a single log file is retained. 197 </p> 198 </li> 199 <li> 200 <p> 201 Restart your application. 202 </p> 203 </li> 204 </ol> 205 </div> 206 </div> 207 <div class="sect2" lang="en" xml:lang="en"> 208 <div class="titlepage"> 209 <div> 210 <div> 211 <h3 class="title"><a id="logregionsize"></a>Configuring the Logging Region Size</h3> 212 </div> 213 </div> 214 </div> 215 <p> 216 The logging subsystem's default region size is 60 KB. The 217 logging region is used to store filenames, and so you may 218 need to increase its size if a large number of files (that 219 is, if you have a very large number of databases) will 220 be opened and registered with DB's log manager. 221 </p> 222 <p> 223 You can set the size of your logging region by using the 224 <span> 225 226 <code class="methodname">DbEnv::set_lg_region()</code> 227 </span> 228 229 method. Note that this method can only be called before the 230 first environment handle for your application is opened. 231 </p> 232 </div> 233 <div class="sect2" lang="en" xml:lang="en"> 234 <div class="titlepage"> 235 <div> 236 <div> 237 <h3 class="title"><a id="inmemorylogging"></a>Configuring In-Memory Logging</h3> 238 </div> 239 </div> 240 </div> 241 <p> 242 It is possible to configure your logging subsystem such 243 that logs are maintained entirely in memory. When 244 you do this, you give up your transactional durability 245 guarantee. Without log files, you have no way to run 246 recovery so any system or software failures that you might 247 experience can corrupt your databases. 248 </p> 249 <p> 250 However, by giving up your durability guarantees, you can 251 greatly improve your application's throughput by avoiding 252 the disk I/O necessary to write logging information to 253 disk. In this case, you still retain your transactional 254 atomicity, consistency, and isolation guarantees. 255 </p> 256 <p> 257 To configure your logging subsystem to maintain your logs 258 entirely in-memory: 259 </p> 260 <div class="itemizedlist"> 261 <ul type="disc"> 262 <li> 263 <p> 264 Make sure your log buffer is capable of holding all 265 log information that can accumulate during the longest 266 running transaction. See <a class="xref" href="logconfig.html#logbuffer" title="Setting the In-Memory Log Buffer Size">Setting the In-Memory Log Buffer Size</a> for details. 267 </p> 268 </li> 269 <li> 270 <p> 271 Do not run normal recovery when you open your environment. In this configuration, there are no 272 log files available against which you can run recovery. As a result, if you specify recovery 273 when you open your environment, it is ignored. 274 </p> 275 </li> 276 <li> 277 <p> 278 Specify 279 <span> 280 <code class="literal">DB_LOG_IN_MEMORY</code> to the 281 282 <code class="methodname">DbEnv::log_set_config()</code> 283 </span> 284 285 method. Note that you must specify this before 286 your application opens its first environment 287 handle. 288 </p> 289 </li> 290 </ul> 291 </div> 292 <p> 293 For example: 294 </p> 295 <pre class="programlisting">#include "db_cxx.h" 296 297... 298 299int main(void) 300{ 301 // Set the normal flags for a transactional subsystem. Note that 302 // we DO NOT specify DB_RECOVER. 303 u_int32_t env_flags = DB_CREATE | // If the environment does not 304 // exist, create it. 305 DB_INIT_LOCK | // Initialize locking 306 DB_INIT_LOG | // Initialize logging 307 DB_INIT_MPOOL | // Initialize the cache 308 DB_THREAD | // Free-thread the env handle 309 DB_INIT_TXN; // Initialize transactions 310 311 std::string envHome("/export1/testEnv"); 312 DbEnv myEnv(0); 313 314 try { 315 316 // Indicate that logging is to be performed only in memory. 317 // Doing this means that we give up our transactional durability 318 // guarantee. 319 myEnv.log_set_config(DB_LOG_IN_MEMORY, 1); 320 321 // Configure the size of our log memory buffer. This must be 322 // large enough to hold all the logging information likely 323 // to be created for our longest running transaction. The 324 // default size for the logging buffer is 1 MB when logging 325 // is performed in-memory. For this example, we arbitrarily 326 // set the logging buffer to 5 MB. 327 myEnv.set_lg_bsize(5 * 1024 * 1024); 328 329 // Open the environment as normal. 330 myEnv.open(envHome.c_str(), env_flags, 0); 331 332 } catch(DbException &e) { 333 std::cerr << "Error opening database and environment: " 334 << file_name << ", " 335 << envHome << std::endl; 336 std::cerr << e.what() << std::endl; 337 } 338 339 // From here, you open databases, create transactions and 340 // perform database operations exactly as you would if you 341 // were logging to disk. This part is omitted for brevity. </pre> 342 </div> 343 <div class="sect2" lang="en" xml:lang="en"> 344 <div class="titlepage"> 345 <div> 346 <div> 347 <h3 class="title"><a id="logbuffer"></a>Setting the In-Memory Log Buffer Size</h3> 348 </div> 349 </div> 350 </div> 351 <p> 352 When your application is configured for on-disk logging 353 (the default behavior for transactional applications), log 354 information is stored in-memory until the storage space 355 fills up, or a transaction commit forces the log 356 information to be flushed to disk. 357 </p> 358 <p> 359 It is possible to increase the amount of memory available 360 to your file log buffer. Doing so improves throughput for 361 long-running transactions, or for transactions that produce 362 a large amount of data. 363 </p> 364 <p> 365 When you have your logging subsystem configured to maintain 366 your log entirely in memory (see 367 <a class="xref" href="logconfig.html#inmemorylogging" title="Configuring In-Memory Logging">Configuring In-Memory Logging</a>), it is very important 368 to configure your log buffer size because the log buffer 369 must be capable of holding all log information that can 370 accumulate during the longest running transaction. 371 You must make sure that the in-memory log buffer size is 372 large enough that no transaction will ever span the entire 373 buffer. You must also avoid a state where the in-memory 374 buffer is full and no space can be freed because a 375 transaction that started the first log "file" is still 376 active. 377 </p> 378 <p> 379 When your logging subsystem is configured for on-disk 380 logging, the default log buffer space is 32 KB. When 381 in-memory logging is configured, the default log buffer 382 space is 1 MB. 383 </p> 384 <p> 385 You can increase your log buffer space using the 386 <span> 387 388 <code class="methodname">DbEnv::set_lg_bsize()</code> 389 </span> 390 391 method. Note that this method can only be called before the 392 first environment handle for your application is opened. 393 </p> 394 </div> 395 </div> 396 <div class="navfooter"> 397 <hr /> 398 <table width="100%" summary="Navigation footer"> 399 <tr> 400 <td width="40%" align="left"><a accesskey="p" href="logfileremoval.html">Prev</a> </td> 401 <td width="20%" align="center"> 402 <a accesskey="u" href="filemanagement.html">Up</a> 403 </td> 404 <td width="40%" align="right"> <a accesskey="n" href="wrapup.html">Next</a></td> 405 </tr> 406 <tr> 407 <td width="40%" align="left" valign="top">Removing Log Files </td> 408 <td width="20%" align="center"> 409 <a accesskey="h" href="index.html">Home</a> 410 </td> 411 <td width="40%" align="right" valign="top"> Chapter 6. Summary and Examples</td> 412 </tr> 413 </table> 414 </div> 415 </body> 416</html> 417