diff options
Diffstat (limited to 'db/docs/ref/program/ram.html')
| -rw-r--r-- | db/docs/ref/program/ram.html | 141 |
1 files changed, 98 insertions, 43 deletions
diff --git a/db/docs/ref/program/ram.html b/db/docs/ref/program/ram.html index d050577b3..787de5075 100644 --- a/db/docs/ref/program/ram.html +++ b/db/docs/ref/program/ram.html @@ -1,62 +1,117 @@ -<!--$Id: ram.so,v 10.6 2004/09/28 13:31:25 bostic Exp $--> -<!--Copyright 1997-2004 by Sleepycat Software, Inc.--> +<!--$Id: ram.so,v 10.10 2006/09/19 15:42:31 bostic Exp $--> +<!--Copyright 1997-2006 by Oracle Corporation--> <!--All rights reserved.--> <!--See the file LICENSE for redistribution information.--> <html> <head> -<title>Berkeley DB Reference Guide: Memory-only configurations</title> +<title>Berkeley DB Reference Guide: Memory-only or Flash configurations</title> <meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> <meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> </head> <body bgcolor=white> +<a name="2"><!--meow--></a> <table width="100%"><tr valign=top> <td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Programmer Notes</dl></h3></td> <td align=right><a href="../program/namespace.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../program/cache.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> <p> -<h3 align=center>Memory-only configurations</h3> -<p>Berkeley DB supports a variety of memory-only configurations, intended for -systems where filesystem space is limited in availability or entirely -replaced by some combination of RAM and Flash. There are three database -environment files that are potentially written to disk: database -environment shared region files, database files and log files. Each of -these file types can be individually configured to be created in memory -rather than on disk.</p> -<p>First, database environment shared region files. Applications can -create RAM-only database environments in two different types of memory: -in application heap memory or in system shared memory. To create the -database environment in heap memory, specify the <a href="../../api_c/env_open.html#DB_PRIVATE">DB_PRIVATE</a> flag -to the <a href="../../api_c/env_open.html">DB_ENV->open</a> method. Database environments created in heap -memory are only accessible to the threads of a single process, however. -To create the database environment in system shared memory, specify the +<h3 align=center>Memory-only or Flash configurations</h3> +<p>Berkeley DB supports a variety of memory-based configurations for systems +where filesystem space is either limited in availability or entirely +replaced by some combination of memory and Flash. In addition, Berkeley DB +can be configured to minimize writes to the filesystem when the +filesystem is backed by Flash memory.</p> +<p>There are three parts of the Berkeley DB database environment normally written +to the filesystem: the database environment region files, the database +files and the database environment log files. Each of these items can +be configured to live in memory rather than in the filesystem:</p> +<dl compact> +<dt>The database environment region files:<dd>Each of the Berkeley DB subsystems in a database environment is described by +one or more regions, or chunks of memory. The regions contain all of the +per-process and per-thread shared information (including mutexes), that +comprise a Berkeley DB environment. By default, these regions are backed by +the filesystem. In situations where filesystem backed regions aren't +optimal, applications can create memory-only database environments in +two different types of memory: either in the application's heap memory +or in system shared memory. +<p>To create the database environment in heap memory, specify the +<a href="../../api_c/env_open.html#DB_PRIVATE">DB_PRIVATE</a> flag to the <a href="../../api_c/env_open.html">DB_ENV->open</a> method. Note that database +environments created in heap memory are only accessible to the threads +of a single process, however.</p> +<p>To create the database environment in system shared memory, specify the <a href="../../api_c/env_open.html#DB_SYSTEM_MEM">DB_SYSTEM_MEM</a> flag to the <a href="../../api_c/env_open.html">DB_ENV->open</a> method. Database environments created in system memory are accessible to multiple -processes. However, database environments created in system shared -memory do create a small (roughly 8 byte) file in the file system read -by the processes to identify the system shared memory segments to use.</p> -<p>Second, database files. Applications can create RAM-only databases by -not specifying a physical filename when opening the database using the -<a href="../../api_c/db_open.html">DB->open</a> method. Normally, if the database environment cache fills -up Berkeley DB will create temporary backing files for RAM-only databases. -Use the <a href="../../api_c/memp_set_flags.html#DB_MPOOL_NOFILE">DB_MPOOL_NOFILE</a> flag to the <a href="../../api_c/memp_set_flags.html">DB_MPOOLFILE->set_flags</a> method to -configure the cache to never create backing files for a database. Use -the <a href="../../api_c/memp_set_maxsize.html">DB_MPOOLFILE->set_maxsize</a> method to limit the size of a database so it -cannot dirty the entire cache.</p> -<p>Third, log files. If a database environment is intended to be -transactionally recoverable after application or system failure (that -is, if it will exhibit the transactional attribute of "durability"), the -databases and the database environment log files must either be written -to the local disk and recovered, or they must be replicated to other -systems. When durability is not desired, or is accomplished through -replication, local database environments can be configured for -transactional behavior without durability, resulting in log files never -being written to disk. To configure a database environment this way, -specify the <a href="../../api_c/env_set_flags.html#DB_LOG_INMEMORY">DB_LOG_INMEMORY</a> flag to the -<a href="../../api_c/env_set_flags.html">DB_ENV->set_flags</a> method. To configure an individual database in a -database environment to be transactional but not durable, specify the -<a href="../../api_c/db_set_flags.html#DB_TXN_NOT_DURABLE">DB_TXN_NOT_DURABLE</a> flag to the <a href="../../api_c/db_set_flags.html">DB->set_flags</a> method.</p> +processes, but note that database environments created in system shared +memory do create a small (roughly 8 byte) file in the filesystem, read +by the processes to identify which system shared memory segments to +use.</p> +<p>For more information, see <a href="../../ref/env/region.html">Shared +memory regions</a>.</p> +<dt>The database files:<dd>By default, databases are periodically flushed from the Berkeley DB memory +cache to backing physical files in the filesystem. To keep databases +from being written to backing physical files, pass the +<a href="../../api_c/memp_set_flags.html#DB_MPOOL_NOFILE">DB_MPOOL_NOFILE</a> flag to the <a href="../../api_c/memp_set_flags.html">DB_MPOOLFILE->set_flags</a> method. This flag +implies the application's databases must fit entirely in the Berkeley DB +cache, of course. To avoid a database file growing to consume the +entire cache, applications can limit the size of individual databases +in the cache by calling the <a href="../../api_c/memp_set_maxsize.html">DB_MPOOLFILE->set_maxsize</a> method. +<dt>The database environment log files:<dd>If a database environment is not intended to be transactionally +recoverable after application or system failure (that is, if it will not +exhibit the transactional attribute of "durability"), applications +should not configure the database environment for logging or +transactions, in which case no log files will be created. If the +database environment is intended to be durable, log files must either +be written to stable storage and recovered after application or system +failure, or they must be replicated to other systems. +<p>In applications running on systems without any form of stable storage, +durability must be accomplished through replication. In this case, +database environments should be configured to maintain database logs in +memory, rather than in the filesystem, by specifying the +<a href="../../api_c/env_set_flags.html#DB_LOG_INMEMORY">DB_LOG_INMEMORY</a> flag to the <a href="../../api_c/env_set_flags.html">DB_ENV->set_flags</a> method.</p> +</dl> +<p>In systems where the filesystem is backed by Flash memory, the number +of times the Flash memory is written may be a concern. Each of the +three parts of the Berkeley DB database environment normally written to the +filesystem can be configured to minimize the number of times the +filesystem is written:</p> +<dl compact> +<dt>The database environment region files:<dd>On a Flash-based filesystem, the database environment should be placed +in heap or system memory, as described previously. +<dt>The database files:<dd>The Berkeley DB library maintains a cache of database pages. The database +pages are only written to backing physical files when the application +checkpoints the database environment with the <a href="../../api_c/txn_checkpoint.html">DB_ENV->txn_checkpoint</a> method, +when database handles are closed with the <a href="../../api_c/db_close.html">DB->close</a> method, or when +the application explicitly flushes the cache with the <a href="../../api_c/db_sync.html">DB->sync</a> +or <a href="../../api_c/memp_sync.html">DB_ENV->memp_sync</a> methods. +<p>To avoid unnecessary writes of Flash memory due to checkpoints, +applications should decrease the frequency of their checkpoints. This +is especially important in applications which repeatedly modify a +specific database page, as repeatedly writing a database page to the +backing physical file will repeatedly update the same blocks of the +filesystem.</p> +<p>To avoid unnecessary writes of the filesystem due to closing a database +handle, applications should specify the <a href="../../api_c/db_close.html#DB_NOSYNC">DB_NOSYNC</a> flag to the +<a href="../../api_c/db_close.html">DB->close</a> method.</p> +<p>To avoid unnecessary writes of the filesystem due to flushing the cache, +applications should not explicitly flush the cache under normal +conditions -- flushing the cache is rarely if ever needed in a +normally-running application.</p> +<dt>The database environment log files:<dd>The Berkeley DB log files do not repeatedly overwrite the same blocks of the +filesystem as the Berkeley DB log files are not implemented as a circular +buffer and log files are not re-used. For this reason, the Berkeley DB log +files should not cause any difficulties for Flash memory configurations. +<p>However, as Berkeley DB does not write log records in filesystem block sized +pieces, it is probable that sequential transaction commits (each of +which flush the log file to the backing filesystem), will write a block +of Flash memory twice, as the last log record from the first commit will +write the same block of Flash memory as the first log record from the +second commit. Applications not requiring absolute durability should +specify the <a href="../../api_c/env_set_flags.html#DB_TXN_WRITE_NOSYNC">DB_TXN_WRITE_NOSYNC</a> or <a href="../../api_c/env_set_flags.html#DB_TXN_NOSYNC">DB_TXN_NOSYNC</a> flags +to the <a href="../../api_c/env_set_flags.html">DB_ENV->set_flags</a> method to avoid this overwrite of a block of +Flash memory.</p> +</dl> <table width="100%"><tr><td><br></td><td align=right><a href="../program/namespace.html"><img src="../../images/prev.gif" alt="Prev"></a><a href="../toc.html"><img src="../../images/ref.gif" alt="Ref"></a><a href="../program/cache.html"><img src="../../images/next.gif" alt="Next"></a> </td></tr></table> -<p><font size=1><a href="../../sleepycat/legal.html">Copyright (c) 1996-2004</a> <a href="http://www.sleepycat.com">Sleepycat Software, Inc.</a> - All rights reserved.</font> +<p><font size=1>Copyright (c) 1996-2006 Oracle Corporation - All rights reserved.</font> </body> </html> |