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Sleepycat Software has provided MySQL with the Berkeley DB transactional storage engine. This storage engine typically is called
BDB for short.
BDB tables may have a greater chance of surviving crashes and are also capable of
ROLLBACK operations on transactions.
Support for the
BDB storage engine is included in MySQL source distributions, which come with a
BDB distribution that is patched to make it work with MySQL. You cannot use a non-patched version of
BDB with MySQL.
We at MySQL AB work in close cooperation with Sleepycat to keep the quality of the MySQL/BDB interface high. (Even though Berkeley DB is in itself very tested and reliable, the MySQL interface is still considered gamma quality. We continue to improve and optimize it.)
When it comes to support for any problems involving
BDB tables, we are committed to helping our users locate the problem and create reproducible test cases. Any such test case is forwarded to Sleepycat, which in turn helps us find and fix the problem. As this is a two-stage operation, any problems with
BDB tables may take a little longer for us to fix than for other storage engines. However, we anticipate no significant difficulties with this procedure because the Berkeley DB code itself is used in many applications other than MySQL.
For general information about Berkeley DB, please visit the Sleepycat Web site, http://www.sleepycat.com/.
Currently, we know that the
BDB storage engine works with the following operating systems:
Linux 2.x Intel
Sun Solaris (SPARC and x86)
FreeBSD 4.x/5.x (x86, sparc64)
IBM AIX 4.3.x
SCO UnixWare 7.1.x
BDB storage engine does not work with the following operating systems:
Linux 2.x Alpha
Linux 2.x AMD64
Linux 2.x IA-64
Linux 2.x s390
Mac OS X
Note: The preceding lists are not complete. We update them as we receive more information.
If you build MySQL from source with support for
BDB tables, but the following error occurs when you start mysqld, it means that the
BDB storage engine is not supported for your architecture:
bdb: architecture lacks fast mutexes: applications cannot be threaded Can't init databases
In this case, you must rebuild MySQL without
BDB support or start the server with the
If you have downloaded a binary version of MySQL that includes support for Berkeley DB, simply follow the usual binary distribution installation instructions.
If you build MySQL from source, you can enable
BDB support by invoking configure with the
--with-berkeley-db option in addition to any other options that you normally use. Download a MySQL 5.0 distribution, change location into its top-level directory, and run this command:
./configure --with-berkeley-db [
The following options to mysqld can be used to change the behavior of the
BDB storage engine. For more information, see Section 5.2.2, “Command Options”.
The base directory for
BDB tables. This should be the same directory that you use for
BDB lock detection method. The option value should be
BDB log file directory.
Do not start Berkeley DB in recover mode.
Don't synchronously flush the
BDB logs. This option is deprecated; use
--skip-sync-bdb-logs instead (see the description for
Start Berkeley DB in multi-process mode. (Do not use
DB_PRIVATE when initializing Berkeley DB.)
BDB temporary file directory.
BDB storage engine.
Synchronously flush the
BDB logs. This option is enabled by default. Use
--skip-sync-bdb-logs to disable it.
If you use the
--skip-bdb option, MySQL does not initialize the Berkeley DB library and this saves a lot of memory. However, if you use this option, you cannot use
BDB tables. If you try to create a
BDB table, MySQL uses the default storage engine instead.
Normally, you should start mysqld without the
--bdb-no-recover option if you intend to use
BDB tables. However, this may cause problems when you try to start mysqld if the
BDB log files are corrupted. See Section 22.214.171.124.3, “Starting and Troubleshooting the MySQL Server”.
bdb_max_lock variable, you can specify the maximum number of locks that can be active on a
BDB table. The default is 10,000. You should increase this if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to execute a query:
bdb: Lock table is out of available locks Got error 12 from ...
You may also want to change the
max_binlog_cache_size variables if you are using large multiple-statement transactions. See Section 5.11.3, “The Binary Log”.
See also Section 5.2.3, “System Variables”.
BDB table is stored on disk in two files. The files have names that begin with the table name and have an extension to indicate the file type. An
.frm file stores the table format, and a
.db file contains the table data and indexes.
To specify explicitly that you want a
BDB table, indicate that with an
ENGINE table option:
CREATE TABLE t (i INT) ENGINE = BDB;
The older term
TYPE is supported as a synonym for
ENGINE for backward compatibility, but
ENGINE is the preferred term and
TYPE is deprecated.
BerkeleyDB is a synonym for
BDB in the
ENGINE table option.
BDB storage engine provides transactional tables. The way you use these tables depends on the autocommit mode:
If you are running with autocommit enabled (which is the default), changes to
BDB tables are committed immediately and cannot be rolled back.
If you are running with autocommit disabled, changes do not become permanent until you execute a
COMMIT statement. Instead of committing, you can execute
ROLLBACK to forget the changes.
You can start a transaction with the
START TRANSACTION or
BEGIN statement to suspend autocommit, or with
SET AUTOCOMMIT=0 to disable autocommit explicitly.
For more information about transactions, see Section 13.4.1, “
BDB storage engine has the following characteristics:
BDB tables can have up to 31 indexes per table, 16 columns per index, and a maximum key size of 1024 bytes.
MySQL requires a primary key in each
BDB table so that each row can be uniquely identified. If you don't create one explicitly by declaring a
PRIMARY KEY, MySQL creates and maintains a hidden primary key for you. The hidden key has a length of five bytes and is incremented for each insert attempt. This key does not appear in the output of
SHOW CREATE TABLE or
The primary key is faster than any other index, because it is stored together with the row data. The other indexes are stored as the key data plus the primary key, so it's important to keep the primary key as short as possible to save disk space and get better speed.
This behavior is similar to that of
InnoDB, where shorter primary keys save space not only in the primary index but in secondary indexes as well.
If all columns that you access in a
BDB table are part of the same index or part of the primary key, MySQL can execute the query without having to access the actual row. In a
MyISAM table, this can be done only if the columns are part of the same index.
Sequential scanning is slower for
BDB tables than for
MyISAM tables because the data in
BDB tables is stored in B-trees and not in a separate data file.
Key values are not prefix- or suffix-compressed like key values in
MyISAM tables. In other words, key information takes a little more space in
BDB tables compared to
There are often holes in the
BDB table to allow you to insert new rows in the middle of the index tree. This makes
BDB tables somewhat larger than
SELECT COUNT(*) FROM is slow for
BDB tables, because no row count is maintained in the table.
The optimizer needs to know the approximate number of rows in the table. MySQL solves this by counting inserts and maintaining this in a separate segment in each
BDB table. If you don't issue a lot of
ROLLBACK statements, this number should be accurate enough for the MySQL optimizer. However, MySQL stores the number only on close, so it may be incorrect if the server terminates unexpectedly. It should not be fatal even if this number is not 100% correct. You can update the row count by using
ANALYZE TABLE or
OPTIMIZE TABLE. See Section 126.96.36.199, “
ANALYZE TABLE Syntax”, and Section 188.8.131.52, “
OPTIMIZE TABLE Syntax”.
Internal locking in
BDB tables is done at the page level.
LOCK TABLES works on
BDB tables as with other tables. If you do not use
LOCK TABLES, MySQL issues an internal multiple-write lock on the table (a lock that does not block other writers) to ensure that the table is properly locked if another thread issues a table lock.
To support transaction rollback, the
BDB storage engine maintains log files. For maximum performance, you can use the
--bdb-logdir option to place the
BDB logs on a different disk than the one where your databases are located.
MySQL performs a checkpoint each time a new
BDB log file is started, and removes any
BDB log files that are not needed for current transactions. You can also use
FLUSH LOGS at any time to checkpoint the Berkeley DB tables.
For disaster recovery, you should use table backups plus MySQL's binary log. See Section 5.9.1, “Database Backups”.
Warning: If you delete old log files that are still in use,
BDB is not able to do recovery at all and you may lose data if something goes wrong.
Applications must always be prepared to handle cases where any change of a
BDB table may cause an automatic rollback and any read may fail with a deadlock error.
If you get a full disk with a
BDB table, you get an error (probably error 28) and the transaction should roll back. This contrasts with
MyISAM tables, for which mysqld waits for sufficient free disk space before continuing.
The following list indicates restrictions that you must observe when using
BDB table stores in its
.db file the path to the file as it was created. This is done to enable detection of locks in a multi-user environment that supports symlinks. As a consequence of this, it is not possible to move
BDB table files from one database directory to another.
When making backups of
BDB tables, you must either use mysqldump or else make a backup that includes the files for each
BDB table (the
.db files) as well as the
BDB log files. The
BDB storage engine stores unfinished transactions in its log files and requires them to be present when mysqld starts. The
BDB logs are the files in the data directory with names of the form
log. (ten digits).
If a column that allows
NULL values has a unique index, only a single
NULL value is allowed. This differs from other storage engines, which allow multiple
NULL values in unique indexes.
If the following error occurs when you start mysqld after upgrading, it means that the current version of
BDB doesn't support the old log file format:
bdb: Ignoring log file: .../log.
NNNNNNNNNN: unsupported log version #
In this case, you must delete all
BDB logs from your data directory (the files that have names of the form
log.) and restart mysqld. We also recommend that you then use mysqldump --opt to dump your
BDB tables, drop the tables, and restore them from the dump file.
If autocommit mode is disabled and you drop a
BDB table that is referenced in another transaction, you may get error messages of the following form in your MySQL error log:
001119 23:43:56 bdb: Missing log fileid entry 001119 23:43:56 bdb: txn_abort: Log undo failed for LSN: 1 3644744: Invalid
This is not fatal, but the fix is not trivial. Until the problem is fixed, we recommend that you not drop
BDB tables except while autocommit mode is enabled.
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