Sleepycat Software has provided MySQL with the Berkeley DB
transactional storage engine. This storage engine typically is
BDB for short.
tables may have a greater chance of surviving crashes and are also
ROLLBACK operations on transactions.
Support for the
BDB storage engine is included
in MySQL source distributions is activated in MySQL-Max binary
distributions. The MySQL source distribution comes 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
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
For general information about Berkeley DB, please visit the Sleepycat Web site, http://www.sleepycat.com/.
Currently, we know that the
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
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
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. (MySQL-Max distributions
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 [
For more information, see Section 5.3, “The mysqld-max Extended MySQL Server”, Section 2.8, “Installing MySQL on Other Unix-Like Systems”, and Section 2.9, “MySQL Installation Using a Source Distribution”.
The following options to mysqld can be used
to change the behavior of the
engine. For more information, see
Section 5.2.1, “mysqld Command Options”.
The base directory for
BDBtables. This should be the same directory that you use for
BDBlock detection method. The option value should be
BDBlog file directory.
Do not start Berkeley DB in recover mode.
Don't synchronously flush the
BDBlogs. This option is deprecated; use
--skip-sync-bdb-logsinstead (see the description for
Start Berkeley DB in multi-process mode. (Do not use
DB_PRIVATEwhen initializing Berkeley DB.)
BDBtemporary file directory.
Synchronously flush the
BDBlogs. This option is enabled by default. Use
--skip-sync-bdb-logsto 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
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 18.104.22.168, “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 5.12.3, “The Binary Log”.
See also Section 5.2.2, “Server 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
To specify explicitly that you want a
table, indicate that with an
CREATE TABLE t (i INT) ENGINE = BDB;
The older term
TYPE is supported as a synonym
ENGINE for backward compatibility, but
ENGINE is the preferred term and
TYPE is deprecated.
BerkeleyDB is a synonym for
BDB in the
BDB storage engine provides transactional
tables. The way you use these tables depends on the autocommit
If you are running with autocommit enabled (which is the default), changes to
BDBtables are committed immediately and cannot be rolled back.
If you are running with autocommit disabled, changes do not become permanent until you execute a
COMMITstatement. Instead of committing, you can execute
ROLLBACKto forget the changes.
You can start a transaction with the
BEGINstatement to suspend autocommit, or with
SET AUTOCOMMIT=0to disable autocommit explicitly.
For more information about transactions, see
Section 13.4.1, “
BDB storage engine has the following
BDBtables 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
BDBtable 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 TABLEor
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
BDBtable 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
MyISAMtable, this can be done only if the columns are part of the same index.
Sequential scanning is slower for
BDBtables than for
MyISAMtables because the data in
BDBtables is stored in B-trees and not in a separate data file.
Key values are not prefix- or suffix-compressed like key values in
MyISAMtables. In other words, key information takes a little more space in
BDBtables compared to
There are often holes in the
BDBtable to allow you to insert new rows in the middle of the index tree. This makes
BDBtables somewhat larger than
SELECT COUNT(*) FROM
tbl_nameis slow for
BDBtables, 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
BDBtable. If you don't issue a lot of
ROLLBACKstatements, 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
OPTIMIZE TABLE. See Section 22.214.171.124, “
ANALYZE TABLESyntax”, and Section 126.96.36.199, “
Internal locking in
BDBtables is done at the page level.
LOCK TABLESworks on
BDBtables 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
BDBstorage engine maintains log files. For maximum performance, you can use the
--bdb-logdiroption to place the
BDBlogs on a different disk than the one where your databases are located.
MySQL performs a checkpoint each time a new
BDBlog file is started, and removes any
BDBlog files that are not needed for current transactions. You can also use
FLUSH LOGSat any time to checkpoint the Berkeley DB tables.
For disaster recovery, you should use table backups plus MySQL's binary log. See Section 5.10.1, “Database Backups”.
Warning: If you delete old log files that are still in use,
BDBis 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
BDBtable may cause an automatic rollback and any read may fail with a deadlock error.
If you get a full disk with a
BDBtable, you get an error (probably error 28) and the transaction should roll back. This contrasts with
MyISAMtables, for which mysqld waits for sufficient free disk space before continuing.
The following list indicates restrictions that you must observe
BDBtable stores in its
.dbfile 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
BDBtable files from one database directory to another.
When making backups of
BDBtables, you must either use mysqldump or else make a backup that includes the files for each
.dbfiles) as well as the
BDBlog files. The
BDBstorage engine stores unfinished transactions in its log files and requires them to be present when mysqld starts. The
BDBlogs are the files in the data directory with names of the form
If a column that allows
NULLvalues has a unique index, only a single
NULLvalue is allowed. This differs from other storage engines, which allow multiple
NULLvalues in unique indexes.
If the following error occurs when you start mysqld after upgrading, it means that the current version of
BDBdoesn't support the old log file format:
bdb: Ignoring log file: .../log.
NNNNNNNNNN: unsupported log version #
In this case, you must delete all
BDBlogs from your data directory (the files that have names of the form
NNNNNNNNNN) and restart mysqld. We also recommend that you then use mysqldump --opt to dump your
BDBtables, drop the tables, and restore them from the dump file.
If autocommit mode is disabled and you drop a
BDBtable 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
BDBtables except while autocommit mode is enabled.