Linux 2.x Intel

Sun Solaris (SPARC and x86)

FreeBSD 4.x/5.x (x86, sparc64)

IBM AIX 4.3.x

SCO OpenServer

SCO UnixWare 7.1.x

Windows NT/2000/XP

Linux 2.x Alpha

Linux 2.x AMD64

Linux 2.x IA-64

Linux 2.x s390

Mac OS X

--bdb-home=path

The base directory for BDB tables. This should be the same directory that you use for --datadir.

--bdb-lock-detect=method

The BDB lock detection method. The option value should be DEFAULT, OLDEST, RANDOM, or YOUNGEST.

--bdb-logdir=file_name

The BDB log file directory.

--bdb-no-recover

Do not start Berkeley DB in recover mode.

--bdb-no-sync

Don't synchronously flush the BDB logs. This option is deprecated; use --skip-sync-bdb-logs instead (see the description for --sync-bdb-logs).

--bdb-shared-data

Start Berkeley DB in multi-process mode. (Do not use DB_PRIVATE when initializing Berkeley DB.)

--bdb-tmpdir=path

The BDB temporary file directory.

--skip-bdb

Disable the BDB storage engine.

--sync-bdb-logs

Synchronously flush the BDB logs. This option is enabled by default. Use --skip-sync-bdb-logs to disable it.

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.

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 DESCRIBE.

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 MyISAM tables.

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 MyISAM tables.

SELECT COUNT(*) FROM tbl_name 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 DELETE or 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 13.5.2.1, “ANALYZE TABLE Syntax”, and Section 13.5.2.5, “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 5.10.1, “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.

Opening many BDB tables at the same time may be quite slow. If you are going to use BDB tables, you should not have a very large table cache (for example, with a size larger than 256) and you should use the --no-auto-rehash option when you use the mysql client.

SHOW TABLE STATUS does not provide some information for BDB tables:

mysql> SHOW TABLE STATUS LIKE 'bdbtest'\G
*************************** 1. row ***************************
           Name: bdbtest
         Engine: BerkeleyDB
        Version: 10
     Row_format: Dynamic
           Rows: 154
 Avg_row_length: 0
    Data_length: 0
Max_data_length: 0
   Index_length: 0
      Data_free: 0
 Auto_increment: NULL
    Create_time: NULL
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options:
        Comment:

Optimize performance.

Change to use no page locks for table scanning operations.

Each 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 .frm and .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.NNNNNNNNNN (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.NNNNNNNNNN) 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.