A Berkeley DB environment is an encapsulation of one or more databases, logfiles, region files, and configuration files. The Berkeley DB environment has its own set of default configuration values for things such as the number of database locks allowed to be taken out at any given time, the maximum size of the journaling logfiles, and so on. Subversion's filesystem logic additionally chooses default values for some of the Berkeley DB configuration options. However, sometimes your particular repository, with its unique collection of data and access patterns, might require a different set of configuration option values.

The producers of Berkeley DB understand that different applications and database environments have different requirements, so they have provided a mechanism for overriding at runtime many of the configuration values for the Berkeley DB environment. BDB checks for the presence of a file named DB_CONFIG in the environment directory (namely, the repository's db subdirectory), and parses the options found in that file.

Subversion creates the DB_CONFIG file when it creates the rest of the repository. The file initially contains some default options, as well as pointers to the Berkeley DB online documentation so that you can read about what those options do.

$ svnadmin create --fstype bdb /var/svn/repos
$ ls /var/svn/repos/db
changes        __db.003   __db.register  log.0000000001   revisions
checksum-reps  __db.004   format         miscellaneous    strings
copies         __db.005   fs-type        node-origins     transactions
__db.001       __db.006   locks          nodes            uuids
__db.002       DB_CONFIG  lock-tokens    representations
$

Of course, you are free to add any of the supported Berkeley DB options to your DB_CONFIG file. Just be aware that while Subversion never attempts to read or interpret the contents of the file and makes no direct use of the option settings in it, you'll want to avoid any configuration changes that may cause Berkeley DB to behave in a fashion that is at odds with what Subversion might expect. Also, changes made to DB_CONFIG won't take effect until you recover the database environment (using svnadmin recover).