Your environment directory contains log files. Berkeley DB log files are used to record all the transactional activity performed against the Berkeley DB database files. This information is used after an application or system failure to automatically restore the database to an up-to-date consistent point.
Your log files are maintained by Berkeley DB's logging subsystem.
There are some aspects of the Berkeley DB logging subsystem that
you can configure using DB_CONFIG
parameters, and (sometimes) by using PRAGMAs.
For most users of the BDB SQL interface, there should not normally be any reason to manage your log files or otherwise worry about them. However, it is important to realize that they can not simply be deleted. Note that when using the Berkeley DB SQL interface, your log files will be automatically deleted by Berkeley DB when they are no longer needed.
The things you can manage for your logging subsystem are:
Size of the log files. See Setting the Log File Size.
Size of the logging subsystem's region. See Configuring the Logging Region Size.
Size of the log buffer in memory. Setting the In-Memory Log Buffer Size.
Whenever a pre-defined amount of data is written to a
log file (10 MB by default), the BDB SQL interface stops using the
current log file and starts writing to a new file.
You can change the maximum amount of data contained in each
log file by using either PRAGMA journal_size_limit or
the set_lg_max DB_CONFIG file parameter.
If you use PRAGMA journal_size_limit,
then using this PRAGMA with no value simply returns the
current journal size limit. Using:
PRAGMA journal_size_limit=N
sets the log size to N bytes. If the PRAGMA is successful, N is returned. If it fails, the previous log file size is returned. Failures can occur if you specify a log file size that is less than 4K bytes, or if you specify a log file size larger than the permitted file size on the system.
If you use the DB_CONFIG file to manage this value,
set_lg_max may be changed without
re-creating the environment. You will, however, have to
restart your application in order for the DB_CONFIG
file to be re-read.
The DB_CONFIG file is described in The DB_CONFIG File.
The set_lg_max parameter is
described in the
Berkeley DB C API
.
For a description of how, when and why you should change the size of your log files, see the Selecting a page size section in the Berkeley DB Programmer's Reference Guide.
The logging subsystem's default region size is 512 KB. The logging region is used to store database and table names, and so you may need to increase its size if you will be using a large number of tables.
You can set the size of your logging region by using the
set_lg_regionmax DB_CONFIG parameter. Note that
to manage this value you must set it before you create your
environment, or you must re-create your environment. See The DB_CONFIG File
for more information.
The set_lg_regionmax parameter is described in the
Berkeley DB C API
.
When using named (persistent) databases, log information is stored in-memory until the storage space fills up, or a transaction commit forces the log information to be flushed to disk.
It is possible to increase the amount of memory available to your file log buffer. Doing so improves throughput for long-running transactions, or for transactions that produce a large amount of data. Note that for named (persistent) databases, the default log buffer space is 32 KB.
You can increase your log buffer space by using the
set_lg_bsize DB_CONFIG parameter.
For the BDB SQL interface, when the logging subsystem is configured for on-disk logging, the
default size of the in-memory log buffer is approximately 64KB.
Note that this
method can only be called before the environment
is first opened, so you will have to set this by creating
your -journal directory, and then
creating your database. See
The DB_CONFIG File
for more information.
The set_lg_bsize parameter is described
in the
Berkeley DB C API
.
When working with in-memory databases, the environment is configured to perform logging in-memory. The log buffer is set to 64 * 1024, and the maximum log size is set to 32 * 1024.