Berkeley DB Reference Guide: Programmer Notes

Copying or moving databases

There are two issues with copying or moving databases: database page log sequence numbers (LSNs), and database file identification strings.

Because database pages contain references to the database environment log records (LSNs), databases cannot be copied or moved from one transactional database environment to another without first clearing the LSNs. Note that this is not a concern for non-transactional database environments and applications, and can be ignored if the database is not being used transactionally. Specifically, databases created and written non-transactionally (for example, as part of a bulk load procedure), can be copied or moved into a transactional database environment without resetting the LSNs. The database's LSNs may be reset in one of three ways: the application can call the DB_ENV->lsn_reset method to reset the LSNs in place, or a system administrator can reset the LSNs in place using the -r option to the db_load utility, or by dumping and reloading the database (using the db_dump and db_load utilities).

Because system file identification information (for example, filenames, device and inode numbers, volume and file IDs, and so on) are not necessarily unique or maintained across system reboots, each Berkeley DB database file contains a unique 20-byte file identification bytestring. When multiple processes or threads open the same database file in Berkeley DB, it is this bytestring that is used to ensure the same underlying pages are updated in the database environment cache, no matter which Berkeley DB handle is used for the operation.

The database file identification string is not a concern when moving databases, and databases may be moved or renamed without resetting the identification string. However, when copying a database, you must ensure there are never two databases with the same file identification bytestring in the same cache at the same time. Copying databases is further complicated because Berkeley DB caches do not discard cached database pages when database handles are closed. Cached pages are only discarded when the database is removed by calling the DB_ENV->remove or DB->remove methods.

Before physically copying a database file, first ensure that all modified pages have been written from the cache to the backing database file. This is done using the DB->sync or DB->close methods.

Before using a copy of a database file in a database environment, you must ensure that all pages from any other database with the same bytestring have been removed from the memory pool cache. If the environment in which you will open the copy of the database has pages from files with identical bytestrings to the copied database, there are a few possible solutions:

1. Remove the environment, either using system utilities or by calling the DB_ENV->remove method. Obviously, this will not allow you to access both the original database and the copy of the database at the same time.

2. Create a new file that will have a new bytestring. The simplest way to create a new file that will have a new bytestring is to call the db_dump utility to dump out the contents of the database and then use the db_load utility to load the dumped output into a new file. This allows you to access both the original and copy of the database at the same time.

3. If your database is too large to be dumped and reloaded, you can copy the database by other means, and then reset the bytestring in the copied database to a new bytestring. There are two ways to reset the bytestring in the copy: the application can call the DB_ENV->fileid_reset method, or a system administrator can use the -r option to the db_load utility. This allows you to access both the original and copy of the database at the same time.

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