Patch Files

The patch file mechanism was designed for situations where transactions are written to a database during a backup. A convenient feature of Microsoft Exchange Server is the ability to back up databases without interrupting service to end users.

During the backup operation, data is read from the .edb files. If a transaction is made to a section of the .edb file that has already been backed up, it is recorded in a .pat (patch) file. If a transaction is made to a section of the .edb file that has not yet been backed up, it is processed and does not need to be written to the patch file. A separate .pat file is used for each database ¾ Priv.pat, Pub.pat, and Dir.pat. Note that these .pat files are exposed only during the backup process. The online backup operation process is as follows:

1. A .pat file is created for the current database.
2. The backup operation for the current .edb file begins.
3. Transactions that must be written to sections of the .edb file that have already been backed up are recorded to both the .edb file and the .pat file.
4. The .pat file is written to the backup tape.
5. The .pat file is deleted from the Mdbdata or Dsadata directory.