Exchange database (.edb) files are the repository for mailbox data. They are accessed by the ESE directly and have a B+tree structure that is designed for quick access, thereby enabling users to access any page of data within four I/O cycles. The Exchange database is composed of multiple B+trees, with ancillary trees that work with the main tree by holding indexing and views.

Note:
Exchange 2007 does not use the stream (.stm) file format that was used in Exchange Server 2003. Data that was formerly divided between .edb and .stm files is now stored only in .edb files.

Exchange 2007 writes operations (such as creating or modifying a message) to a log (.log) file for that database's storage group. Committed transactions are later written to the database itself (in an .edb file). This approach guarantees that all completed and in-progress transactions are logged, so data integrity is maintained in case of a service interruption. The databases in a storage group share a single set of transaction logs that are named with consecutive numbers (for example, E0000000001.log and E0000000002.log).

Checkpoint (.chk) files store information that indicates when a transaction is successfully saved to the database files on the hard disk. Exchange 2007 uses checkpoint files to allow an instance of the ESE to automatically replay log files into an inconsistent database when recovering from a service interruption, starting with the next unwritten transaction.

For more information about transaction logging, see Understanding Transaction Logging.