Logging Explained

Exchange Server maintains several database files (stores) in a structure transparent to the end user. The information store, for instance, consists of two databases: private (PRIV.EDB) and public (PUB.EDB), both managed by the information store service. The Exchange directory is stored in DIR.EDB. The Exchange Server services use transaction log files for each of these databases.

Exchange database technology implements log files to accept, track, and maintain data. To enhance performance and recoverability, all message transactions are written first to log files and memory and then to the respective database files. Client performance is boosted because log files are written to sequentially (eliminating seek time) and Exchange Server writes message transactions to log files immediately. Log files are always appended to the end of the file however, and Exchange database files (PUB.EDB, PRIV.EDB, and DIR.EDB) are written to randomly (making seek time a performance factor).

Recoverability is boosted because log files can be used to recover message transaction data if a hardware failure corrupts the information store or directory database files, provided that the logs are backed up and intact. Log files are typically kept on a separate physical disk drive from the information store and directory database files, so a failure that affects the database files probably will not affect the log files. Any data that has not been backed up but that has been recorded in the transaction logs can be "played" back to restore the database file.

The directory and information store services use transaction logs, previous logs, checkpoint files, reserved logs, and patch files.

Fast Automatic Recovery Using Transaction Rollback

When an Exchange Server information store or directory service is started after abnormal server shutdown, the transaction log file is scanned to see if there were any incomplete transactions. If there are, they are rolled back automatically to their pre-shutdown state. This automatic recovery operation is relatively quick because only the most recent transactions in the log have to be checked.

These recoverability differences are analogous to those between production DBMS servers such as Microsoft SQL Server or Oracle and end-user databases such as Microsoft Access or Lotus Approach.

Single-Instance Storage with Automatic Referential Integrity

Single-instance storage is a key requirement from customers who wish to store users' mail centrally, on the server. If 100 users on the same server receive the same message, only a single instance of the message is stored on the server: pointers to the message are placed in the users' mailboxes. Single-instance storage can save space and boost server performance.

The information store design of Exchange Server has built-in single-instance storage: it is always in effect, and requires no special configuration or administration. When a message or user mailbox is deleted, messages cannot be orphaned or lost. Pointers cannot become out of synchronization between files because everything is stored in a single file and referential integrity is handled internally by the database engine. Exchange Server is optimized for efficient, reliable storage of messages on the server.

Single-Instance Storage with Per-User Storage Limits

Research shows that one of the most common reasons for mail system outages is simply the inability to limit user storage, which eventually causes servers to fill up and cease working. To prevent this, Exchange Server allows administrators to set and enforce disk quotas at any level from user to system. Users can be given a warning as well as a "hard" limit that prohibits them from sending any mail until they clean out their mailbox. This prevents users from missing critical incoming e-mail and does not penalize other users by failing to deliver mail they send to "warned" users.

Live Online Backup to Tape for 24x7 Operation

Exchange Server has built-in support for online backups directly to tape. The server does not have to be shut down, nor do users have to log off. Furthermore, Exchange Server backup is integrated with Windows NT Server backup, so you can back up both types of servers from the same location. You can perform full, incremental, or differential backups directly to a wide variety of tape devices, from -inch cartridges to high-capacity DAT systems.