To provide fault tolerance in the event of a hard disk failure, keep your Exchange 2000 transaction log files and database files on separate physical hard disks. Furthermore, if you keep these log files and database files on separate disks, hard disk I/O performance is significantly increased.

Note To track the operations made on every database within a storage group, each storage group has its own set of transaction log files. Transaction logs maintain a sequential record of every operation that is performed on a database. Transaction logs are not deleted until a Normal or Incremental backup is performed for all the databases in a storage group.

The following scenarios include steps you should perform if you lose the disks containing your Exchange 2000 databases or your transaction logs:

• If you lose the hard disk containing the Exchange 2000 databases, you can replace the damaged disk, and then restore the most recent databases backups. After you restore the databases, an automatic log file replay of all transactions that occurred after the backup transfers the recorded transactions from the log files to the databases on disk. This process is known as hard recovery.
• If you lose the hard disk containing the transaction logs, but not the disk containing your databases, you do not have to restore any Exchange 2000 data from backup. However, losing the hard disk containing the transaction logs is more dangerous than losing the hard disk containing the databases because you cannot replay transactions that are recorded to log files but not recorded to the physical database files on disk. As a result, there is an increased chance of losing data that is not preserved in either the log files or in the last backup.

After this event, you should immediately create a new normal backup of any storage group that lost its log files. This new normal backup backs up the databases that no longer have log files, thereby preserving the transactions that were made since the last Normal backup.

Important If you keep your Exchange 2000 databases and transaction log files on the same physical hard disk and that disk fails, you can recover only the existing data up to your last backup.

Furthermore, you can minimise the time it takes to recover from a hard disk failure if you keep each of your Exchange 2000 storage groups on a separate hard disk. If only one disk fails, and you have each storage group located on a separate physical hard disk, you need only to restore the storage group that is kept on the failed disk .

For more information about Exchange 2000 transaction log files, databases, and storage groups, see Understanding Exchange 2000 Database Technology.

The information in this article applies to:

• Microsoft Exchange Server 2003 Enterprise Edition
• Microsoft Exchange Server 2003 Standard Edition
• Microsoft Windows Small Business Server 2003, Premium Edition
• Microsoft Windows Small Business Server 2003, Standard Edition
• Microsoft Exchange 2000 Server