Is it OK to use a disk-defragmentation utility on a disk that holds only Exchange Server databases? I plan to dismount the stores, defrag the disk, then remount the stores.

You can defrag the disk as you plan, but doing so might not lead to the results you expect. Most defragmentation tools (including the ones that come with Windows) calculate fragmentation percentages according to the number of files on the disk. For example, suppose you have one disk that contains one storage group's (SG's) worth of databases: a couple of .edb and .stm files, plus a few other files. (For convenience, we'll assume that the logs are elsewhere.) If four files out of eight are fragmented, a defragmentation tool will calculate the disk fragmentation as 50 percent, which seems fairly severe. But defragging the disk will take your Exchange databases offline for a long time, and the performance gains in this situation will probably be minimal. Although defragmentation software is generally robust, I recommend against using it on volumes that hold your Exchange databases or log files.

You might be confusing fragmentation of the database files' storage extents on disk and fragmentation within those database files. As Exchange uses and releases pages inside the .edb file, the Store can become fragmented. This is why online defragmentation occurs daily as part of Exchange's scheduled maintenance; event ID 1221 in the event logs indicates the completion of the defragmentation. (Take a look at the Web-exclusive article "Exchange's Daily Scheduled Maintenance," September 2003, InstantDoc ID 40359, for more information about Exchange's automatic maintenance.)