Troubleshooter: Deciding Whether to Use a Disk Defragmenter with Exchange

Can I use a disk defragmenter on my Exchange Server system? Should I do so?

Disk defragmenters move file fragments around until the pieces of each file (or of as many files as possible) on the disk are contiguous and in order. Defragmented files speed access to the data in the files. Most defragmenters also work on the NTFS Master File Table (MFT), thus improving access times for directory data.

Exchange has an internal defragmentation process (i.e., the online defragmentation that occurs as part of the daily Information Store—IS—maintenance process), but that process doesn't touch the MFT or physically defragment disk data—it simply defragments the database pages inside each file. In his excellent book "Scaling Microsoft Exchange 2000" (Digital Press, 2001), Pierre Bijaoui points out two reasons to defragment Exchange servers physically. First, fragmentation decreases backup throughput. Because the backup process must provide pages in the correct order, backups of fragmented disks run slowly. Second, when an application asks NTFS to extend a file's size, NTFS does so by adding an extent to the file. The more extents per file, the longer the Chkdsk utility takes to run after an unexpected reboot. For large database files, fragmented disks can dramatically increase the time required to complete a Chkdsk pass.

You'll find one significant obstacle to using a disk defragmenter with Exchange, however: Exchange keeps its mounted database files open. Defragmenters can't defragment open database files. Bijaoui's book recommends that you periodically back up and restore your Exchange database files to defragment them. Although this process incurs some downtime, the improvements in backup and Chkdsk times might be worth it. Be sure, however, to weigh the potential improvements against the actual downtime you'll incur. Eseutil usually processes about 4GB per hour, so defragging an 80GB IS will take around 20 hours of downtime.