Most mornings, I get up to work at 5:00 a.m., while my three young sons are still sleeping. That way, I can do some productive work and get a good head start on the most important tasks of the day. Exchange Server has a similar special time: the daily scheduled maintenance period. This period, which by default occurs from 2:00 a.m. to 6:00 a.m., gives the Information Store (IS) a chance to perform several vital maintenance tasks--online defragmentation, tombstone maintenance, Deleted Item Retention store cleanup, Deleted Mailbox store cleanup, and index purging--that help keep your Exchange mailbox and public folder databases free of unnecessary clutter. Let's examine these basic tasks.
- Online defragmentation. As users add and remove messages, the amount of used space in Exchange's database files fluctuates. The IS conscientiously recycles free space in the database file, only increasing the size of the file when there isn't enough unallocated space. During an online defragmentation, the IS reallocates and rearranges pages to put all the free space in a contiguous block at the end of the database file. Doing so makes future allocations more efficient. After a defrag, you can check the event log for event ID 1221, which tells you how much free space a particular database file contains. This information in turn helps you estimate how long you have before the IS increases the database file size.
- Tombstone maintenance. This task sounds like something from an old John Ford movie, but in reality it's something altogether different. When a user marks an item for deletion from the IS, Exchange doesn't immediately delete the item. Rather, Exchange replaces the deleted item with a tombstone--basically, Exchange sets an "Is this deleted?" flag on the object and replicates the object (along with the flag) to other servers. After a 30- or 60-day period (depending on the object type), Exchange removes the tombstone and truly deletes the item during tombstone maintenance.
- Deleted Item Retention store cleanup. If you've turned on Deleted Item Retention, Exchange places deleted items in the Deleted Item Retention store (aka the dumpster). Deleted items stay in the dumpster until the end of the specified Deleted Item Retention period, after which Exchange deletes them.
- Deleted Mailbox store cleanup. Exchange Server 2003 and Exchange 2000 Server support a dumpster for deleted mailboxes. This mechanism can be a real lifesaver if you accidentally delete an important mailbox. The process is the same as for deleted items.
- Message table checking. The IS performs some limited consistency checking to examine the message tables in each mailbox and update those tables as needed. Don't confuse this process with the more comprehensive, and much slower, message table tests that Isinteg performs.
- Index purging. For efficiency, the IS dynamically builds database indexes when users change the view criteria in Outlook. However, keeping a limitless store of view information around is impossible, so after a variable period (usually about 7 days) old views expire and Exchange removes them.
The best part about these maintenance tasks is that they're completely automatic. Although you can change the times when the tasks run, you can't (and shouldn't) turn them off entirely. Exchange lets you control the allocated time window for maintenance, so if you set a window that's too brief, Exchange will defer some maintenance tasks until the next window. This delay can lead to IS instability or storage-utilization problems, so check the event log to verify that all your daily maintenance tasks are finishing. If they aren't, use the mailbox database or public folder database Properties dialog box's Database tab to lengthen the schedule window. Next week, I'll discuss other maintenance tasks that Exchange sometimes requires.
