By default Exchange Server performs online maintenance of you IS between 1:00am and 6:00am. Unfortunately this is usually the period in which we schedule our backups. Good practise is to schedule this in low periods of activity, but not during backups. Whilst Exchange is quite good at doing more than one thing at once, backups and Online Maintenance are not the things you want to be happening at the same time. To understand why not, let's look at what happens during Online Maintenance.

• Index Aging - Index's that have not been used for a certain period of time are cleaned up.

• Tombstone Maintenance - Compaction of deleted message information used for local and Public Folder (PF) replication.

• Message Expiration in PF - Expiring of messages in PF's that are older than their specified time.

• Age Folder Tombstones (PF) - Removing of folder tombstone entries that are older than a specific time. The default setting is 180 days. This is used by PF replication.

• Update Server Versions (PF) - The Directory Store (DS) contains the version numbers of all Exchange Servers it knows about. This is kept to control interoperability between older and newer versions of Exchange. This process keeps this information up to date.

After Online Maintenance has run, then Exchange runs the Online Defragmentation. This runs for a minimum of 15 minutes and a maximum of 1 hour. If during Online Maintenance some of the tasks have not been completed, then Online Defragmentation will not occur, if it previously ran on the previous schedule.

This runs as a low priority task and can be interrupted of another process requires access to the database, such as a backups!