Chapter 2: Capacity and Availability Management 27
scenario, the online maintenance occurs during the night (by default) when very few users
are logged on, so the load on the Active Directory servers should be very low. The extra
domain controller load created by online maintenance should not be a problem in this
scenario.
If Exchange 2000 is installed in a global data center, serving customers from multiple time
zones, the default online maintenance time could become an issue. The effect that online
maintenance has on Active Directory is proportional to the number of users in each of the
server’s databases. A check for a deleted mailbox is performed against each user in a
database. Thus, if you have 10,000 users in a database, it will perform 10,000 LDAP
searches against Active Directory at the beginning of that database’s online maintenance.
If Active Directory servers are under moderate load at all times, it is necessary to stagger
the online maintenance (set each database to start maintenance at a different time on the
server-configuration object). This is especially critical if you have hundreds of thousands
of users spread across dozens of servers and hundreds of databases.
Message Deletion and Online Defragmentation
These are very disk-intensive tasks and only affect the server on which the maintenance
is being run. During this portion of online maintenance, the server might be perceived by
users as sluggish if many databases are set to perform online maintenance at the same time.
Again, in corporate scenarios this would occur at night where the server can easily handle
the extra load. In a global data center, it might be a good idea to stagger the databases (in
respect to each other on a single server) to spread the disk-intensive tasks over a greater
amount of time.
Defragmenting the database consists of 18 separate tasks. After a task has started, it
must complete fully before the process exits. Therefore, online maintenance can run over
the time window. The next task will execute during the next online maintenance window.
Depending upon the run window and the backup schedule, it might take a number of
days before a full defragmentation completes.
Online Backups
Online backups complicate online maintenance even further. Backing up an Exchange 2000
database halts the maintenance of any database within that storage group, although it will
restart if the backup is finished before the maintenance interval has been passed. If you have
two databases in a single storage group and one is running online maintenance, the online
defragmentation on the database that is running online maintenance will stop when a
backup is started against either database.