MORE INFORMATION
When a public or private Messaging Database (MDB) is
started, a maintenance task is scheduled to run for that MDB.
When
the maintenance task (also known as a thread) runs for the MDB, it exhibits the
behavior that is described in the following sections.
Read maintenance schedule
Exchange Server reads the maintenance schedule from the internal
(in-store memory) MDB object if it is available, or from Active Directory if
the MDB object is not available.
The schedule uses one of the following styles:
Initially, the MDB object is marked so that it requires that
the Active Directory object be queried for the schedule style (and to be
queried for the schedule details if the schedule style is "Selected"). When the
information store detects that the MDB object in Active Directory has been
changed, the MDB object is marked so that it requires a refresh operation from
Active Directory.
If the schedule style is "Never," the maintenance
task is skipped.
If the schedule style is "Selected," the program
determines whether the maintenance task runs in the current 15-minute period.
If not, the maintenance task is skipped.
If the maintenance schedule
cannot be obtained, the default schedule of 12:00 midnight to 5:00 A.M. is
used.
Determine maintenance task to perform
The maintenance task (thread) next reads a table in the
Extensible Storage Engine (ESE) database to determine the last maintenance
subtask that was performed.
This value is used to determine which
subtask is initiated first, and it makes sure that all of the maintenance
subtasks are eventually run in a round-robin fashion.
The following list describes the maintenance subtasks in the
order in which they run:
- Purge Indices Indexes that are created in database tables
by the client to be used for views and that have not been used for a specified
time are cleaned up when this subtask occurs.
For more information about index aging, click the following
article number to view the article in the Microsoft Knowledge Base: 159197
Controlling folder index aging
- Tombstone Maintenance This subtask compacts the deleted
message information that is used for local and public folder
replication.
By default, this task runs only one time in a 24-hour
period.
- Dumpster Cleanup This subtask cleans up any messages that
have passed their deleted item retention date.
By default, this task
runs only one time in a 24-hour period. To override this frequency, use the Deletion Thread Period registry value.
- Public Folder Expiry This subtask expires messages that are
in public folders and that are older than a specified time value. The setup for
message expiration is on the Age Limits tab in the public information store container in the Microsoft
Exchange Server Administrator program.
By default, this task runs
only one time in a 24-hour period and only on the public folder
MDB.
- Age Folder Tombstone This subtask removes folder tombstone
entries that are older than a specified time (the default is 180 days). Folder
tombstone information is used by public folder replication. The aging prevents
the folder tombstone list from growing without limits.
By default,
this task runs only one time in a 24-hour period and only on the public folder
MDB.
- Folder Conflict Cleanup By default, this task runs only one
time in a 24-hour period and only on the public folder
MDB.
- Update Server Versions By default, this task runs only one
time in a 24-hour period and only on the public folder
MDB.
- Secure Folders Cleanup By default, this task runs only one
time in a 24-hour period and only on the public folder MDB. To override this
frequency, use the Secure Folder Aging Task Frequency registry value.
- Site Folder Check By default, this task runs only one time
in a 24-hour period and only on the public folder MDB.
- Deleted Mailbox Cleanup This subtask checks Active
Directory to determine whether there are any deleted mailboxes. The information
store performs an Active Directory lookup for each user in the MDB. The number
of users who are in each MDB determines the number of Active Directory
Lightweight Directory Access Protocol (LDAP) searches. These searches are used
to keep the information store synchronized with any Active Directory changes
(specifically, to look for deleted mailboxes). The performance cost of this
task is negligible on the Exchange server, but the performance cost can be
significant on the Active Directory server, depending on the number of users,
number of MDBs, and the online maintenance times of each MDB.
By
default, in a corporate scenario, the online maintenance occurs during the
night when very few users are logged on, so that the load on the Active
Directory servers is very low. The extra domain controller load that is created
by online maintenance is typically not a problem in this scenario.
If
Exchange Server is installed in a global data center that serves customers who
are in multiple time zones, the default online maintenance time may become an
issue. The effect that online maintenance has on Active Directory is
proportional to the number of users in each of the MDBs on the servers. The
maintenance task looks for a deleted mailbox for each user in an MDB.
Therefore, if you have 10,000 users in an MDB, the task performs 10,000 LDAP
searches of Active Directory when the online maintenance for that MDB begins.
If the Active Directory servers are continually under moderate load, you may
have to stagger the online maintenance, for example, by setting each MDB to
start online maintenance at a different time. This is particularly critical if
you have hundreds of thousands of users spread across dozens of servers and
hundreds of MDBs.
By default, this task runs only one time in a
24-hour period.
The start of the maintenance task is indicated when the
information store service logs the following event ID message:
Event ID 1208 - "Starting the IS Maintenance tasks."
Perform maintenance subtasks
A selected maintenance subtask is evaluated to determine whether
it is appropriate to run at the current time. A subtask can start at any time
during a specified maintenance window, but the subtask is not guaranteed to
finish before the end of the maintenance window.
After a subtask is
initiated, that subtask runs until it is complete. After a subtask is complete,
the completed subtask is evaluated to determine whether it did the work that it
was designed to do. For example, evaluation of the Purge Indices subtask will
return a value of
False if there were no indexes to clean up.
After a subtask
is complete, the following event ID message is logged by the information store
service:
Event ID 1210 - "The IS Maintenance task subtask name completed."
The information store continues to
operate on the next subtask unless that subtask is not scheduled to run in the
current time interval or unless the service has completed all of the tasks one
time during the current maintenance window.
When the last subtask is
run for the current maintenance window, the maintenance subtask that is
performed is saved to a table in the ESE database so that it can be read when
maintenance is started again.
Most tasks run only one time in a 24-hour period no matter
how many information store maintenance intervals are scheduled.
Maintenance tasks end
When the program determines that no more subtasks are scheduled
for the current maintenance window, the following event ID message is logged by
the information store service:
Event ID 1209 - "The IS Maintenance tasks completed."
Online defragmentation
If at least one subtask completed successfully and performed work
that resulted in a database change, online defragmentation runs after
information store maintenance is complete.
By default, online
defragmentation runs for a minimum of 15 minutes and a maximum of 1 hour after
the information store maintenance period.
To override the minimum
time for Online Defragmentation, use the
OLD Minimum RunTime registry value.
To override the length of time that
online defragmentation can run beyond maintenance, use the
OLD Completion Time registry value.
If the maintenance schedule style is
not "Always," then a check is performed to see how long online defragmentation
can run. The amount of time left in the maintenance window is determined and
added to the
OLD Completion Time value (by default, 1 hour). This accounts for situations where
the last subtask runs a little longer than the configured maintenance schedule.
If the calculated value is less than the
OLD Minimum RunTime value, the calculated value is set to
OLD Minimum RunTime.
After online defragmentation is complete, a store routine
calculates the number of free megabytes left in the database and logs the
following event ID message:
Event ID: 1221 - The database name has amount megabytes of free space after online defragmentation has terminated.
After the online defragmentation process is complete, the next
time period for maintenance to run is calculated and scheduled.
How to enable logging for a particular Exchange Server service
To enable logging for a particular Exchange Server service, follow
these steps:
- Right-click the server in Exchange System Manager, and then
click Properties.
- On the Diagnostics Logging tab, set the
Event Logging Level to Minimum for the
following:
- MSExchangeIS\Mailbox\General
(Information Store Mailbox)
- MSExchangeIS\Public
Folder\General (Information Store Public Folders)
Note MSExchangeIS\Mailbox
and MSExchangeIS\Public Folder
have many categories. You
can set the Logging Level to Minimum for the
categories as required.
After you set the
Logging Level to
Minimum, events 1208, 1209, 1210 are logged when you perform
the related operations.