XADM: Database Size After Offline Defrag Not Same as Total K

When you perform an offline defragmentation on the Priv.edb or the Pub.edb, the size of your database after defragmentation may not coincide with the Total K of all items in Mailbox Resources or Public Folder Resources.

There are several reasons why the actual database size may not match the total size of Mailbox Resources. For example, if you add the Total K for each mailbox listed in Mailbox Resources, and it equals 1 GB, and then you run EDBUTIL /D on the Priv.edb, the resulting database size may be larger than 1 GB. This difference could range from a few megabytes to several gigabytes, depending on several factors on the Exchange Server computer, described in 1-4 below:

NOTE: In Microsoft Exchange versions 4.0 and 5.0, you use the EDBUTIL /D command to perform an offline defragmentation of a database. In Microsoft Exchange version 5.5, you use the ESEUTIL /D command to perform an offline defragmentation of a database.

1. The Priv.edb stores more than mailbox messages and folders. Inbox Rules, Views, and other items associated with the mailbox are stored here as well but are not represented in the Mailbox Resources page.
2. After an item is deleted or moved, it is not immediately wiped from the store. A background cleanup process has to run first in order to fully commit the change. For more information about background cleanup, see the following article in the Microsoft Knowledge Base:
   159306� XADM: IS Maintenance Tasks Not on IS Schedule.
   Running EDBUTIL /D; after this will give you a better count of what is actually in the database.
3. Because of the Single Instance Storage feature, one message can belong to many mailboxes. For example, if you send a message to several recipients who reside on the same Exchange Server computer, the message is only stored once in the information store, and the recipients just receive a pointer to the message. If one recipient deletes the message from his or her mailbox, the message is not deleted from the information store, only the pointer to it. The message itself is not deleted until the last recipient deletes it. This also means that all messages associated with a user may not be removed from the Exchange Server computer when you move the user's mailbox to another Exchange Server computer. The message sent to several recipients will remain on the Exchange Server computer until all recipients of the message have been moved to another Exchange Server computer or they all delete their pointer.
4. The Exchange database engine uses sector level allocation. Every operation that requests new memory and requires that a new extent be allocated, is given a new extent of 16 pages (a page is 4 KB) -- regardless of the actual number of pages being requested (may be one page). This leads to a relatively sparse database structure. Therefore, it's generally true that the size of a Exchange database will always be larger than the amount of data it contains.

NOTE: Before you perform an EDBUTIL /d on any Microsoft Exchange database, make sure the drive that the utility is running against has available free disk space equal to twice the size of the database that is being defragmented.