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Q178931: XADM: Database Size After Offline Defrag Not Same as Total K

Article: Q178931
Product(s): Microsoft Exchange
Version(s): WINDOWS:4.0,5.0,5.5
Operating System(s): 
Keyword(s): kbusage
Last Modified: 22-MAR-1999

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The information in this article applies to:

- Microsoft Exchange Server, versions 4.0, 5.0, 5.5 
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SYMPTOMS
========

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.

CAUSE
=====

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:

  Q159306 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.

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Keywords          : kbusage 
Technology        : kbExchangeSearch kbExchange500 kbExchange550 kbExchange400 kbZNotKeyword2
Version           : WINDOWS:4.0,5.0,5.5
Issue type        : kbprb

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