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Important This article contains information about how to modify the registry. Make sure that you back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/256986/ )Description of the Microsoft Windows registry
Internet e-mail messages are typically structured in MIME format. In some cases, Microsoft Exchange Server must convert MIME messages to MAPI format.
For Post Office Protocol version 3 (POP3) or Internet Message Access Protocol, version 4rev1 (IMAP4) clients to gain access to that e-mail, the MAPI-formatted content must be converted back to MIME format before the clients can log on. This conversion allows the exact message size to be calculated, although the MIME-converted content is not persisted in the database. If the file is larger than 4 kilobytes (KB), the mail cannot be converted in memory. Therefore, Microsoft Exchange 2000 Server writes a temporary file to the Windows Tmp folder.
Mail is converted to MAPI during the following operations:
If timeouts occur during the conversion, the following event ID messages are logged in the Application event log:
Event Type: Error
Conditions under which client latencies occurIf all of the users on an Exchange 2000 server are using POP3 or IMAP4 clients, a large number of the messages in the mailboxes are frequently stored in MIME format. If the mailboxes are moved, Exchange 2000 converts all of these messages to MAPI.
Client latencies may occur in the following scenario:
Client latencies may also occur in the following scenario:
How to avoid client latenciesYou cannot prevent Exchange 2000 from converting messages to MAPI when mailboxes are moved or when public folder data is replicated. Additionally, you cannot set the folder that Exchange 2000 uses to convert the messages in Exchange 2000. Exchange 2000 must use the folder that either the Microsoft Windows TMP system variable or the Windows TMP user variable specifies.
The TMP system variable is used on stand-alone Exchange 2000 servers. The cluster service account user's TMP user variable is used on clustered servers. To avoid the adverse effects of this behavior, change the TMP folder variable that is used to a location represented by a drive that has a high-performance caching controller connected to it and enough spindles to handle the conversions.
Note that on a cluster, when you relocate the Tmp folder to a shared cluster drive, this may cause problems during failover. These problems occur because disk ownership is transferred to a surviving node during failover. Therefore, Microsoft recommends that you do not move the folder to a shared cluster drive. If you can, move the Tmp folder to locally attached storage.
If the Exchange 2000 server has either very limited local disk storage or no local disk storage, and all of the external storage is allocated as shared cluster resources, you may not be able to move the Tmp folder to locally attached storage. As a last resort, either reallocate some of the shared storage as a non-cluster resource, or add additional storage area network (SAN) storage that is not shared among the nodes, so that there is a location that the Tmp folder can be moved to.
To permit users to log on faster, you can also set the following registry keys to turn off exact message-size calculation. One of the keys is for POP3 clients, and the other is for IMAP4 clients.
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
IMAP4 clientsEnable fast message retrieval for IMAP4 users. To do this, follow these steps:
How to determine whether access to the Tmp folder is causing client latenciesTo determine whether access to the Tmp folder on the Exchange 2000 server is causing client latencies, use System Monitor to monitor disk I/O activity on the disk where the Tmp folder is located. On the disk where the Tmp folder is located, you may notice the following behaviors:
(http://support.microsoft.com/kb/248345/EN-US/ )How to Create a Log Using System Monitor in Windows 2000
How to determine how many disk spindles you needIf the average message size is 45 KB, the server does about 3 TMP writes for each RETR (POP3) or FETCH (IMAP4) when the server converts from MAPI to MIME. You can use this value to determine how many disk spindles are necessary on any computer.
For example, assume that a server has 1,000 users. Each user has an Inbox that contains 500 messages, and all of the mailboxes have just been moved. After the move, when users log on and RETR or FETCH their mail at a rate of 42 messages per second, the server performs about 126 writes per second to the TMP drive (3 writes per RETR multiplied by 42 RETR commands per second). One spindle can handle about 100 writes per second. Therefore, two Raid0 spindles are necessary, or four Raid0+1 spindles. This example was tested on a 4 x 450 megahertz (MHz) Exchange 2000 server with 4 gigabytes (GB) of RAM.