This article tells you why personal folder files (.pst files) are unsupported over a local area network (LAN) or wide area network (WAN) link. The .pst files are not meant to be a long-term, continuous-use method of storing messages in an enterprise environment.

The article also suggests two alternatives to .pst files. You can use .ost files or Microsoft Windows Terminal Services.

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The Microsoft Exchange Server 4.0 team created .pst files with the intent of letting a person maintain a copy of their messages on their local computer. The .pst files also serve the purpose of a message store for users who do not have access to a Microsoft Exchange Server computer (for example, Microsoft Outlook Internet Mail Only (IMO) mode users).

However, .pst files are not meant to be an enterprise network solution. Although it is possible to specify a network directory or a Universal Naming Convention (UNC) path as a storage location for a .pst file, network usage is not meant to be a long-term, continuous-use method of storing messages in an enterprise environment.

A .pst file is a file-access-driven method of message storage. File-access-driven means that the computer uses special file access commands that the operating system provides to read and write data to the file.

This is not efficient on WAN or LAN links because WAN/LAN links use network-access-driven methods, commands the operating system provides to send data to or receive from another networked computer. If there is a remote .pst (over a network link), Microsoft Outlook tries to use the file commands to read from the file or write to the file, but the operating system then has to send those commands over the network because the file is not on the local computer. This creates a great deal of overhead and increases the time it takes to read and write to the file. Additionally, the use a .pst file over a network connection may result in a corrupted .pst file if the connection degrades or fails.

For more information about issues that you may experience with the network storage devices or with servers that house shares, visit the following Web site: Other Behaviors of .pst Files over WAN/LAN Links

•	All operations take longer.
•	Write operations can take approximately four times longer than read operations.
•	Outlook has slower performance than the Exchange Client.

For the reasons mentioned above, Offline Folders (.ost) files and personal address book (.pab) files on a network share that are accessed remotely are also unsupported configurations.

If there are roaming users in the organization, visit the following Microsoft Web site:

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Recommendations

Microsoft recommends the following solutions, instead of .pst files over a LAN or WAN.

Exchange Server with .ost File

When you are working over a WAN or LAN, it is better to use the offline folder files (.ost files) to allow the remote client to work without being connected to the server. .ost files support local replication, which means that all folders and their data can be replicated to the .ost, not just e-mail messages, as is the case when you use remote mail. The use of .ost files is therefore more efficient and more useful. .ost files also do not have a dependency on the availability of the Exchange Server computer (except to synchronize new data from the server to the client and vice versa), because the information is cached in the local .ost file. This improves performance because the information being viewed is stored on the local drive while the master copy of the data remains on the server, where it can be accessed and backed up. .ost files also provide data redundancy, which ensures greater integrity and recoverability of the data.

Microsoft Terminal Services

If an enterprise wants to use Outlook over WAN or LAN links, it is highly efficient to run the Microsoft Windows Terminal Server service. With Terminal Services, only enough information required to update a display is transferred. The potential benefits in having many remote users based on Terminal Services, instead of using either .pst files or .ost files, would be significant in any network bandwidth conservation analysis.

Considerations when you store .pst files

When you store .pst files, shares may stop responding. This behavior may cause several client-side problems, such as causing Outlook to stop responding or freezing desktops on client computers. Queuing in the Server service work queues is what causes this temporary condition. The Server service uses work items, such as a request to extend a .pst file, to handle I/O requests that come in over the network. These work items are queued in the Server service work queues. From there, they are handled by the Server service worker threads. The work items are allocated from a kernel resource that is called the nonpaged pool (NPP). The Server service sends these I/O requests to the disk subsystem. If, for reasons that are mentioned above, the disk subsystem does not respond in time, the incoming I/O requests are queued by using work items in the server work queues. Because these work items are allocated from the NPP, this resource eventually runs out. Running out of NPP causes systems to eventually stop responding and to log event ID 2019.

If you troubleshoot this issue, you can usually find evidence of a problem in Poolmon and Perfmon captures. For example, you may see the LSwn pool tag allocation climb in a Poolmon trace. These allocations are made by the Srv.sys program. The size of the allocation is configurable by using the SizReqBuf registry value. One allocation is made for each work item that is used by the Server service. When you use Perfmon to troubleshoot this issue, you see a steady decrease in the "Available Work Items" counter. If "Available Work Items" reaches zero, clients may be unable to access files. You may also experience event ID 2019 errors if the problem is LSwn allocations (NPP depletion). Another tag that indicates .pst file issues is the MmSt tag. This tag represents the Mm section object prototype PTEs, a memory management-related structure that is used for mapped files. (This is the pool tag that is used to map the operating system memory that is used to track shared files.) MmSt issues frequently manifest as paged pool depletion (Event ID 2020).

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For more information, click the following article numbers to view the articles in the Microsoft Knowledge Base:

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