Pro-Exchange,Lync & Office 365
Belgian Microsoft Unified Communications Professionals
Microsoft Exchange Server, Microsoft Lync Server & Office 365
PST Problems all solved by Exchange 2010

A customer of mine was having issues with PST files being replicated over DFS and causing WAN outages.

When I told him PSTs were actually not supported on LAN or WAN he was not aware of that problem.

So I decided to put a blogpost together with more information about PSTs and LAN, WAN, DFS and Offline Caching.


One general rule: PSTs "ARE" supported when used on a "LOCAL STORAGE" device.

What is not Supported for PSTs

  • PSTs NOT Supported on LAN/WAN : Limits to using personal folders (.pst) files over LAN and WAN links

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 of a .pst file over a network connection may result in a corrupted .pst file if the connection degrades or fails."


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  • PSTs NOT Supported on LAN/WAN (explained): Network Stored PST files ... don't do it!

imagine if you had a couple of hundred users who each have two or three PST files. These users have been with the company for a while, and they rarely (if ever!) delete their email from their PST files. The files continue to grow in size - let's use an average of 1 GB as the size of the PST file. Now consider that when each user launches Outlook, they make a request for two (or three) files, each of them being about 1 GB in size. Then consider what happens when 200 users all launch Outlook around the same time when they get to work. 200 x 3 x 1 = 600 GB of data being requested at the same time. That's an awful lot of Disk & Network I/O to process simultaneously. This is a very common scenario - the file server "freezing" for a few minutes at a time while it tries to service these requests.

The queuing in the server service work queues is what causes this temporary hang. The server service uses work items to handle I/O requests that come in over the network - for example: a request to extend a PST file. These work items are queued in the server service work queues, and from there they are handled by the server service worker threads. The work items are allocated from a kernel resource called Non-Paged Pool (NPP).

The server service sends these I/O requests down to the disk subsystem. If, for reasons mentioned above, the disk subsystem does not respond in time, the incoming I/O requests are queued via work items in the server work queues. Since these work items are allocated from NPP, eventually this resource runs empty. Running out of NPP causes systems to hang eventually (logging an Event ID 2019 in the process).


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  • PSTs NOT Supported on for DFS & Replication : Can DFS Replication replicate Outlook .pst or Microsoft Office Access database files?

DFS Replication can safely replicate Microsoft Outlook personal folder files (.pst) and Microsoft Access files only if they are stored for archival purposes and are not accessed across the network by using a client such as Outlook or Access (to open .pst or Access files, first copy the files to a local storage device). The reasons for this are as follows:

  • Opening .pst files over network connections could lead to data corruption in the .pst files. For more information about why .pst files cannot be safely accessed from across a network, see article 297019 in the Microsoft Knowledge Base (
  • .pst and Access files tend to stay open for long periods of time while being accessed by a client such as Outlook or Office Access. This prevents DFS Replication from replicating these files until they are closed.


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  • PSTs NOT Supported with Client side caching (Windows XP, Vista & Windows 7) : Error message: "Files of this type cannot be made available offline"

Certain file types are excluded by default as a means to prevent file conflicts and data loss. The mechanism that the Offline Files feature uses is Client-Side Caching (CSC). CSC can recognize when synchronization issues exist (for example, if the copy of a file on the server has been modified since the last synchronization). In such a case, you can select which version to use, but you cannot merge the contents.


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  • PSTs NOT Supported to sync between computers : How to manage a .pst file for two different computers in Outlook

You cannot automatically synchronize personal folders (.pst) files between two different computers in Microsoft Outlook.

If you are using Microsoft Exchange Server as your e-mail service, the best way to synchronize data between two computers is to use an .ost file


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The Overall Solution:

Exchange 2010 to the Rescue with 2 Valid Options



Posted 10-19-2011 2:08 by Johan Delimon