Thursday, November 21, 2013

Pst... Storing PST's on a network share? Still a no-no...

Limits to using personal folders (.pst) files over LAN and WAN links

This article contains guidance on the limitations of using of personal folders (.pst) files over a local area network (LAN) or wide area network (WAN) link. Note that .pst files are not meant to be a long-term, continuous-use method of storing messages in an enterprise environment.


The Microsoft Exchange Server 4.0 team created .pst files in order to let users maintain a copy of their messages on their local computers. The .pst files also serve as a message store for users who do not have access to a Microsoft Exchange Server computer (for example, POP3 or IMAP email users). 

However, .pst files are not intended as 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 and LAN links use network-access-driven methods. These are commands that the operating system provides to send data to, or receive from, another networked computer. If there is a remote .pst file (over a network link), Outlook tries to use the file commands to read from the file or write to the file. However, the operating system must then send those commands over the network because the file is not located on the local computer. This creates lots of overhead and increases the time that is required 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.

For more information about issues that you may experience with network storage devices or with servers that host shares, visit the following Microsoft website:

Other behaviors of .pst files over WAN and 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.

Because of these behaviors, Offline Folders (.ost) files and Personal Address Book (.pab) files on a network share that are accessed remotely are also unsupported configurations.


What to consider 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 earlier, 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).


That said, there is ONE scenario that is supported;

Outlook 2013 or Outlook 2010 hosted remotely by using Windows Server 2008 R2 Remote Desktop Session Host (RDSH) or Virtual Desktop Infrastructure (VDI) configuration

Outlook 2013 and Outlook 2010 functionality is supported when networked .pst or .ost files are used under the following conditions: 
  • A high bandwidth/low latency network connection is used.
  • There is single client access per file (one Outlook client per .pst or .ost).
  • Either Windows Server 2008 R2 Remote Desktop Session Host (RDSH) or Windows Server 2008 R2 Virtual Desktop Infrastructure (VDI) is used to run Outlook remotely.


In general, network share hosting of PST's is "bad" and the long you have them open, the more likely your going to jack them up. Also the impact on your file servers is pretty killer... BTW, this isn't new, there's guidance going back a decade that this is "bad."

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