VDI and Microsoft Outlook, analysing the variables

VDI is a multitude of technologies that must work together in a premeditated way; and this we already know. However, couple weeks ago an interesting discussion in one of VMware’s internal forums sparkled my attention. The topic demonstrates clearly the level of attention and detail that a good VDI design should have. The central point of the question was about Microsoft Outlook configuration for VDI environments. Sounds simple, but let’s have a look.



The practice recommends that PSTs be avoided in VDI, whenever possible. The reason for that is that the existence of PSTs in a given virtual desktop will tie that virtual desktop to a user, which is fine in the case of persistent desktops. What about if the desktop needs to be recomposed?

Well, a possible solution for that would be PST files in a Persistent Disk (UDD). If users are allowed to create PSTs in Persistent Disks, in a Persistent Desktop it’s then indispensable to carry out backups of these PSTs. The backup would have to be disk based and not agent based, since the user may leave Microsoft Outlook open inside the virtual machine. Of course there are way to solve that, like executing a automatic logoff or using a agent that support open files.

Let’s not forget that the main idea behind the creation of PST is due to the existence of the mailbox limit size imposed by administrators to save storage space on Exchange Servers. However, letting users create PST files they would be consuming the same amount of storage space from a same or different storage array. The Persistent Disk could be eventually located in a cheaper storage tier such as a NAS or SATA disks.

Anyway, seems like a good idea not to let users create PST on local disks, therefore increasing storage allocation for the Exchange Servers. In this way desktops could be part of floating pools and the users would have their independence from a single desktop that contains their PST.



The second problem to be solved are the OST files that are created when using Microsoft Outlook in Cached Mode. These cache files must be local and Microsoft does not support them in network shares. OST files are offline copies of server content that are locally indexed and synced with the server creating a high demand for disk traffic and IOPS. Additionally, every first use of Microsoft Outlook with Cached Mode enabled will force Outlook to sync server contents, causing a huge burst in network traffic and disk throughput and IOPS.

In a similar fashion to PSTs it is possible to accommodate OSTs in Persistent Disks, however the same constraints and issues apply. To keep up with the amount of IOPS a possible solution to consider is the use of SSDs, but the additional storage costs would not make sense from a TCO perspective.

Seems like it’s best to disable Cached Mode.


Disabling Cached Mode

Disabling Cached Mode we will be eliminating OST files, therefore getting rid of the persistent disk (UDD) requirement and also reducing network and storage bursts during the replication and indexing periods. However, now we have transferred the workload to Exchange Server.

Because OST files don’t exist anymore Microsoft Outlook will solely rely on being Online with the Exchange server for the user to send and receive and access mail, and this will put the IOPS burden on the Exchange Server, possibly impacting all users in the organisation should the server is not properly sized for the workload.

Another downside of Disabling Cached Mode is that Outlook Instant Search is unavailable if OSTs are disabled, and it will not index against Exchange Server. Since most users use advanced find to search for anything, the overall IO load could be much higher than it would have been if left on the virtual desktop. In summary, may be better to enable Cached Mode.

An interesting point raised during the conversations is that it is easier to manage Microsoft Exchange IOPS and network since it’s a single entity. Virtual Desktops are multiple and each individual desktop could have different requirements.


Looks like there is no right or wrong,and the VDI architect will need to ponder all variables and the organisation’s context before making a decision. What is your take on this discussion?


I would like to thank the following people for contributing for such interesting topic: Chris Drake, David Noonon, John Dodge, Michael Francis, Russel Wilkinson and Stephane Asselin.


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  1. Hi – Nice compilation of points to ponder w.r.t Outlook and VDI.

    1) One of the options can be that of increasing the mailbox size of End-users and using PST only for archival. Why not use the PST space that we allocate from Storage to VDI directly to Exchange to increase their mailbox size ? Flip side would be increased administration efforts and Backup time window on the Exchange front.

    2)Allocate a separate drive for PST with pre-defined quota on the size allowed for PST per user. This would give us a flexibility to have this on a RAID1/RAID10 spindles if performance of outlook is an criteria.

    3) Study the mail exchange pattern on the Exchange Server before designing a solution.


  2. Yet, another option would be to not allow the use of Microsoft Outlook at all. The full web versions provided in Exchange 2010 is great! If you really want to reduce CAPEX, Zimbra might be a good move.

    • Mihai on 02/14/2011 at 2:26 am

    Difficult choices indeed. I don’t think real Outlook users would accept online mode unfortunately so a persistent desktop is necessary; only task workers whose tasks does not include email would be happy without instant search :).

  3. Once organizations move to Exchange 2010, the I/O requirements for Online Mode go down significantly. I could see this being a problem in Exchange 2003 and possibly 2007, but in 2010 the I/O requirements are greatly reduced so it’s not as big of a deal. Provided you size your Exchange servers properly you’re looking at a very low amount of IOPS per user in Online Mode.

    I don’t think that Instant Search is as big of an issue. Isn’t it a best practice to disable any kind of guest OS indexing in a VDI VM to reduce the potential storage performance impact? Remember that you can still search in Exchange, it just requires an online search instead of a search from a local index. Again Exchange 2010 helps here as they’ve improved the mechanism and performance of indexing so the lack of Instant Search is less of an issue.

    Not trying to be a commercial for Exchange 2010 here. I just think that Exchange 2010 makes the Outlook in VDI discussion a lot easier.

    PSTs are a bad idea in general. There are all sorts of eDiscovery/compliance issues with PSTs that make them a nightmare for many industries (legal/finance especially), not to mention the backup issues mentioned in this article. In my experience with VDI we’ve been using Outlook in Online Mode without performance issues. You lose some of the benefits of seamless failover/mobility of the Exchange roles when using Online mode, but it’s not a big issue either way.


  4. @Matt Liebowitz
    Good to hear that Exchange 2010 address some of the issues when running Outlook in VDI environments. However, we really need to think if does make sense to have a full blown email reader loaded into all virtual desktops, when WEB solutions can provide us with the same or very similar functionality.

    I am starting to think that Outlook is a no go for VDI environments, and I will do some research on solutions available on the market that provide same features – the ones I have previously used are OWA (WEB Exchange), Zimbra and Kerio. Perhaps this questioning will drive us even deeper on the stack – do we really need Exchange Server? What about MS Office 365?

    Using WEB based apps we should be able to reduce resources required to run VDI, therefore increase consolidation ratio e reduce overall cost of the solutions. My Outlook ATM is using almost 100Mb of RAM.

    Just thoughts, as we all know there is no right or wrong. Most designs will be aligned to customers existing infrastructure anyway.

  5. I agree that cloud based email solutions are coming and they’ll change the way we use email. Though at least for the customers I work with, I wouldn’t expect that to change for them in the immediate future. Many are upgrading to Exchange 2010 and plan to stay there for several years.

    The issue with going away from Outlook is application integration. Many applications integrate into Outlook for things like document management, PDF creation, metadata removal, SharePoint lists, etc. It is going to take these application vendors a while before they can integrate these tools into web based solutions or otherwise get them out of Outlook. Until they can do that, I can’t see folks moving away from Outlook even if they go with hosted, cloud based email solutions.

    Outlook uses a lot of RAM, application add-ins are the most common cause of Outlook crashes and instability, and it makes our lives complicated when deploying VDI. Unfortunately I just don’t see it changing until there is less of a dependency on Outlook integrated applications.

    Good discussion, thanks as always Andre..

  6. Our organization runs Exchange 2010, and with Online Mode, we dont haev any performance impact on our VDI client, and even the search against the server is working with WDS 4.0 well configured.
    Anyway, the problem with have is trying to manage the View Online Mode and the need for an OST file 🙁
    Did anybody solve this riddle yet ?

    • pesos on 05/07/2011 at 12:22 am

    I commented on Matt’s recent blog post regarding this over at Kraft Kennedy. As a xenapp shop, our main reason for considering a shift to VDI/xendesktop would be to leverage cached exchange mode and advanced search. Yes disk reqs are lower on ex2010 and ol2010 search has improved a bit, but it still doesn’t compare to local indexed searches – I use both almost daily and instant search is still far better.

    The trick is to design a workable storage solution for VDI and cached mode – and ideally with relatively inexpensive block storage (especially for a smaller shop like us who won’t have more than 300 desktops ever). Have any of you worked with Virsto’s solution? I am still working on getting pricing, but it seems like it may be a good way to go to alleviate capacity and performance issues…

  7. Microsoft released a document on the subject. “Cached Exchange Mode in a Remote Desktop Session Host environment: planning considerations” http://www.microsoft.com/download/en/confirmation.aspx?id=15238

    IMHO the document does not bring any conclusion or help to the discussion.

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  1. […] Since we are here, whether users are using PST and if they are storing in some central location. Here is reference on this topic. This post also provide you likely […]

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