Dealing with Outlook in a VDI environment (PST, OST ETC)

Being in the desktop virtualization industry is like watching a rerun sometimes. Many of the same problems that first arose in the TS era (which is NOT over by the way!) now also seem to arise in VDI environments. How to deal with outlook and PSTs etc is a perfect example. This article talks about those problems and possible solutions.

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.

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.

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.

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.


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  1. Hello my friend,

    Today I support a environment that works with Xendesktop 7.1. We work with roam OST, and I realized that sometimes the OST´s of some users on File Server are duplicated. I know, Microsoft don´t support Outlook on this way, but we have other clients that work of same way and works fine. Do you have some tip about this situation? I used GPO to cached mode.

    Thanks for your help!

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