This article basically answers the question “if user data is retained at logoff, why can’t we use Cached Exchange Mode in non-persistent VDI environments” and does a small IOPS comparison as well
The short version is this – Exchange/Outlook best practices do not necessarily work in a VDI environment.
When using Outlook in an Exchange environment, it is recommended to use Cached Exchange Mode. In this mode a copy of the user’s mailbox is downloaded into an OST file and stored offline on the user’s desktop. This mode offers better performance for the end user and reduces utilization on the Exchange environment as well. In addition, using Cached Exchange Mode allows users to use Outlook Instant Search for fast searching of items in their mailbox. Instant Search works by indexing the contents of the OST file so all searches occur locally and not on the Exchange server, further improving performance and reducing utilization on Exchange.
VDI environments that we see at our clients are typically configured as non-persistent or floating pools of desktops. That is, each user connects to a pool of identical desktops and grabs whatever desktop is available. When the user logs off, any changes written to the VDI desktop are discarded and the desktop returns to a pristine state. There are mechanisms and tools in place to make sure user data is retained at logoff.
So if user data is retained at logoff, why can’t we use Cached Exchange Mode in non-persistent VDI environments?
- The OST file is equal in size to the user’s mailbox so storing a 15-30GB OST (not unusual at our clients) is not that practical from a performance or storage perspective. If this data is being stored on the SAN, then you’re essentially doubling your Exchange storage (which may already be doubled or tripled if you’re using Exchange 2010 w/ DAGs). In addition, the length of time it would take to download that file every time and the I/O impact that would cause makes it completely impractical.
- OST files are not supported when stored on network shares, so redirecting the OST to a home directory is out.
- Indexing of files on virtual desktops is typically disabled to reduce I/O demands. This would prevent the use of Outlook Instant Search even if the OST was present.
For these and other reasons, Outlook is typically configured in Online mode when used with VDI. This keeps all mailbox operations and searches on the Exchange server, placing the processing and I/O burden solely on the Exchange environment. That sounds bad, but advances in Exchange technology specifically with Exchange 2010 have made this much less of an issue. In fact, Microsoft states that IOPS requirements for Cached Exchange Mode and Online mode are essentially equal now, meaning there is no I/O “penalty” for using Online mode.