Exchange 2013 storage considerations when virtualizing on Hyper-V

The Exchange Server virtualization scenarios available today are much broader than those of a few years ago. Restrictions remain, but they are far less obtrusive. This year, many companies will examine virtualizing Exchange 2013 on Microsoft Hyper-V. The most glaring restrictions here are the storage requirements, which are critical to Exchange VM mailbox databases and transport queues.

If you're migrating to the newest version of Exchange and preparing for a virtual setup, you should know exactly how virtualized Exchange 2013 storage will work. Here are the five most important aspects to keep in mind:

1. Virtual disk size for all Exchange instances must be fixed

Like many hypervisor and virtual-machine products,

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Microsoft Hyper-V lets you create virtual disks that will dynamically expand up to a predetermined size. For example, if you designate a virtual disk with a 250 GB maximum size, but you're only using 100 GB, the corresponding dynamically expanding virtual disk file will only be 100 GB.

Fixed-size virtual disks, however, use a virtual disk file. Virtual disk files pre-allocate all of the space required for the virtual disk. In other words, a 250 GB virtual disk will use a 250 GB file. Fixed disks will help eliminate the performance issues you may see with dynamically expanding disks.

2. Block-level storage is required for all direct-attached devices

Hyper-V allows for direct-attached storage (DAS), which in turn lets a VM directly write to a storage device instead of to a virtual-disk file on said device. This increases performance. However, any such direct-attached devices must use block-level storage; they can't be network-attached storage (NAS) devices. However, they can be iSCSI, Fibre Channel over Ethernet, or another storage-area network-type device.

The reason behind this this is simple. For Exchange 2013 storage to function properly, Exchange needs low-level access to the device because of how it works with its databases; NAS doesn't expose enough low-level functionality for Exchange. Additionally, exposing a NAS to the hypervisor, and then sharing it out to Exchange 2013 as a storage medium doesn't work.

3. Avoid using differencing storage mechanisms

This is especially true for snapshotting and differencing virtual hard disks (VHDs) in Microsoft Hyper-V. The problem with snapshots and differencing VHDs is that they never completely capture the entire state of a given Exchange installation, which is often spread out across multiple storage devices at once.

Snapshotting an Exchange installation and then attempting to roll it back might create inconsistencies across the various databases, with a broken Exchange installation as a potential end result. Until Hyper-V's snapshotting becomes application-aware -- which I don't see happening for a while -- you should avoid using differencing VHDs or snapshots in virtualized Exchange 2013.

4. Allocate enough space for each virtualized instance of Exchange

For each instance of Exchange you plan on virtualizing, Microsoft recommends allocating disk space using the following formula:

"15 GB + the amount of virtual memory allocated to the VM in question"

Therefore, a VM with 32 GB of RAM would need at least 47 GB of disk space for the OS, the paging file and Exchange 2013's own files. If you can throw more space at the problem, go for it, but this is the bare minimum.

Note: This space does not include Exchange's databases; it's just the minimum amount of space required for Exchange 2013 plus the OS. Databases should always be on a separate volume anyway.

5. Your Exchange 2010 storage plans should still work.

When prepping for Exchange 2013 storage, remember that Microsoft recommends roughly the same planning requirements that were laid out for Exchange 2010. If you still have the storage calculator numbers you used to plan your Exchange 2010 setup, they should still prove useful. That said, any plans devised under earlier editions of Exchange Server should be revamped, especially if you're performing a physical to virtual migration.

Final thoughts

The points discussed above indicate a number of possible future changes for how Exchange 2013 storage will be handled when virtualized on Hyper-V. The most important take away here is how Hyper-V could be made application-aware to allow snapshotting of Exchange instances. That said, the actual scenarios where this sort of functionality would be useful don't involve Exchange itself, but are experiments involving Exchange indirectly. For example, think of multiple iterations of the same setup to gauge differences between them.

Right now, Hyper-V supports more than enough of the functionality you will need to create and maintain a solid instance of Exchange 2013, as long as you keep in mind the rules for allocating storage for both the hypervisor and Exchange Server itself.

About the author:
Serdar Yegulalp has been writing about personal computing and IT for more than 15 years for a variety of publications, including Windows Magazine, InformationWeek and the TechTarget family of sites.

This was first published in January 2013

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