Storage Sizing with VMFS Volumes

For VMware VI3 and vSphere environments, there is no clear guidance on how storage should be sized. We do have one firm piece of information, and that is the configuration maximums document. For both platforms, the only firm guidance is virtually the same for VMFS-3 volumes, in that a single logical unit number (LUN) cannot exceed 2 TB. With extents, they can be spanned 32 times to 64 TB. This became relevant to me while reading a post by Duncan Epping at Yellow-Bricks.com. He's right -- it just depends.

For most environments, sizing scenarios that flirt with the maximums are rare. There are plenty of planning points to determining the right size however. One piece of guidance is the allocation unit size for the formatting of the VMFS-3 volume. This will determine how big a single file on the VMFS volume will be, explained here. With this information, if you know you will have a VM with a 600 GB VMDK virtual disk, then an allocation unit of 4 MB or higher is required.

Another factor that can influence how to size a LUN is the desired number of VMs per LUN. If you want a nice, even number like 10, 15 or 20 VMs per LUN, you can take Duncan's advice and determine an average size and multiply accordingly. This is where a curve ball comes into play. Many organizations are now deploying Windows Server 2008 as a VM, and the storage requirement is higher compared to a Windows Server 2003 system. This will skew the average as new VMs are created.

The hardest factor to use as a determination for sizing is I/O contention for VMs. This becomes a layer of abstraction that is incredibly difficult to manage in aggregate per LUN, per host and per I/O-intensive VM basis. VMware has plans for that and more with their roadmap technology PARDA. Duncan again is right on point with a recommendation that very large VMs use a raw device mapping (RDM); this may be applicable to I/O-intensive VMs as well.

Finally, I think it is very important to reserve the right to change the sizing strategy. For example, when a VI3 or vSphere environment is initially implemented, it would make sense for 400 or 500 GB LUNs to hold 10 average VMs. If the environment grows to over 1,000 VMs, do you want to manage 100 LUNs? It is pretty easy for the VMware administrator, but consider earning back the respect of your SAN administrator and consolidating to larger LUNs and returning the smaller LUNs back to the SAN. This can keep your management footprint reasonable with the SAN administrator.

The answer still is that it depends, but these are my thoughts on the topic based on experience. Share your comments on LUN sizing below or shoot me a message with your strategy.

Posted by Rick Vanover on 06/29/2009 at 12:47 PM