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Religious Issue #4: Number of VMs Per Volume

I am not sure if I'm being increasingly brave here, but I'll say that I was inspired to weigh in with my comments on the contentious issue regarding the number of virtual machines per volume. It's good timing too. Recently, I just came across another great post by Scott Drummonds in his Pivot Point blog, Storage Consolidation (or: How Many VMDKs Per Volume?).

Scott probes the age-old question: how many virtual machines do we put per datastore? The catch-all answer of course is, "It depends."

One factor that I think is critical to how this question is approached and isn't entirely addressed in Scott's post is the "natural" datastore size that makes sense from the storage processor. Most storage controllers in the modular storage space have a number of options in how to determine the best size of a logical unit number (LUN) to present to the vSphere environment.

The de-facto standard is often 2 TB as a common maximum, but there are times when smaller sizes make more sense. Take into account other factors such as storage tiers, hard drive sizes, RAID levels and number of hard drives in an array; and 2 TB may not be the magic number for a datastore size. Larger storage systems can provision a LUN as a datastore across a high number of disks and can be totally abstracted from these details, making the comfortable LUN size something smaller, such as 500 GB or even less.

Scott also mentions cautioned use of a configuration command to increase the queue length for a datastore from the default value of 32. I mentioned this value in a previous post (see On the Prowl for vSphere Performance Tweaks). Like Scott, I issue caution and testing to see if it make sense for each environment. The simple example is, if there will be a small number of datastores, this command makes perfect sense. But if the vSphere environment would be highly consolidated (i.e. 60 or more virtual machines per host) across a high number of datastores (i.e. 60 or more separate datastores), the risk of the microburst phenomena may be introduced. There is no defined threshold of virtual machines or datastores, and this example (60) isn't even that extreme any more.

My general practice is to put lightweight virtual machines in the 25 or less range per 2 TB datastore for lower tiers of storage. These workloads are the basic applications that we all have and hate; yet don't really require much in terms of throughput or storage requirements. I utilize all of the good stuff such as thin provisioning and would like to always keep headroom of about 40-50 percent free space on the datastore.

For the larger virtual machines, I usually go the datastore-per-VM or datastore-per-few-VMs approach on higher tiers of storage. I'll still utilize thin provisioning, but with a smaller amount of virtual machines per datastore the free space is kept at a minimum, as usually these applications have a more defined growth requirement.

I've always thought that capacity planning (and I'd lump this into a form of capacity planning) takes a certain amount of swagger and finesse. Further, this isn't always compiled of discreet information from application requirements, storage details and a clear roadmap for growth. But we all have that clearly spelled out for us, right?

Posted by Rick Vanover on 11/09/2010 at 12:48 PM


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