How many VMs should you put on a LUN?

Rob McShinsky

With the best logical unit number (LUN) configuration for your infrastructure, you can improve virtual machine (VM) performance. Keep in mind disk types, I/O optimization, hardware, RAID level and more as you configure LUNs.

But the number of VMs you put on those LUNs depends on the size of your infrastructure and whether your environment is for testing and development.

Large number of VMs in production implementations
In medium or large infrastructures, with anywhere from 100 to 1,000 VMs, there are a few ways to glean the best virtual machine performance. In most cases, you can get away with a RAID 5 configuration if you have a SAN with two to four controllers, a larger disk cache and a 10 Gb iSCSI or at least a 4 Gb Fibre Channel transport. This VM storage strategy has proven to provide a good balance of virtual machine performance and cost for production workloads.

In my own infrastructure, which uses 4 Gb Fibre Channel to a HP EVA 8400 SAN with 300 GB 15K SCSI drives, I can get 20 to 25 VMs per 1 TB RAID 5 LUN. The VMs range in size and I/O resource demands, but in general they have one or two processors, 2 GB to 4 GB of RAM and between 25 and 60 GB of virtual disk.

As the number of VMs grows, the LUN configuration tends to scale out to many LUNs on multiple SANs. That improves virtual machine performance and provides greater resiliency.

Small number of VMs or test/development environments
Small or test environments, with fewer than 100 VMs and low I/O utilization, are where many admins tend to struggle with LUN configuration. Many fledgling virtual infrastructures grow faster than anticipated, or admins have unrealistic I/O performance expectations. 

That said, any VM storage method -- local storage, a lower-end direct-attached SCSI or even an iSCSI SAN with SATA drives -- can work well. In small infrastructures, where controller CPUs and disk caches have lower resource usage, look to RAID 10 to provide an extra boost in virtual machine performance.

The ideal number of VMs for a LUN configuration in smaller infrastructures varies greatly. Anywhere from four to 15 VMs per LUN is possible, but if you assign more than your infrastructure can handle, disk I/O could be saturated quickly.

It’s also important to appropriately size your host resources to meet the expected performance of your VM storage subsystem. If you buy an excessive number of CPU cores or large amounts of memory, for instance, these resources will go to waste because disk I/O will be exhausted well before the others.

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