Advanced virtual machine (VM) storage options can improve performance, but their benefits will go only so far if your physical logical unit numbers (LUNs) are not configured with best practices in mind.
Only when a LUN configuration meets the needs of your VM workloads can you significantly improve virtual machine performance. When it comes to LUN configuration, hardware choices, I/O optimization and VM placement are all important considerations.
Hardware and LUN configuration
The hardware on which you house your LUNs can make all the difference in VM performance. To avoid an overtaxed disk subsystem, choose hardware with similar resource levels as your host systems. It does no good to design a cluster of servers with two six-core processors and 128 GB of RAM and attach it to an iSCSI Serial-Attached Technology Advancement (SATA) storage area network (SAN) over a 1 GB link. That arrangement can create a storage bottleneck at either the transport or disk-latency level.
As you set up the LUN configuration, correctly sizing your disk subsystem is the key to ensuring acceptable performance. Going cheaper on one component may save you money up front; but if a resulting bottleneck reduces overall VM storage capacity or stability, it could ultimately cost you much more.
Disk type
To improve virtual machine performance, choose disk types for VM storage based on workload. Lower speed, lower duty cycle and higher latency drives such as SATA/FATA may be good for development environments. These drives usually range from 7,200 RPM to 10,000 RPM. For production workloads, or those with low latency needs, various SCSI/SAS alternatives give a good balance of VM performance, cost and resiliency. These drives range from 10,000 RPM to 15,000 RPM.
Solid-state drives are also a realistic option. For most workloads, these kinds of drives may be overkill technically and financially, but they provide low latency I/O response.
I/O optimization
To ensure a stable and consistent I/O response, maximize the number of VM storage disks available. You can maximize the disk number in your LUN configuration whether you use local disks or SAN-based (iSCSI or Fibre Channel) disks. This strategy enables you to spread disk reads and writes across multiple disks at once, which reduces the strain on a smaller number of drives and allows for greater throughput and response times. Controller and transport speeds affect VM performance, but maximizing the number of disks allows for faster reads and resource-intensive writes.
RAID level
The RAID level you choose for your LUN configuration can further optimize VM performance. But there’s a cost-vs.-functionality component to consider. RAID 0+1 and 1+0 will give you the best virtual machine performance but will come at a higher cost, because they utilize only 50% of all allocated disks.
RAID 5 will give you more gigabytes per dollar, but it requires you to write parity bits across drives. On large SANs, any VM performance drawback will often go unnoticed because of powerful controllers and large cache sizes. But in less-powerful SANs or on local VM storage, this resource deficit with RAID 5 can create a bottleneck.
Still, on many modern SANs, you can change RAID levels for a particular LUN configuration. This capability is a great fallback if you’ve over-spec’d or under-spec’d the performance levels your VMs require.
Transport
Whether the connectivity between host servers and LUNs is local, iSCSI or Fibre Channel, it can create resource contention. The specific protocol determines how quickly data can traverse between the host and disk subsystem. Fibre Channel and iSCSI are the most common transports used in virtual infrastructures, but even within these designations there are different classes, for example 1/10 Gb iSCSI and 4/8 Gb Fibre Channel.
Thin provisioning
Thin provisioning technologies do not necessarily increase virtual machine performance, but they allow for more efficient use of SANs, because only the data on each LUN counts toward total utilization. This method treats total disk space as a pool that’s available to all LUNs, allowing for greater space utilization on the SAN. With greater utilization comes greater cost savings.
Block-level deduplication
Block-level deduplication is still an emerging technology among most mainstream SAN vendors. Again, this technology does not improve virtual machine performance through the LUN configuration, but it does allow data to be stored only once on the physical disk. That means large virtual infrastructures can save many terabytes of data because of similarities in VM workloads and the amount of blank space inherent with fixed-size virtual hard disks.
Number of VMs on a LUN
With the best 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.
So what does this all mean? For optimum VM performance and cost savings, use a healthy combination of the previously mentioned options. Using the best possible resources and LUN configuration is ideal, but it’s not practical or necessary for the majority of virtual infrastructures.
