Monday, August 8, 2011

VDI and storage; can't find one without the other

During a recent design engagement building a scalable VDI environment, storage quickly became a core consideration. Those of you who have designed and operated large VDI environments know that storage I/O can easily cause performance issues if not properly planned for. There were a few interesting things we learned as we worked with different SAN vendors to ensure their solution addressed some of the unique characteristics of our VDI workload.

What is unusual about virtual desktop environments is two very different disk I/O conditions; operational and burst I/O. Burst I/O is more common in VDI environments because operational requirements necessitate large reboots of desktop operating systems not typical in virtual server environments. Operational I/O can also be problematic if things such as virus scanning activities are synchronized based on time vs. randomized to reduce the performance hit on the VMs.

Some storage vendors have a very utilitarian view of storage services; they do not view virtual desktop workloads as any more unique then other virtual workloads. The limitation with SAN vendors who do not differentiate between server and desktop virtualization environments is that in order to guarantee good throughput you may have to consider their enterprise class storage systems to ensure good performance. This can be a tough sell hosting virtual desktop operating systems.

Others storage vendors provide mid-tier solutions and provide Solid State Drives in order to deal with burst I/O. While better, it still requires you to adjust your design so that high I/O requirements are segregated onto volumes made up of SSD. This leads to a very static design where you may or may not make good use of high performance drives.

Most recently we have seen storage vendors start to build mid-tier storage systems that have some of the features of enterprise class systems such as Dynamic Tiering. Dynamic Tiering is the ability to move hot or data that is in demand to high performance drives so that the SAN delivers great performance. This can typically be done on the fly or scheduled to happen periodically during the day. These solutions are ideal for Virtual Desktop environments as they do not require the premium of enterprise class storage systems but still deliver the features. EMC has clearly targeted the VNX line to provide features that make them ideally suited for virtual workloads. Of course companies such as NetApp have been using Programmable Acceleration Modules (PAM) cards for years to deal with burst I/O. Whichever solution you select here are a few general considerations for putting together your design.

1. Each SAN vendor has very different numbers when est. I/O’s for virtual/virtual desktop workloads. It is best to have your own reference numbers based on internal testing. Use these to make sure the estimates provided meet your requirements.

2. Burst I/O and Operational I/O are treated distinctly by most storage vendors. For example if your numbers est. that your environment may generate 15,000 burst I/O’s and required 4 TB of storage the vendor may suggest 6 X SSD drives (6 x 2500 IOs each = 15K burst, excluding RAID considerations) and SAS drives to meet your storage/operational requirements.

3. Ensure that your virtual desktop design incorporates the SAN environment. A good design should provide consistent performance over the lifetime of the solution (typically 3 years). This is not possible if you build a great VDI design that does not set specific requirements for storage. While your VDI environment may run great during the first year you may see high SAN utilization lead to problems over time.

4. Separate your expected read and write I/Os. Take the number of writes and ensure you factor the number by 4 to allow for an I/O penalty on writes. For example if you expect 2000 Reads and 2000 writes, multiply the writes X 4 for a total of 10,000 expected I/Os (2000 Read I/O + 8000 Write I/O) .

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