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Determining storage requirements
To properly design a VDI infrastructure you need to understand the resource requirements needed by virtual desktop users. Don't make assumptions; to properly calculate resource requirements you need actual statistics from the users whose desktops will be virtualized. Profiling the users and measuring their resource usage is the key to determining storage requirements. Products from vendors like Lakeside Software Inc. and Liquidware Labs Inc. can collect data from users' desktops so you can perform an assessment of your environment and determine your needs. The longer you collect data, the less likely it will be affected by unusual or periodic activities.
The key measurement for storage is IOPS. A number of factors can affect IOPS (caching, block size), but the base calculation is derived from hard drive mechanics: rotational speed (rpm), latency and seek time. A typical 7,200 rpm drive might be capable of 75 IOPS, a 10K drive 125 IOPS, a 15K drive 175 IOPS and a solid-state drive 5,000 IOPS. Spread across a RAID group, you can multiply the number of drives in the RAID group times the IOPS of the drive to get the total IOPS the RAID group is capable of (e.g., six 15K drives x 175 IOPS = 1,050 IOPS). There are other factors, such as caching, that can increase IOPS, while RAID overhead and latency in network storage protocols can decrease it.
You should always measure actual user resource
|Click here to get a PDF of the Accepted user resource averages.|
Don't design your VDI storage to handle just average I/O loads; it has to accommodate peak I/O loads to provide a good user experience. Having enough storage capacity is obviously important, but how it performs is more important. Because the number of spindles plays a big part in a storage array's performance, you may end up with more capacity than you need just to get the required IOPS.
Fibre Channel vs. iSCSI vs. NAS
The type of storage is often dictated by budgets and available existing storage infrastructure. A Fibre Channel (FC) SAN would provide ample performance, but acquiring it may make VDI too expensive to implement. iSCSI and NAS (NFS) are attractive alternatives, but you need to ensure they can meet I/O requirements. Using 10 Gb Ethernet (10 GbE) can dramatically increase the throughput to iSCSI and NAS devices, but if you haven't implemented 10 GbE yet it could be just as expensive as implementing FC.
Peak IOPS loads may exceed the number of IOPS an iSCSI or NAS (NFS) device can handle. But adding cache or an accelerator in front of the storage device may improve performance sufficiently. Both iSCSI and NFS add CPU overhead to the host server; for iSCSI this can be offset with hardware initiators. Accelerator solutions typically won't work with NAS, but there are other caching solutions available for NAS (NFS).
This was first published in March 2011