RAID Cache and VDI

For the past few days I’ve been doing a little performance testing to see just how much an influence RAID cache makes when provisioning Virtual Desktops.

The test was to create 20 virtual desktops and see how long it takes to get the first one spun up and ready for use and then to see how long it took to get all of the desktops ready for use.

The tests were conducted using-

Citrix XenServer
Citrix VDI-in-a-Box
HP DL385 G7 Server
8 core 2.0ghz HE AMD Opteron Processor
44GB DDR3 RAM
4x15k SFF 72GB SAS 6Gbs HDDs
HP P410i RAID Controller (1Gb cache)

the results (all times in mins and seconds) were-

So why did we get these results?

The RAID Cache/Accelerator allows frequently used information from the hard drives to be temporarily stored (under flash backup) in DDR2 RAM (HP currently supports 1GB for the P410 and 2GB for the new P420 controllers).

There is a strong case for caching in VDI as all of the desktops are very similar and so share many common files and as such the cache can be used to obtain a significant performance boost.

RAID0 with cache is the best option for speed as there is no need to calculate/store the backup data on separate drives as with RAID5.

In direct comparison between RAID5 with 512mb assigned to read and 512mb assigned to write and RAID5 with the cache module disabled it is clear that the system without cache is performing very very very badly as all the data is being taken right off the hard drives.

So what should I do?

If you are going to be running a VDI setup which uses local drives (instead of a SAN) just as Citrix VDI-in-a-Box does then you will be best off including some RAID cache in there – as much as possible in fact!

Most of the HP DL3xx servers either don’t come with any cache or a very small amount (256mb) but that is no problem as a 1GB flash module (HP PN 534562-B21) goes for about £200-£250 from most suppliers (link to BT Business Direct).