Costing Citrix PVS Solutions – Look Beyond NAS

We have been doing a lot of costing exercises lately to try to drive down our cost per user to provide VDI.  One of the biggest costs is of course on the Storage side.  The numbers can get really fascinating and compared to a fat VDI implementation the PVS options are pretty wide-ranging.  Interestingly the option that comes out the cheapest (for us anyway) is to use local SSD on the blades and put your write cache there.  The problem with that is of course that we end up with a static VM shell.  You can kiss DRS goodbye, which means anything that takes that blade out of service will take that capacity with it until you get the host back up.  In our typical N+1 planning scenario the +1 becomes fairly redundant unless you have live VMs there ready to take the load you lose.  ESX 5.1 brings live storage migration as part of DRS but then you are dragging those local write cache files back and forth across the network.  Not ideal to say the least.  The other big loss is you can no longer use the PVS wizard to provision easily.  Because the wizard relies on a local template in ESX, you would have to have a template per host and build them 1 host at a time.  It’s possible but painful.

Another really interesting choice is putting it all in RAM.  Although this is significantly cheaper than it used to be, it’s still not exactly cheap.  On a G8 HP blade for instance to support 120 users with a 2GB VM/2GB Write Cache you would need to put 32GB DIMMs in.  Those things are, at a minimum, 3x the cost of the 16GB DIMMs.  Ultimately it is still certainly cheaper than full fat NAS or SAN but on a cost per user basis it is 35% more expensive than local SSD (your pricing may vary!).  You do regain DRS and the ability to use the PVS wizard to create the shells, but you are a lot more limited on the write cache space.  You need to be very sure of your typical write cache size because running out will result in a blue screen.  On the plus side it is blazingly fast!

At the end of the day looking purely at CAPEX costs, local SSD far outstrips traditional NAS and RAM-based write cache.  You just have to be really sure you can live with the limitations.  OPEX costs become more interesting, since you have no spinning disks in SSD or RAM.  Your heat and power costs should drop significantly with both solutions.  We ran the same numbers for XenApp PVS as well, and came up with even more cost savings.  So before you get stuck buying more storage, take a look at your options!  It’s worth it.

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