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I've been asked for an opinion about a purchase of a blade server that should be used for running virtual machines. I have very little experience in this, so what things should be considered in terms of system specs?

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The answers below are good, but to get better answers you might want to post what virtual machines you plan to run. Building a host to support 4 virtual machines that are used for software development testing, is way different than building a host to support 4 virtual machines made up of exchange, sql, sharepoint, etc. One requires very little disk IOPS consideration, the other it is very important, etc... –  Aaron May 1 '09 at 12:45
    
@Aaron: We're going to use it to set up virtual machines mainly for software development (both Windows and Linux, having version control and issue tracking) and whenever we need a new temporary machine just to test things out. I kept the question general enough for it to be useful for others who are in the same situation. So the more answers with more considerations, the better. –  Spoike May 1 '09 at 20:11

5 Answers 5

up vote 4 down vote accepted

I agree with Bravax's points. In addition to that I would pay attention to the following things:

  • Server CPU should support VT technology
  • 64 Bit is key for a virtual server (you want a lot of memory)
  • Fast and good quality network cards
  • Ram, Ram, Ram
  • A good quality raid controller and some fast (at least 7200rpm) hard disks (I prefer a striped RAID set on my virtual servers)

Also, think about your backup strategy in advance. If you decide to backup your complete virtual machines rather then running backups "in" your virtual machines during the night you will need the appropriate backup space.

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Pick a manufacturer (I like HP and IBM over just about anybody else but your mileage may vary), speak to their sales engineers, they're not too busy right now, tell them what you want, then post back their recommendations on here and we'll strip away all the unrequired kit they think you need :)

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  • The ability to run the virtualization software. (Some hardware can't, or isn't supported.).
  • Whether you can increase the amount of Memory, CPUs and storage easily.
  • What the network and network card configuration will be, as this needs to support the VM's

I would suggest 1 dedicated core per VM, and you may want more.

Some of the calculations depend on the virtualization software you're using too.
A rough calculation you can do is spec each of the VMs as normal servers, and multiply their requirements by 1.5, so you get some leeway for new VMs, as well as flexibility.

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I agree with Flo's points and the key is what are you using the VMs for.

The major problem we have with running ESX and ESXi on blades is storage capacity and disk i/o. Most blades can support two local HDD and have RAID1 or RAID0. Depending on your disk space and i/o requirements a blade may not be the best choice. If you already have a SAN in place then these two points aren't valid, but if your budget doesn't allow for that then consider a traditional rackmount or tower type server where you can add RAID cards and gobs of disk for cheap.

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IO is quite often the major bottleneck for VMs, especially when you don't have the money to run a SAN or other high performance array. If you're looking at a single setup, you'll probably end up with RAID0,1 or 10 (depending on your needs). The particular issue you run into is typically not pure throughput limits, but rather the number of IO operations/sec that the array can process (IOPS).

Before you move machines to a VM, it's very much worth monitoring your machines before and seeing what their base IOPS rates are. It's very easy to create a VM setup where you have plenty of RAM and tons of unused CPU, but slow response due to overloaded drives.

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