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Thin Client and Virtual Machines Thread, Hyper-V Live Migration and CPU Allocation / Oversubscription in Technical; @sonofsanta: We went down the Xenserver route with 3 hosts 32GB Ram in each and worked on the theory of ...
  1. #16

    bossman's Avatar
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    @sonofsanta:

    We went down the Xenserver route with 3 hosts 32GB Ram in each and worked on the theory of utilising a third of the memory on each host so in the event of 2 hosts failing (Chances are slim) the remaining host could still deal with the load.

    I am working on getting our hosts up to 128GB Ram in each.

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    Arthur, although I haven't added a VM with to many processors recently (since SP1) I can say that when I tried to add a dual core VM to a host with 8 threads and already 6 virtual cores in use but loads of RAM free, I was met with the error that there was not enough resources available. I actually took a core from another VM on the host taking it to 5 of 8 used and then the VM could be made.

    This was an older server but the problem existed. Recently I also had a host fail overnight (my newest and best host) and the next day one server was down, there was plenty of RAM free on other servers but a lack of virtual cores and this server refused to start, infact it actuall moved itself (when starting with the cluster manager) to a server that had spare cores (even with though sufficient RAM was available on the host it was on) once I moved some VMs about.

    Maybe this is due to System Centre Virtual Machine Manager or maybe these limits are due to Failover Cluster Manager, and not actually the physical hardware limitation I don't know and I haven't looked into it as I have sufficient cores available anyway as long as I don't have VMs all over the place. I work with 3 older hosts and 3 new hosts, so it makes life a little bit trickier.

    I also before that this core problem is one of the major benefits of the AMD processors with physical 12 cores (no hyperthreading), as you get more cores for your money and as such they make a better host. All my servers are intel based though, so no experience with them.

  3. #18
    jamesfed's Avatar
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    16 core AMD Opterons....dual socket - 32 REAL cores how easy can it be - with 8:1 that is 256 VMs.

    To be honest we over subscribe our Hyper-V CPUs and as others have said CPU is rarely the bottleneck in virtualisation, most often its in RAM (where dynamic allocation and just having loads of the stuff comes in use) and in the disk IO (where SSDs come in use).

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    sonofsanta's Avatar
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    Quote Originally Posted by Arthur View Post
    The formula used to work out the maximum number of virtual processors your Hyper-V server supports is this...

    Code:
    (Number of processors) * (Number of cores) * (Number of threads per core) * 8
    Source: Hyper-V VM Density, VP:LP Ratio, Cores and Threads... - Windows Virtualization Team Blog - Site Home - TechNet Blogs

    Therefore, if you have a dual quad core Xeon with hyper-threading, you should be able to use upto 128 virtual processors per host.
    I don't think I'll get anywhere near that limit - I think there'll only be a half dozen VMs running to be honest - I'm just annoyed/surprised that the VM guest OS can only have 4 threads given over to it, seems a strange thing to limit.

  5. #20

    sonofsanta's Avatar
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    Quote Originally Posted by jamesfed View Post
    To be honest we over subscribe our Hyper-V CPUs and as others have said CPU is rarely the bottleneck in virtualisation, most often its in RAM (where dynamic allocation and just having loads of the stuff comes in use) and in the disk IO (where SSDs come in use).
    How do you allocate them then? As said before, I've got 24 threads total on each of two hosts, so 22 available to guests on each - if you allocate (say) 30 cores, what happens when one server falls over or is taken down for maintenance? What @Achandler is saying jives with what I've found on TechNet and around the web so far, and there's not much point in having a failover cluster if not all your VMs failover, I fear :/
    Last edited by sonofsanta; 3rd February 2012 at 09:07 AM. Reason: gramammar!

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    One of the things I woudl lvoe to see, if that you can specify unneeded machines in event of a failure, to allow oversubscribing resources better. I would like to specify that if I lost say 2 hosts or 3 hosts then shutdown all servers except the exchange server, main DC and fileserver for example. As this is the core of what I need to keep people working, they can not print, and have printers mapped locally if that was really necessary while work is completed.

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    garethedmondson's Avatar
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    Quote Originally Posted by Arthur View Post
    The formula used to work out the maximum number of virtual processors your Hyper-V server supports is this...

    Code:
    (Number of processors) * (Number of cores) * (Number of threads per core) * 8
    Source: Hyper-V VM Density, VP:LP Ratio, Cores and Threads... - Windows Virtualization Team Blog - Site Home - TechNet Blogs

    Therefore, if you have a dual quad core Xeon with hyper-threading, you should be able to use upto 128 virtual processors per host.
    So if we have E5645 chip with 'IPMI X2 6 Core E5645 2.4Ghz Xeon CPU's' then it would be:

    2 x 6 x 12 x 8 = 1152 virtual processors? Seems a lot. That would also be *2 because we have two servers.

    Or am I missing the point.

    GJE

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    As I have stated earlier, I don't believe Microsoft at all, one of my Virtual Machines Properties, it has 4 Virtual Processors assigned, and clearly states it is using 25% of the machines resources. This machine has 8 cores and 16 threads, so 25% would be right.

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