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CPU and Disk "overselling" (not really) in Xen?

shovenoseshovenose Member, Host Rep
edited March 2013 in General

Two things I need to determine:
1. How does CPU work in Xen? Does each VPS get all of one core? so let's say I have 8 cores on the node, I can only fit 4 VPS if they have 2 cores each? Or can I share them? Also how do I give higher VPS plans more priority?
2. It is such a waste of disk space to allocate a lot of disk, that is not going to be used. I'm tempted just to use OpenVZ because of this. I am assuming due to how Xen creates a virtual disk for every VPS it can not be oversold? But OpenVZ has a poor reputation :(

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Comments

  • SpencerSpencer Member
    edited March 2013

    You can oversell HDD on Xen, you just have to know how ;)

    Thanked by 1Maxsam4
  • texteditortexteditor Member

    Every virtual machine gets 1 cpu, the other 4 would be wasted. 8 cores means you can have a max of 8 VMs with no leftover CPUs

  • Dragoon0309Dragoon0309 Member
    edited March 2013

    http://bit.ly/Wn0Eb5

    Anyway, I'm pretty sure it has something to do with balloons. Maybe the more helium the more you can fit? Idk.

  • SpencerSpencer Member

    @texteditor said: Every virtual machine gets 1 cpu, the other 4 would be wasted. 8 cores means you can have a max of 8 VMs with no leftover CPUs

    That is why you don't really find XEN VPS anymore :(

  • nickvanwnickvanw Member

    Yeah it's one core per VM, that's why Xen is usually better, but more expensive. It's probably best if you get something cheap like a dual L5420 because then you can fit 8 xen vms on a cheap server.

  • Dragoon0309Dragoon0309 Member

    @Spencer I know right! I think that's why everyone went KVM, as it lets you use hyperthreaded cores as real cores, so you can fit 16 to a server instead of 8

  • dmmcintyre3dmmcintyre3 Member

    @Dragoon0309 said: I know right! I think that's why everyone went KVM, as it lets you use hyperthreaded cores as real cores, so you can fit 16 to a server instead of 8

    you don't have to dedicate a core to a KVM VPS.

  • shovenoseshovenose Member, Host Rep

    Wait, so I can't share cores? Really?

  • texteditortexteditor Member

    Kinda if you have hyperthreading support, but it's not 100% stable if I remember

  • SpencerSpencer Member

    @shovenose said: Wait, so I can't share cores? Really?

    Yes...
    That is why everybody is doing KVM now

  • Dragoon0309Dragoon0309 Member

    @shovenose Nope! KVM let's you use hyperthreaded cores though. That's why the LEB market is small for xen and at least a bit bigger for KVM.

  • bitronictechbitronictech Member
    edited March 2013

    I haven't really run into this issue with Xen. I have 16 VMs running on a box with 8 cores although it does have 16 threads. Also the Dom0 with 0 problems under Xen.

  • shovenoseshovenose Member, Host Rep

    KVM you are still stuck with one per core right?

  • shovenoseshovenose Member, Host Rep

    @bitronictech how do you give them a certain amount of clock speed rather than cores? Can I give each person like 1GHz and if the server is 3GHz Quad Core with HyperThreading I could put 24 VPS?

  • Dragoon0309Dragoon0309 Member
    edited March 2013

    @Shovenose yes, but you can use hyperthreaded cores as a core.

    Also idk how @bitronictech does it, he may be thinking of Xen HVM.

    But you cant assign clock speed per vps.

  • Awmusic12635Awmusic12635 Member, Host Rep

    Quad AMDs = 64 cores = 64 VMs, enjoy

  • bitronictechbitronictech Member

    CPU cap :) but it only works on KVM.

  • shovenoseshovenose Member, Host Rep

    How does BurstNET make a profit?
    http://burst.net/xenvps.shtml

  • Awmusic12635Awmusic12635 Member, Host Rep

    @shovenose Large company

  • Dragoon0309Dragoon0309 Member

    @shovenose using really old hardware that they already own. They also own the datacenter and land and everything. Stuff is cheaper when you own it and don't have to rent!

  • SpencerSpencer Member

    @shovenose said: How does BurstNET make a profit?

    And AFAIK a lot of their nodes are those like AMD 64 core ones

  • Awmusic12635Awmusic12635 Member, Host Rep

    Quad AMD 6274 to be precise

  • dmmcintyre3dmmcintyre3 Member
    edited March 2013 
    virsh # nodeinfo  
CPU model:           x86_64
CPU(s):              4
CPU frequency:       3400 MHz
CPU socket(s):       1
Core(s) per socket:  4
Thread(s) per core:  1
NUMA cell(s):        1
Memory size:         32851016 KiB

virsh # list
 Id    Name                           State
----------------------------------------------------
 1     kvm117                         running
 2     kvm139                         running
 5     kvm138                         running
 7     kvm124                         running
 8     kvm141                         running
 9     kvm118                         running
 10    kvm119                         running
 13    kvm132                         running
 14    kvm140                         running
 15    kvm127                         running
 16    kvm125                         running
 18    kvm152                         running
 20    kvm150                         running
 21    kvm143                         running
 22    kvm135                         running
 23    kvm122                         running
 24    kvm112                         running
 26    kvm148                         running
 27    kvm153                         running
 29    kvm154                         running
 33    kvm149                         running
 34    kvm130                         running
 37    kvm151                         running
 56    kvm144                         running
 58    kvm123                         running
 62    kvm146                         running
 64    kvm147                         running
 74    kvm142                         running
 78    kvm155                         running
 83    kvm156                         running
 87    kvm109                         running
 89    kvm136                         running
 91    kvm158                         running

    A lot more than 1 KVM VPS per core running on one of my KVM nodes. Most of the VMs have 4 cores assigned too.

  • shovenoseshovenose Member, Host Rep

    Hmm, @dmmcintyre3 how is that possible? Do you think it is easy or hard to set up KVM node? Also can I oversell disk, RAM, etc. or not?

  • dmmcintyre3dmmcintyre3 Member

    RAM can be oversold, but most VMs will use almost all of what they are assigned. KSM can help, but not much. Disk can't be oversold easily (SolusVM uses LVM). I didn't do anything special to put more than 1 VM on each core. I just made the VMs from SolusVM.

  • Dragoon0309Dragoon0309 Member

    @shovenose @dmmcintyre3 he must be joking because its not possible or its not KVM. You can't oversell memory (that's why you said they take up almost all what they are assigned.) CPU's can't be oversold either. Its just how it works.

  • krokro Member

    You can oversell all resources hdd cpu ram on kvm vz xen. Stop being silly.

    with xen, you can share cores like normal. Vcpu/cpu/cpus look @ xen docs bro.

    This is a mean dump too, wipe time

  • yomeroyomero Member

    You can't share cores in Xen??? Then how the xen providers like Inception @AnthonySmith can do that pricing scheme? Sounds weird

  • OttoYiuOttoYiu Member
    edited March 2013

    @Dragoon0309 said: @shovenose @dmmcintyre3 he must be joking because its not possible or its not KVM. You can't oversell memory (that's why you said they take up almost all what they are assigned.) CPU's can't be oversold either. Its just how it works.

    What you said is incorrect; The cores are virtualized on Xen, and on KVM, so it is possible to assign each VM with virtual cores up to the number of physical cores available on the node (eg. you can have 10 VMs with 8 virtual cores each, if you are running on an 8 core CPU). Due to the scheduling overhead, you will most likely take a performance hit for assigning too many virtual cores.

  • mpkossenmpkossen Member

    @Dragoon0309 said: @shovenose @dmmcintyre3 he must be joking because its not possible or its not KVM. You can't oversell memory (that's why you said they take up almost all what they are assigned.) CPU's can't be oversold either. Its just how it works.

    You can oversell memory on KVM. I can't remember how it was done, but I remember an example with an SSD for swap and just overload the node.

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