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Old 07-29-2008, 10:47 AM   #1
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Virtualisation with KVM really bad performance..

Hi everyone,

I have just configured 2 virtual machines using KVM. Those are the commands I used:
kvm -cdrom ../liveCD.iso -boot d -net nic -net socket,listen=:8010 /dev/sda11
kvm -cdrom ../liveCD.iso -m 1024 -boot d -net nic -net socket,connect=:8010 /dev/sda12

after that I built a local network between the machines and transferred a file using FTP. The transfer speed was 270kb/s! Can anyone tell me why it is so slow? My hardware configuration is: Intel Core 2 QUAD 2.6ghz with 2gb of ram and an SATA hard drive. As I copy a file between the virtual machines I am actually copying it from one hard drive partition to another(wright?). It shouldn't be so slow. I also tried to copy a file (using FTP again) from the local host on one of the virtual machines(practically downloading the file from my self) the speed was around 6mb/s which is acceptable. But if I transfer something between both machines it is really slow... Can someone tell me if I am doing something wrong? Or if you have any idea why is the speed not good. Or maybe KVM is not the wright Virtualisation Software for me? Do you know which Virtualisation software will give me the best transfer speed in that case?
ok, that's it for now. I appreciate any help..
thanks in advance.
Old 07-30-2008, 05:05 AM   #2
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Unluckily I didn't have yet a look at the network transfer speed inside a kvm-vm, but:

Perhaps kvm is running in full emulation without any help from the linux kernel?
I run from time to time WindowsXP on my notebook using kvm, and when I forget to first load the kernel modules of kvm (kvm_intel or kvm_amd, depending on your machine), the OS uses a lot of CPU and is very very slow.
Old 08-25-2008, 08:36 PM   #3
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RE: Virtualisation with KVM really bad performance..

One of the gotchas here is that although newer machines have Virtualisation hardware, in Intels this is turned off by default. If you can load the kernel module (kvm-intel) then you have the virtualisation hardware turned on.
Old 08-26-2008, 06:27 PM   #4
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well, it appears that I have loaded the modules correctly(kvm_intell is loaded). So that is not the problem.... After checking that, I heard that there is a special network driver in kvm, for the virtual machines. So I recompiled the kernel of the VMs, so that they support that virtual network driver, and tried again... Unfortunately once again the speed was very low... a little bit better than before, but still really slow.. so I gave up testing kvm and I think that my older solution(xen) is still better than kvm. Maybe they still have issues to solve in kvm. And maybe in time they will get much better.. I don't know.. Anyway, if anyone still has an idea of, what I might be doing wrong, I would appreciate your opinion...

Old 08-27-2008, 02:53 AM   #5
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kvm relies heavily on the VT extension, if you don't have it enabled in the BIOS it will run really slow.


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