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I recently installed a new Debian Wheezy server with KVM (the "KVM Host") then installed a Debian VM on the KVM (the "VM"), and got very bad performance on the VM. Looking through dmesg on the VM, I noticed lot's of messages like:

task foo:1234 blocked for more than 120 seconds

(even for tasks like scp, apt-get).

After much Googling and reading, I decided I would first experiment with changing the kernel task scheduler on the VM, but I couldn't work out how to do that. So my first question:

  • how do you change the kernel task scheduler on Linux? Is it via some setting in /sys/kernel, compiling a custom kernel, or some other way?

My KVM Host filesystem is ext4 on top of md RAID1 (mirroring). Reading suggested maybe my problems are caused by ext4/mirroring. So my second question:

  • any suggestions or guides on recommended ways to setup KVM?

I couldn't find anything useful on the KVM website. A Youtube search on "kvm performance" produced lots of videos, but all at the ultra-geek level, nothing basic. The KVM doco has a couple of books in German, but my German consists of "nein" and "ja" from Shultz in Hogan's Heroes :-)

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Why don't you post the VM configuration details, instead of jumping to conclusions? –  dyasny Oct 24 '13 at 19:52

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