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I use KVM to manage my virtual machines. I am trying to limit the resources provided for VMs. I am able to limit the CPU and memory allocated for VMs using libvirt/cgroups. Now I want to control the disk time allotted for each VM by applying some weights. I looked at blkio control from cgroups. Since VM is just a Linux process, I will be able to use cgroups but I am not sure whether it will work for asynchronous IO too. If not, can someone suggest a good alternative solution?

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Blkio in cgroup terminology stands for access to I/O on block devices. It does not seem to be about regulating all the different ways software developers have at hand for I/O-related purposes.

It seems to be targeted mainly to I/O on devices, not on the way software has access to devices. It can limit the number of iops, the bandwidth or a weight with other processes, in other things. It seems that buffered write is not supported by blockio at the moment. It's in the official documentation:

Currently, the Block I/O subsystem does not work for buffered write operations. It is primarily targeted at direct I/O, although it works for buffered read operations.

If you take a look at this presentation from Linda Wang and Bob Kozdemba of Red Hat, on page 20+, you'll see that the graph is about the device bandwidth per VM, not about random vs blocking vs asynchronous I/O.

It seems there has been recent work by Red Hat to implement it directly into virsh. It has been released last week in libvirt 0.9.9. In a few months, you'll be able to do something like this in your favorite distribution:

virsh blkiotune domA --device-weights /dev/sda,250 virsh blkiotune domB --device-weights /dev/sda,750
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You may want to use ionice, it's like nice but for IO.

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    Is ionice a better alternative than using blkio from cgroups? Does it provide something the OP's current solution does not? Do you know how it relates to asynchronous IO? Commented Aug 4, 2011 at 11:45
  • Caleb it will be great if you can just give a gist on what is asynchronous IO and how does it differ from synchronous IO with respect to an operating system. Commented Aug 5, 2011 at 20:33
  • @Caleb: See above. Commented Dec 9, 2011 at 9:23

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