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    SubjectRe: Support for global block IO throttling?
    >> Hi Vivek,
    >> >> Are there any plans to support global (per cgroup) IO throttling in
    >> >> the near future? I'm in need of this and would gladly work on it if
    >> >> nobody else is and if people think this is a useful feature. What I
    >> >> envision is that one can specify the limits of individual devices in a
    >> >> group but also specify the overall aggregate limit of the whole group.
    >> >> I just started looking at the throttling code and haven't quite
    >> >> figured it out yet so it might take me a bit to get up to speed.
    >> >
    >> > Hi Juerg,
    >> >
    >> > What's the use case? Why do you need global block IO throttling?
    >> Resource management for guest VMs. I want to make sure a VM cannot
    >> exceed a certain IO limit. If the VM does IOs to only one device, it
    >> gets the full specified bandwidth to that single device but if the VM
    >> talks to multiple devices, it only gets a fraction of the global
    >> bandwidth per device, not exceeded the global limit.
    > Can't we specify per device limit for VM if VM is doing IO to multiple
    > devices?

    Yes we can but it's not exact. As I mentioned, the way I want to
    specify it is so that the VMs alloted bandwidth is shared across all
    the drives presented to that VM.

    > I know it is not exact replacement of global limits as one has to know
    > the topology and figure out to what all devices IO is happening. This
    > might get even more difficult if VM's disk is a file on filesystem on
    > host and not a device directly eported to guest.
    > Somebody also had mentioned that if we have mutiple layers of qemu
    > snapshots and if snapshots are sitting on different logical devices
    > then it becomes hard to figure out where all VM traffic is going
    > and how to even divide global limits in per device limits
    > Also current IO controller does not work for NFS. So if a global limit
    > is imposed in higher layers it will solve that issue too.
    > Also if you implement it above page cache then it should automatically
    > handle the case of async writes too. The downside is that if limits
    > are low, buffered writers will be throttled very frequently.
    > So in general, I think global limits is a useful concept. Do you have
    > a implementation ready?

    Err... No... I was hoping the community can give me some pointers and
    infos to get me started. Just like the details you mentioned above
    which are very useful.


    > Thanks
    > Vivek

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