Re: [Qemu-devel] [RFC]QEMU disk I/O limits

[Sasha Levin]

Hi,

On Mon, 2011-05-30 at 13:09 +0800, Zhi Yong Wu wrote:
> Hello, all,
> 
>     I have prepared to work on a feature called "Disk I/O limits" for 
> qemu-kvm projeect.
>     This feature will enable the user to cap disk I/O amount performed by a 
> VM.It is important for some storage resources to be shared among multi-VMs. 
> As you've known, if some of VMs are doing excessive disk I/O, they will hurt 
> the performance of other VMs.
> 
>     More detail is available here:
>     http://wiki.qemu.org/Features/DiskIOLimits
> 
>     1.) Why we need per-drive disk I/O limits 
>     As you've known, for linux, cgroup blkio-controller has supported I/O 
> throttling on block devices. More importantly, there is no single mechanism 
> for disk I/O throttling across all underlying storage types (image file, LVM, 
> NFS, Ceph) and for some types there is no way to throttle at all. 
> 
>     Disk I/O limits feature introduces QEMU block layer I/O limits together 
> with command-line and QMP interfaces for configuring limits. This allows I/O 
> limits to be imposed across all underlying storage types using a single 
> interface.
> 
>     2.) How disk I/O limits will be implemented
>     QEMU block layer will introduce a per-drive disk I/O request queue for 
> those disks whose "disk I/O limits" feature is enabled. It can control disk 
> I/O limits individually for each disk when multiple disks are attached to a 
> VM, and enable use cases like unlimited local disk access but shared storage 
> access with limits. 
>     In mutliple I/O threads scenario, when an application in a VM issues a 
> block I/O request, this request will be intercepted by QEMU block layer, then 
> it will calculate disk runtime I/O rate and determine if it has go beyond its 
> limits. If yes, this I/O request will enqueue to that introduced queue; 
> otherwise it will be serviced.
> 
>     3.) How the users enable and play with it
>     QEMU -drive option will be extended so that disk I/O limits can be 
> specified on its command line, such as -drive [iops=xxx,][throughput=xxx] or 
> -drive [iops_rd=xxx,][iops_wr=xxx,][throughput=xxx] etc. When this argument 
> is specified, it means that "disk I/O limits" feature is enabled for this 
> drive disk.
>     The feature will also provide users with the ability to change per-drive 
> disk I/O limits at runtime using QMP commands.

I'm wondering if you've considered adding a 'burst' parameter -
something which will not limit (or limit less) the io ops or the
throughput for the first 'x' ms in a given time window.

> Regards,
> 
> Zhiyong Wu
> 

-- 

Sasha.