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

[Sasha Levin]

On Thu, 2011-06-02 at 14:29 +0800, Zhi Yong Wu wrote:
> On Thu, Jun 02, 2011 at 09:17:06AM +0300, Sasha Levin wrote:
> >Date: Thu, 02 Jun 2011 09:17:06 +0300
> >From: Sasha Levin <address@hidden>
> >To: Zhi Yong Wu <address@hidden>
> >Cc: address@hidden, address@hidden, address@hidden,
> >     address@hidden, address@hidden,
> >     address@hidden, address@hidden, address@hidden,
> >     address@hidden, address@hidden, address@hidden,
> >     address@hidden, address@hidden, address@hidden
> >Subject: Re: [Qemu-devel] [RFC]QEMU disk I/O limits
> >X-Mailer: Evolution 2.32.2 
> >
> >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.
> Currently no, Do you let us know what scenario it will make sense to?

My assumption is that most guests are not doing constant disk I/O
access. Instead, the operations are usually short and happen on small
scale (relatively small amount of bytes accessed).

For example: Multiple table DB lookup, serving a website, file servers.

Basically, if I need to do a DB lookup which needs 50MB of data from a
disk which is limited to 10MB/s, I'd rather let it burst for 1 second
and complete the lookup faster instead of having it read data for 5
seconds.

If the guest now starts running multiple lookups one after the other,
thats when I would like to limit.

> Regards,
> 
> Zhiyong Wu
> >
> >> Regards,
> >> 
> >> Zhiyong Wu
> >> 
> >
> >-- 
> >
> >Sasha.
> >

-- 

Sasha.