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

[Sasha Levin <levinsasha928@gmail.com>]

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 <levinsasha928@gmail.com>
> >To: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
> >Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, kwolf@redhat.com,
> >	aliguori@us.ibm.com, herbert@gondor.apana.org.au,
> >	guijianfeng@cn.fujitsu.com, wuzhy@cn.ibm.com, luowenj@cn.ibm.com,
> >	zhanx@cn.ibm.com, zhaoyang@cn.ibm.com, llim@redhat.com,
> >	raharper@us.ibm.com, vgoyal@redhat.com, stefanha@linux.vnet.ibm.com
> >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.