Re: [Qemu-devel] [PATCH v5 3/4] block: add block timer and block throttling algorithm

[Zhi Yong Wu]

On Fri, Aug 12, 2011 at 1:06 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Fri, Aug 12, 2011 at 6:00 AM, Zhi Yong Wu <address@hidden> wrote:
>> On Wed, Aug 10, 2011 at 7:00 PM, Stefan Hajnoczi <address@hidden> wrote:
>>> On Wed, Aug 10, 2011 at 7:57 AM, Zhi Yong Wu <address@hidden> wrote:
>>>> On Tue, Aug 9, 2011 at 11:19 PM, Stefan Hajnoczi <address@hidden> wrote:
>>>>> On Tue, Aug 9, 2011 at 5:17 AM, Zhi Yong Wu <address@hidden> wrote:
>>>>>> Note:
>>>>>>      1.) When bps/iops limits are specified to a small value such as 511 
>>>>>> bytes/s, this VM will hang up. We are considering how to handle this 
>>>>>> senario.
>>>>>
>>>>> If an I/O request is larger than the limit itself then I think we
>>>>> should let it through with a warning that the limit is too low.  This
>>>> If it will print a waring, it seems to be not a nice way, the
>>>> throtting function will take no effect on this scenario.
>>>
>>> Setting the limit below the max request size is a misconfiguration.
>>> It's a problem that the user needs to be aware of and fix, so a
>>> warning is appropriate.  Unfortunately the max request size is not
>>> easy to determine from the block layer and may vary between guest
>>> OSes, that's why we need to do something at runtime.
>>>
>>> We cannot leave this case unhandled because it results in the guest
>>> timing out I/O without any information about the problem - that makes
>>> it hard to troubleshoot for the user.  Any other ideas on how to
>>> handle this case?
>> Can we constrain the io limits specified by the user? When the limits
>> is smaller than some value such as 1MB/s, one error is provide to the
>> user and remind he/her that the limits is too small.
>
> It would be an arbitrary limit because the maximum request size
> depends on the storage interface or on the host block device.  Guest
> OSes may limit the max request size further.  There is no single
> constant that we can choose ahead of time.  Any constant value risks
> not allowing the user to set a small valid limit or being less than
> the max request size and therefore causing I/O to stall again.
Let us think at first. When anyone of us has some better idea, we will
rediscuss this.:)
>
> Stefan
>



-- 
Regards,

Zhi Yong Wu