Setting disk parameters in a VM

Setting disk parameters in a VM

[Bill Davidsen]

Based on a discussion in another list on ioschedulers, I added a few 
lines to my rc.local to set the scheduler to "noop" for disks with model 
QEMU*, to the scheduling in the VM will not be computing with the 
scheduling in the host.

I wonder if the same thinking applies to the "rotational" feature added 
to /sys/block in 2.6.29, intended for SSD but probably appropriate for 
virtual and nbd devices as well.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark 

Re: Setting disk parameters in a VM

[Avi Kivity]

Bill Davidsen wrote:
> Based on a discussion in another list on ioschedulers, I added a few 
> lines to my rc.local to set the scheduler to "noop" for disks with 
> model QEMU*, to the scheduling in the VM will not be computing with 
> the scheduling in the host.
>
> I wonder if the same thinking applies to the "rotational" feature 
> added to /sys/block in 2.6.29, intended for SSD but probably 
> appropriate for virtual and nbd devices as well.

A virtual disk backed by a rotational physical disk probably should be 
marked rotational, at least with cache=off.

-- 
error compiling committee.c: too many arguments to function

Re: Setting disk parameters in a VM

[Bill Davidsen]

Avi Kivity wrote:
> Bill Davidsen wrote:
>> Based on a discussion in another list on ioschedulers, I added a few 
>> lines to my rc.local to set the scheduler to "noop" for disks with 
>> model QEMU*, to the scheduling in the VM will not be computing with 
>> the scheduling in the host.
>>
>> I wonder if the same thinking applies to the "rotational" feature 
>> added to /sys/block in 2.6.29, intended for SSD but probably 
>> appropriate for virtual and nbd devices as well.
>
> A virtual disk backed by a rotational physical disk probably should be 
> marked rotational, at least with cache=off.
>
The question was more regarding virtual drives backed by files, but even 
so I would think the bad case would be to let the host decide what 
methods should be used, rather than the VM, which is more likely to do 
things wrong based on incomplete information.

Feel free to clarify if you think there's a case where the VM and the 
host should both be doing caching and scheduling. The only one I can 
imagine is if a USB port is passed to the VM and it has a storage device 
attached. That's not really a common configuration, I would guess.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark