Re: [Qemu-devel] KVM call agenda for October 25

[Kevin Wolf <kwolf@redhat.com>]

Am 25.10.2011 15:05, schrieb Anthony Liguori:
> On 10/25/2011 07:35 AM, Kevin Wolf wrote:
>> Am 24.10.2011 13:35, schrieb Paolo Bonzini:
>>> On 10/24/2011 01:04 PM, Juan Quintela wrote:
>>>>
>>>> Hi
>>>>
>>>> Please send in any agenda items you are interested in covering.
>>>
>>> - What's left to merge for 1.0.
>>
>> I would still like to cache the default cache mode (probably to
>> cache=writeback). We don't allow guests to toggle WCE yet which Anthony
>> would have liked to see before doing the change. Is it a strict requirement?
> 
> I don't see a way around it.  If the default mode is cache=writeback, then we're 
> open to data corruption in any guest where barrier=0.  With guest togglable WCE, 
> it ends up being a guest configuration issue so we can more or less defer 
> responsibility.

So do you think that offering a WCE inside the guest would be a real
solution or just a way to have an excuse?

Christoph said that OSes don't usually change this by themselves, it
would need an administrator manually changing the setting. But if we
require that, we can just as well require that the administrator set
cache=writethrough on the qemu command line.

> Do you think it's a good idea to change the default mode w/o guest WCE toggle 
> support?  What's your view about older guests if we change the default mode? 
> What's your main motivation for wanting to change the default mode?

Because people are constantly complaining about the awful
(cache=writethrough) performance they get before they are told they
should use a different cache option. And they are right. The
out-of-the-box experience with qemu's block performance really sucks.

> I'd be much more open to changing the default mode to cache=none FWIW since the 
> risk of data loss there is much, much lower.

I think people said that they'd rather not have cache=none as default
because O_DIRECT doesn't work everywhere.

Kevin