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

[Kevin Wolf <kwolf@redhat.com>]

Am 25.10.2011 16:06, schrieb Anthony Liguori:
> On 10/25/2011 08:56 AM, Kevin Wolf wrote:
>> 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?
> 
> No, it offers a mechanism to "fix mistakes" at run-time verses at start up time. 

This is true (in both directions). But I think it's independent from the
right default.

>   It also means that you can make template images that understand that they 
> don't support barriers and change the WCE setting appropriately.

Isn't that really a job for management tools?

>> 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.
> 
> The administrator of the guest != the administrator of the host.

But the administrator of the guest == the owner of the qemu instance,
no? He should be the one to use the management tools and configure his VMs.

>>> 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.
> 
> With qcow2 you mean, right?

No, with any format, including raw. Which isn't surprising at all,
O_SYNC makes writes very expensive.

>>> 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.
> 
> Where doesn't it work these days?  I know it doesn't work on tmpfs.  I know it 
> works on ext[234], btrfs, nfs.

I think tmpfs was named (and failing to start with default settings on
tmpfs would be nasty enough), but iirc Alex had another one.

Kevin