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

[Alexander Graf]

On 25.10.2011, at 17:32, Kevin Wolf <address@hidden> wrote:

> 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.

Yeah, IIRC NFS also failed on me :)

Alex

> 
> Kevin