Re: [Qemu-devel] qcow2 - safe on kill? safe on power fail?

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
> Avi Kivity wrote:
>> Anthony Liguori wrote:
>>>
>>> Sure you can.  If you don't have a battery backed disk cache and are 
>>> using write-back (which is usually the default), you can definitely 
>>> get corruption of the journal.  Likewise, under the right scenarios, 
>>> you will get journal corruption with the default mount options of 
>>> ext3 because it doesn't use barriers.
>>>
>>
>> What about SCSI or SATA NCQ?  On these, barriers don't impact 
>> performance greatly.
>
> Good question, I don't know the answer.  But ext3 doesn't autodetect 
> SCSI/NCQ or anything.  It disabled barriers by default.  Some distros 
> have changed this behavior historically (SLES I believe).
>

This ought to be on the driver level.  SCSI and NCQ disks should report 
barrier support; old IDE should report no barriers unless the user sets 
dont_care_about_performance_and_have_unlimited_warranty=1.  ext* should 
use barriers if available.

Of course this is linux-kernel material, not really on topic for this list.

>>> This is very hard to see happen in practice though because these 
>>> windows are very small--just like with QEMU.
>>>
>>
>> The exposure window with qemu is not small.  It's as large as the 
>> page cache of the host.
>
> Note I was careful to qualify my statements that cache=off was required.

Ah, okay then.

Qemu should be written assuming the underlying layers are sane; trying 
to work around Linux bugs is madness.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.