Re: Question: data consistency on fail-over using shared disk

[Avi Kivity <avi@redhat.com>]

On 07/21/2010 10:43 AM, Takuya Yoshikawa wrote:
>
>> If the VM can't do anything, you're safe. Of course a crash doesn't mean
>> a VM can't do anything, you typically need to isolate it by resetting
>> the host or having a switch disconnect its storage and network.
>>
>>     
> Thanks,
> We are starting to manage the "only VM crashed" case by HA.
> So yes, we need to reset the host.
>   

It may be sufficient to issue a sync(2).


>> I believe nothing special is needed, as long as a crash means both the
>> qemu and host kernel crashed. If only qemu crashed, then the writeback
>> case needs buffers flushed.
>>
>>     
> My main concern is only qemu crashed and host is alive case.
>
> Host memory is typically large, so we want to eliminate flushing everything
> if possible.
>
> So we don't need to flush in the case of writethrough = RHEL default ?
>   

No. But you do need to unmount the underlying host filesystem.

I recommend using cache=none and direct volume access, this is easier to
guarantee integrity.

-- 
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