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

[Avi Kivity <avi@redhat.com>]

On 07/21/2010 09:27 AM, Takuya Yoshikawa wrote:
> Hi,
>
>
> We are now checking about what we should do on vm fail-over.
>
> Concerning this, does anybody know about any danger about data
> consistency when we are using shared disk?
>
>
> What I'm concerning is if crashed VM-side host is still holding
> buffered data, starting a new VM instance on another node may
> result in file system corruption.
>
>   This problem may similar to live-migration but little bit different
>   in the sense that VM is crashed -> cannot do anything from that point.
>   

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.

>
> How about the combination of old or new guest OS and the following
> settings?
>
>  - writethrough
>  - writeback
>  - none
>
> If needed, we'll do sync by HA-side scripts before starting a new VM
> instance.
>   

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.

-- 
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