Re: [Xen-API] [Xen-devel] driver domain crash and reconnect handling

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/01/13 15:01, Zoltan Kiss wrote:
> On 24/01/13 14:06, George Shuklin wrote:
>> 24.01.2013 17:25, Paul Durrant ÐÐÑÐÑ:
>>>> Some notes about guest suspend during IO.
>>>>
>>>> I tested that way for storage reboot (pause all domains, reboot ISCSI 
>>>> storage
>>>> and resume every domain). If pause is short (less that 2 minutes), guest 
>>>> can
>>>> survive. If pause is longer than 2 minutes, guests in state of waiting for 
>>>> io
>>>> completion, detects IO timeout after resuming  and cause IO error on 
>>>> virtual
>>>> block devices. (PV).
>>>>
>>> To be clear here: do you mean you *paused* and then unpaused the VMs, or 
>>> *suspended* and then resumed the VMs? I suspect you mean the former.
>>>
>>>     Paul
>> Pause, of cause. My bad.
>>
> If you would do a suspend, the frontend driver flush out disk IO 
> operations before suspend reached, and therefore there won't be anything 
> to timeout after resume. However, if the storage driver domain just 
> crashed, I guess the guest would crash at suspend. Maybe we can try out 
> something to save the the ring buffer, and replay them back once the 
> backend come back (but before resuming the guest). But I'm not sure 
> whether the guest would handle the timeouts after the resume first, or 
> cancel them if the requests were succesfully responded.
>
> Zoli

Perhaps I am making this harder, but might it be best to wait for a
short while (15-30 seconds) for the device driver domain to come back,
and if it takes longer than that, pause the VM.

This way, if the driver domain is fast to come back, all the guest
notices is transitorily blocked IO, and if the driver domain is too slow
(but does come back), all the guest might notices is a pause.

Ultimately, if the driver domain never comes back, then we are in a no
worse position than currently.

~Andrew

>
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