Re: [Qemu-devel] virtio-scsi and error handling

[Paolo Bonzini]

Il 12/06/2013 03:56, Stefan Hajnoczi ha scritto:
> On Tue, Jun 11, 2013 at 01:41:38PM +0200, Hannes Reinecke wrote:
>> I currently playing around with improving SCSI EH, optimizing
>> command aborts and the like.
>>
>> And, supposing it to be a nice testbed, tried to make things work
>> with virtio_scsi.
>>
>> However, looking at the code there I've found virtscsi_tmf() just
>> uses 'wait_for_completion', with no timeout specified. So in effect
>> any abort might stall forever.
>>
>> Wouldn't it be more sensible to use 'wait_for_completion_timeout'
>> here, to allow the error escalation to continue?
>> This would especially be useful when running with multipathing,
>> as the underlying device might stall, and aio_cancel() doesn't work
>> reliably, if at all.
> 
> Hi,
> I agree that we need a timeout.  bdrv_aio_cancel() is not guaranteed to
> complete in bounded time.

I also agree that we need a timeout, but then note that host reset could
also not complete in bounded time if I/O doesn't terminate in the host.

Last time I checked the io_cancel system call was basically a no-op (for
aio=native), and for aio=threads the worker might stay in D state for an
unbounded time too.

Paolo

>> Also I've found that there is no host reset. Currently the virtio
>> semantics seem to require reliable communication, ie for every
>> command send there _has_ to be a response.
>>
>> Long and painful experience with RAID HBAs has shown that this model
>> works okay for the lower-level escalations, but you absolutely need
>> a host reset to restore communication.
>> In the case of virtio I would think that a virtio-level reset for
>> host_reset would be a sensible idea.
> 
> One thing to watch out for is that a virtio-scsi reset will likely hang
> too because it resets all pending requests.
> 
> Paolo Bonzini has done the lion's share of virtio-scsi work over the
> past year (or two?).  He might have some more thoughts.
> 
> Stefan
>