[Qemu-devel] virtio-scsi and error handling

[Hannes Reinecke]

Hi Stefan,

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.

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.

Any opinions from your side?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                   zSeries & Storage
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