Re: virtio scsi host draft specification, v3
From: Hannes Reinecke
Date: Tue Jun 14 2011 - 11:30:36 EST
On 06/12/2011 09:51 AM, Michael S. Tsirkin wrote:
On Fri, Jun 10, 2011 at 02:55:35PM +0200, Hannes Reinecke wrote:
Device operation: request queues
--------------------------------
The driver queues requests to an arbitrary request queue, and they are
used by the device on that same queue.
What about request ordering?
If requests are placed on arbitrary queues you'll inevitably run on
locking issues to ensure strict request ordering.
I would add here:
If a device uses more than one queue it is the responsibility of the
device to ensure strict request ordering.
Maybe I misunderstand - how can this be the responsibility of
the device if the device does not get the information about
the original ordering of the requests?
For example, if the driver is crazy enough to put
all write requests on one queue and all barriers
on another one, how is the device supposed to ensure
ordering?
Which is exactly the problem I was referring to.
When using more than one channel the request ordering
_as seen by the initiator_ has to be preserved.
This is quite hard to do from a device's perspective;
it might be able to process the requests _in the order_ they've
arrived, but it won't be able to figure out the latency of each
request, ie how it'll take the request to be delivered to the initiator.
What we need to do here is to ensure that virtio will deliver
the requests in-order across all virtqueues. Not sure whether it
does this already.
Cheers,
Hannes
--
Dr. Hannes Reinecke zSeries & Storage
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SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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