[dm-devel] block: Deprecate QUEUE_FLAG_CLUSTER and use queue_limits instead

[Mike Snitzer]

On Thu, Nov 25 2010 at  7:37pm -0500,
Martin K. Petersen <martin.petersen at oracle.com> wrote:

> New version below, fixing the things you and Matthew pointed out...
> 
> 
> 
> block: Deprecate QUEUE_FLAG_CLUSTER and use queue_limits instead
> 
> When stacking devices, a request_queue is not always available. This
> forced us to have a no_cluster flag in the queue_limits that could be
> used as a carrier until the request_queue had been set up for a
> metadevice.
> 
> There were several problems with that approach. First of all it was up
> to the stacking device to remember to set queue flag after stacking had
> completed. 

But that was already done properly, so that wasn't a problem that needed
fixing (it just had potential to be overlooked if/when there is a new
stacking driver).

> Also, the queue flag and the queue limits had to be kept in
> sync at all times. We got that wrong, which could lead to us issuing
> commands that went beyond the max scatterlist limit set by the driver.

It took me a bit to see exactly where we got it wrong.  Looks like
__scsi_alloc_queue was only concerned with the queue flag.  So a
minimalist fix would've been to also set no_cluster = 1 in
__scsi_alloc_queue?

(OK, I just reviewed Ed's initial report and that is exactly what his
proposed patch did! :)

This patch's header could stand to be a bit more explicit about where
the real problem was ...

But I agree that this patch cleans things up nicely.

Acked-by: Mike Snitzer <snitzer at redhat.com>

Thanks,
Mike