Re: queue_depth tracking from LLD

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Apr 16, 2009 at 10:13:42AM -0400, James Smart wrote:
> However, for arrays, with multiple luns, the queue depth is usually a 
> target-level resource,
> so the midlayer/block-layer's implementation falls on its face fairly 
> quickly. I brought this

If the problem were as simple as the resource being target-level instead
of LUN-level, it would be fairly easy to fix (we could do accounting
per-target instead of per-LUN).  The problem, AIUI, is multi-initiator
where you can't know whether resources are in use or not.

> up 2 yrs ago at storage summit.  What needs to happen is the creation of 
> queue ramp-down
> and ramp-up policies that can be selected on a per-lun basis, and have 
> these implemented
> in the midlayer (why should the LLDD ever look at scsi command 
> results).  What will make
> this difficult is the ramp-up policies, as it can be very target 
> device-specific or configuration/load
> centric.

While not disagreeing that it's complex, I don't think putting it in the
driver makes it less complex.  I completely agree that LLDDs should not
be snooping scsi commands or scsi command results.  It should all be in
the midlayer so we all share the same bugs ;-)

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."