Re: [PATCH] scsi: Allow activation of scsi-mq per-driver

[Johannes Thumshirn <jthumshirn@suse.de>]

On Mon, Jan 25, 2016 at 02:05:25AM -0800, Christoph Hellwig wrote:
> On Fri, Jan 22, 2016 at 02:41:28PM +0100, Johannes Thumshirn wrote:
> > Allow the activation of the scsi-mq feature on a per-driver bassis as opposed
> > to the current stack global (de)activation.
> > 
> > This allows us to have setups which can combine "slow" rotational media and
> > fast media on two different HBA types.
> 
> It's still the wrong thing to do.  One of your co-workers has been
> working on slicing for blk-mq to better support slower adapters.

I know but there are no results yet.

> 
> Anything that prolongs the misery of keeping the old request code around
> is a bad idea.

Agreed, but this patch is a "quick fix" for the problem until the nice solution
is in place. The way it currently works is unacceptable from a distribution
point of view.

> 
> > The following is from a host with rotational disks behind a HP SAS Adapter and
> > a fibre channel array behind a Emulex FC Adapter. The hpsa driver does not
> > support scsi-mq yet (and has rotational disks attached to it), but the lpfc
> > does. This patch allows an optimal combination of the scsi-mq enabled lpfc
> > driver and the hpsa driver which still uses a single queue scsi layer and thus
> > can make use of IO schedulers.
> 
> And this is another reason why this is bad.  There are very slow dumb
> FC array arounds as well as really fast ones.  And there are really slow
> SATA disks behind SAS controllers as well as really fast SSDs.
> 
> The host is simply the wrong place to decide these things.


Yes, but this way the admin can _decide_ on which driver's he/she is going to
activate scsi-mq and on which not. Of casue this only solves the issue if
multiple drivers are involved, but still better than a global on/off switch.

-- 
Johannes Thumshirn                                          Storage
jthumshirn@suse.de                                +49 911 74053 689
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