Re: [PATCH 4/4] scsi: core: don't limit per-LUN queue depth for SSD

["Ewan D. Milne" <emilne@redhat.com>]

On Wed, 2019-11-20 at 11:05 +0100, Hannes Reinecke wrote:
> 
> Hmm.
> 
> I must admit I patently don't like this explicit dependency on
> blk_nonrot(). Having a conditional counter is just an open invitation to
> getting things wrong...
> 

This concerns me as well, it seems like the SCSI ML should have it's
own per-device attribute if we actually need to control this per-device
instead of on a per-host or per-driver basis.  And it seems like this
is something that is specific to high-performance drivers, so changing
the way this works for all drivers seems a bit much.

Ordinarily I'd prefer a host template attribute as Sumanesh proposed,
but I dislike wrapping the examination of that and the queue flag in
a macro that makes it not obvious how the behavior is affected.
(Plus Hannes just submitted submitted the patches to remove .use_cmd_list,
which was another piece of ML functionality used by only a few drivers.)

Ming's patch does freeze the queue if NONROT is changed by sysfs, but
the flag can be changed by other kernel code, e.g. sd_revalidate_disk()
clears it and then calls sd_read_block_characteristics() which may set
it again.  So it's not clear to me how reliable this is.

-Ewan