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

[Sumanesh Samanta <sumanesh.samanta@broadcom.com>]

>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.

I think we can have a host template attribute and discard this check
altogether, that is not check device_busy for both SDD and HDDs.

As both you and Hannes mentioned, this change affects high end controllers
most, and most of them have some storage IO size limit. Also, for HDD
sequential IO is almost always large and
touch the controller max IO size limit. Thus, I am not sure merge matters
for these kind of controllers. Database use REDO log and small sequential
IO, but those are targeted to SSDs, where latency and IOPs are far more
important than IO merging.
If this patch is opt-in for drivers, so any LLD that cannot take advantage
of the flag need not set it, and would work as-is


Thanks,

Sumanesh

On 11/20/2019 10:00 AM, Ewan D. Milne wrote:
> 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
>
>