Re: [DO NOT APPLY] sd take advantage of rotation speed

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Jun 19 2008, Matthew Wilcox wrote:
> Use the noop elevator by default for drives that do not spin
> 
> [Not for applying]
> 
> SSDs do not benefit from the elevator.  It just wastes precious CPU cycles.
> By selecting the noop elevator by default, we can shave a few microseconds
> off each IO.
> 
> I've brazenly stolen sd_vpd_inquiry from mkp's patch here:
> 
> http://marc.info/?l=linux-scsi&m=121264354724277&w=2
> 
> No need to have two copies of that ... but this will conflict with his code.
> 
> On to the self-criticism:
> 
> I don't intend the final version of this patch to include a printk for
> the RPM or even a printk to say we switched IO elevator.  I think we're
> too verbose in SCSI as it is.
> 
> I think there's an opportunity to improve sd_vpd_inquiry() to remove
> some of the duplicate code between sd_set_elevator() and sd_block_limits,
> but it's not terribly important.
> 
> The switching of the elevators isn't particularly nice.  I assume that
> elevator_init("noop") cannot fail, which isn't true.  It would be nice
> to use the #if 0 block instead, but that causes a null ptr dereference
> inside sysfs -- I suspect something isn't set up correctly.

I disagree with this approach. For now, lets just add a queue flag that
says the device doesn't have a seek penalty and let the io schedulers do
what they need to avoid that (it'd be a one-liner change to cfq and as).
There's more to io scheduling than just seek reduction, so this is the
wrong direction to take imo.

-- 
Jens Axboe