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

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

On Wed, Jun 25 2008, Boaz Harrosh wrote:
> Ric Wheeler wrote:
> > Jens Axboe wrote:
> >> 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.
> >>
> >>   
> > Very true - you still will get a significant win by coalescing IO's (say 
> > for example, to do larger, aligned writes to flash devices).
> > 
> > ric
> > 
> And to not let HUGE writers hug the machine. A scheduler ...

Precisely, merging and fairness is a big part of it. Plus, doing this in
sd is just a blatant layer violation.

-- 
Jens Axboe