Re: [RFC PATCH] block: Change default IO scheduler to deadline except SATA

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Apr 10, 2012 at 12:13:07PM -0400, Mike Snitzer wrote:

[..]
> > So we are back to the question of can scsi devices find out if a Lun
> > is backed by single disk or multiple disks.
> 
> I'm not aware of any discrete attribute (comparable to 'rotational'
> flag) that SCSI devices will advertise that indicates "I'm a raid
> array".
> 
> That said, we can have a _very_ good hint that a SCSI device is a raid array if:
> 
> 1) optimal_io_size is not zero, minimum_io_size is not equal to
> optimal_io_size, and optimal_io_size is a multiple of minimum_io_size
> 
> 2) WCE=0 (higher-end arrays with a writeback cache)
> 
> Determining 1 could be enough, we should probably ignore 2 as it isn't
> an absolute indication that a device is composed of multiple devices
> (especially not if considered independently of 1).

Umm..., somehow relying on optimal_io_size != minimum_io_size sounds odd
to me (assuming it works).

I checked bunch of Luns exported to me and all of them have
optimal_io_size=0.

I have few FC Luns exported from two array vendors. I have few iscsi Luns
exported from two separate vendors and all these luns have
optimal_io_size=0.

Thanks
Vivek