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Date: Tue, 10 Apr 2012 13:28:28 -0400
From: Vivek Goyal <vgoyal@redhat.com>
To: Mike Snitzer <snitzer@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
        linux kernel mailing list <linux-kernel@vger.kernel.org>,
        martin.petersen@oracle.com
Subject: Re: [RFC PATCH] block: Change default IO scheduler to deadline
 except SATA
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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