From owner-freebsd-hackers  Fri Mar 21 04:20:33 1997
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To: "A JOSEPH KOSHY" <koshy@india.hp.com>
cc: freebsd-hackers@freebsd.org
Subject: Re: SDDISKSORT 
In-reply-to: Your message of "Fri, 21 Mar 1997 16:44:18 +0500."
             <199703211144.AA094094658@fakir.india.hp.com> 
From: David Greenman <dg@root.com>
Reply-To: dg@root.com
Date: Fri, 21 Mar 1997 04:19:24 -0800
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>I noticed that by default we never turn on disk sorting for SCSI drives.

   Yes, that is correct.

>What would be the reason for doing so.  I understand that if a drive supports
>command queueing then presorting the request list may not be effective.
>However not all hardware support command queueing.
>
>Is not sorting the disk I/Os beneficial for all SCSI drives?

   The block sort algorithm in FreeBSD is highly inferior to the algorithms
used in modern SCSI drives. Enabling the FreeBSD algorithm in this situation
substantially reduces disk performance for drives/controllers that support
tagged command queueing, which is why 'disksort' is disabled by default in
FreeBSD. For people who have controllers that don't support tagged queueing,
they can enable the disksort algorithm with the SDDISKSORT kernel option.

-DG

David Greenman
Core-team/Principal Architect, The FreeBSD Project