defragmentation hard drive scsi elevator seek

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From - Fri Sep  7 06:54:17 2001
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From: Fulko Hew <fulko@wecan.com>
Subject: Re: Defrag Software For Sco
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Bill Vermillion <bill@wjv.com> wrote:

> Tony Lawrence  <tony@aplawrence.com> wrote:
> >Bill Vermillion wrote:
> 
> >> Unless you have an extremely active system you won't see "instant
> >> fragmentation" like the MS systems.  For most instances it is just
> >> not the problem it is in the MS world.
> 
> >There's also the point that multiuser systems are apt to be jumping
> >all around the hard drive anyway just because the users need data
> >from different places.
> 
> And in a decent system - eg SCSI on Unix - the elevator seeking
> mechanism is surely more efficient than the 'jump all over the
> place' methods used in the MS world.
> 
> Which now brings me to ask this. Are any of the IDE system capable
> of doing this? Elevator seeks? Is anyone building add-on controllers
> that do this?
> 
> This 'jump all over the place' is one reason why the seek time seemed
> to be one of the major requisites for performance in the MS arena.
> Since the base design sprang from a single-user mentality I can see
> this would be a logical paradigm to follow given the heritage of the
> original OS [if you can really call the original a 'system'] upon
> which it was based.

But now-a-days with SCSI drives and IDE drives that 'virtualize' the
physical characteristics of the drive, elevator seeking algorithms
are broken by the drives themselves.  When a cylinder used to be a
cylinder, and incrementing that number actually put you on a different
head, everybody was happy and fast.  But today, everything is cached
by the drive, and you never 'really' know when the drive will cross
a physical cylinder boundary on your behalf.

So elevator seeks only worked when you had control, in SCSI you only
specify the logical sector, and in IDE you only 'think' you know
what cylinder your on.

Correct me if I'm wrong.

Fulko

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Subject: Re: Defrag Software For Sco
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From: spcecdt@deeptht.armory.com (John DuBois)
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In article <10109050940.aa09571@wecan.com>, Fulko Hew  <fulko@wecan.com> wrote:
>So elevator seeks only worked when you had control, in SCSI you only
>specify the logical sector, and in IDE you only 'think' you know
>what cylinder your on.

Note that with SCSI you can have multiple outstanding block requests, which
gives the drive itself the liberty to optimize their gathering in whatever
manner is most effective.

        John
-- 
John DuBois  spcecdt@armory.com.  KC6QKZ/AE  http://www.armory.com./~spcecdt/
 

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