Re: SCSI vs S-ATA

From: Andrew H (ajhpms_at_hotmail.com)
Date: 08/04/04 

Date: Wed, 4 Aug 2004 11:07:55 +0200

Thank you, root. It really helps when you lay out your reasoning rather than
just abruptly comment "nonsense" to a previous poster. Please continue to
do so.

"root" <postmaster@buchanangc.com> wrote in message
news:%23qSXmQZeEHA.2532@TK2MSFTNGP09.phx.gbl...
>
> "Jerry Dubuke" <jdubuke@not.gpdservices.com> wrote in message
> news:ulMEECXeEHA.1644@tk2msftngp13.phx.gbl...
> > I have asked the following question of various resellers of hard disks
ans
> > NAS solutions, and gotten cryptic answers at best...
> > So, the question is:
> > How does SATA perform relative to SCSI in a multi-user environment?
> > I am under the impression that SATA is still a "parallel" process,
whereby
> > only one request can be handled at a time, vs SCSI, which can handle
> > multiple simultaneous requests.
>
> Any disk has only a single headset and therefore any HD can only be doing
a
> single seek and I/O at any one instant. The difference is more subtle. A
> SCSI HD may queue up a number of disk I/O requests onboard and choose the
> optimal order to do the I/Os. That optimization when several I/Os are
> queued onboard a SCSI HD can provide up to a 30% performance boost when
> doing small record random I/O.
>
> For SCSI to support onboard command queuing and other features of SCSI,
> SCSI has a much higher single queued command overhead than [S]ATA. That's
> why late model SATA HDs actually beat SCSI performance wise on single user
> workstations.
>
> On transaction servers where the load of small record random I/O can keep
a
> SCSI HD's onboard queue non-empty then such a SCSI HD system can pump out
> the I/Os per second well above CURRENT [S]ATA configurations. On
> configurations not experiencing such saturated small record disk I/O,
SCSI
> offers little performance advantage over top SATA HDs like the WDC Raptor
> unless of course one buys triple cost late model 15K RPM SCSI HDs like the
> Fujitsu MAS3735 which fly under ALL circumstances.
>
> > Is this still a true statement? If so, why would anyone want SATA in a
> > server, (or a NAS for that matter)?
>
> Cost.
>
> > Along the same vein, does SATA use the main CPU for access the same was
> that
> > IDE does, or does the "controller" have on-board dedicated CPU like a
SCSI
> > card does?
>
> SCSI cards do NOT have onboard CPUs that handle any load over [S]ATA
> implementations. Neither [S]ATA nor SCSI I/O puts a significant burden on
> the host's CPU. That changes for RAID 5.
>
> The host CPU issue is relevant for RAID. HW RAID controllers, whether
SCSI
> or [S]ATA, have onboard CPUs to offload the host CPU. That is important
> for any RAID like RAID 5 that does parity/ECC type calculations. For RAID
1
> and RAID 0 that do NOT do any such calculations then HW RAID offers fewer
> advantages over SW/firmware RAID. SW/firmware RAID uses the host's CPU to
> do all processing but there is so little for RAID 1 and RAID 0 that it
makes
> little difference performance wise. Therefore SBS2003's intrinsic SW RAID
1
> is a great way to go.
>
>

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