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List:       sas-l
Subject:    Re: Selecting a brand new computer system
From:       Kevin Myers <KevinMyers () AUSTIN ! RR ! COM>
Date:       2002-10-31 20:29:35
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Hi George -

I'm not sure where you got the impression that I was saying that SAS is not
I/O intensive.  I have repeatedly said exactly the opposite!!!

Yes, I was saying that SCSI is not much faster then IDE in my experience,
where realistic *workstation* configurations are concerned.  However, it
does appear that my SCSI information was a little outdated.  Last time I
messed with it, SCSI was still limited to 160MBps (less SCSI command
overhead), so that would affect my conclusions a little bit.  BUT...

The Seagate drive that you referenced is certainly blazingly fast.  With
15000RPM spindle speed and 3.2ms average seek, it's average data access
latency is spectacularly low.  That will be fantastic for a typical server
application where a single drive or RAID array is handling I/O requests from
multiple processes.  However, the Seagate's sustained data transfer rate
(69MBps) is not that much better than the IBM which you referenced (56MBps),
and it is the sustained data transfer rate that will have the biggest impact
on typical SAS processes, which as you noted are typically very heavy with
sequential I/O.  Also, note that neither one of these drive's sustained
transfer rates approach that of their interfaces.  So, it would be probably
be reasonable to put 4 of the Seagates per SCSI 320 channel, or a pair of
the IBMs per ATA 133 channel.  (FWIW I based my prior info on top end
Western Digital and Maxtor drive specs, which aren't much different from the
IBM.)

Though the multiple device and cabling restrictions that you mentioned are
accurate, I don't believe those will have any impact on this user's system,
which was intended to be a workstation, not a server.  As such, I
anticipated that it would only have a maximum of 2 CPUs and a single high
speed PCI bus to keep the cost within reason, and therefore a maximum of 4
to 8 high speed drives.  If those limits don't apply, then the advantages of
a SCSI based system that you mention might become more important.

All in all, I'll stick to my prior recommendation, while admitting that it
is slightly biased toward lower cost rather than absolute maximum
performance.  As an independent engineer who has to foot my own hardware
bills, I tend to think that way, namely maximum bang for my somewhat limited
bucks...

s/KAM


----- Original Message -----
From: "George Chalissery" <gchalissery@dgapartners.com>
To: <KevinMyers@AUSTIN.RR.COM>
Sent: Thursday, October 31, 2002 1:23 PM
Subject: Re: Selecting a brand new computer system


> "Kevin Myers" <KevinMyers@AUSTIN.RR.COM> wrote in message
> news:<05c201c2810d$346b4b40$66c8a8c0@gusher>...
> > [... stuff deleted]
> > SCSI has a *reputation* of being faster than IDE, but IMHO that
reputation
> > doesn't hold up for the type of application that was being considered
> here.
> > [... more stuff deleted]
>
> Kevin,
>
> I appears that you are saying one of the following or both:
>
> a) SAS is not I/O intensive
> b) SCSI is not much faster than IDE
>
> I think that a) is not true based on my experience and the general
concensus
> of posts to the SAS list. As for b) let us compare the specs of the
fastest
> SCSI disk the Seagate Cheetah 15.3 and the fastest ATA/IDE disk (for
server
> applications) the IBM 180GXP. Instead of presenting the numbers here, here
> are links to the data sheets:
>
http://www.seagate.com/cda/products/discsales/marketing/detail/0,1081,551,00
> .html
> http://www.storage.ibm.com/hdd/desk/ds180gxp.htm
>
> I think that you will find that the Seagate will offer you significantly
> better performance. Whether that is worth the additional cost depends on
the
> users needs, but the Seagate is faster in all aspects. The only drawbacks
of
> SCSI disks that is significant (in my humble opinion) is that the highest
> capacity offered by SCSI is still under 200GB while IDE is at 320GB today
> and the price per GB for IDE is under $2. That $2 mark is a general
average
> (I am saying things like that to a SAS user!!!) which is just incredible.
We
> used to pay that much for a MB ten years ago.
>
> The bandwidth argument you made is not entirely true as SCSI is at 320MB/s
> which ATA is at 133MB/s. SATA is at 150MB/s and is a dedicated link with
no
> sharing. You also have to keep in mind that it will take 4 Cheetahs to
just
> about saturate a 320MB/s bus which transferring sequentially (SAS does
that
> a lot) from the outer tracks of the disk. Also SCSI can scale to 15 disks
on
> a single bus, while ATA is limited to 2. There are serious cable length
> limitations that will make it just about impossible to use more than 12
ATA
> disks in a single server even if you have multiple controllers.
>
> All this said, SATA and future (not current) SATA disks may change all
this
> quite significantly. It may help others if you post a note to the list
with
> your findings.
>
> Thanks,
>
> George.

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