On Mon, 17 Nov 2003 00:26:36 -0600 Jeff Walther <[EMAIL PROTECTED]> wrote:
> Interface speed is the speed of the electronic interface between the
> drive and the computer (or SCSI card). Sustained data rate is how
> fast the hard drive is actually capable of delivering data to that
> electronic interface.
...
> Or the example in my original post a few weeks ago--the Seagate
> Barracuda. The early Barracudas were the ST32550 and ST15150. They
> spin at 7200 RPM, among the very first mass market drives to do so.
> They deliver 6 MB/s and 4.5 MB/s sustained data rates. Would you
> rather have an ST32550 or an IBM 75 GXP? They both spin at 7200
> RPM, but one delivers five times the real world data rate as the
> other. And the early Barracudas were hot and loud...
aha, but there's a joker in the deck.
SCSI is much better at certain things, which may mitigate the difference
in data rate that you cite.
specifically, SCSI has a concept of multiple reads and writes in progress
at the same time. it can keep track of the requested I/O transactions,
and report individually on their completion status.
IDE (other than the not-quite-soup-yet serial ATA) cannot do this. fast
IDE drivers queue I/O transactions up in onboard RAM, wait for the
platter and heads to get to the correct position, and just write -- but
before they do this, they lie to the CPU and claim that the write is
already done.
this is why serious server guys, especially serious database guys,
won't give IDE the time of day. if you pull the plug on a busy IDE
based database server while it's running, your database will be
hash come reboot time. not so with a real database on SCSI drives.
the only way to prevent this is to disable the write caching on the
IDE drives, but then your write performance goes in the toilet.
it's not such a big deal for a home user, and can be reasonably
mitigated for the home user with a good UPS. these aren't,
however, adequate for the needs of someone doing high
availability server work.
the other thing to be aware of is that with daisy chained SCSI
drives, the drives themselves may be slower than the SCSI
bus speed, but the aggregate drive performance may approach
the interface speed -- and if you're striping across an array
using RAID 0 or some other striping pattern, this can pay off
nicely.
richard
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Richard Welty [EMAIL PROTECTED]
Averill Park Networking 518-573-7592
Java, PHP, PostgreSQL, Unix, Linux, IP Network Engineering, Security
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