[rescue] Macs & IDE vs. SCSI

Phil Stracchino alaric at caerllewys.net
Sat Apr 12 11:32:48 CDT 2003

On Sat, Apr 12, 2003 at 06:58:37AM -0700, Lionel Peterson wrote:
> Why is it that an external IDE drive, hanging off a firewire to IDE
> converter/interface is better than an IDE drive running natively?
> The IDE drive can only run as fast as it's IDE interface, so I guess
> what you are saying is that IDE is fine, but the actual IDE chipset on
> your MoBo is slow...

Frankly, beyond a certain point, as far as the actual speed of the
drive is concerned it's all moot anyway.  Once you fill (or empty) the
drive's buffer, you're back to the native sustained transfer rate of the
disk mechanism, which is nowhere close to keeping pace with any current
disk interface.

Ironically, this is something that's been limited by the continuing
shrinkage of hard disk storage.  These days, it's rare for a drive to
have more than one or two platters...  if disk drives still had eight,
twelve, fifteen platters, it would be theoretically possible to design
head arrays with per-head MEMS temperature-compensation mechanisms and
stream data through all the drive's heads in parallel instead of reading
and writing through only one head at a time.  This technique has the
potential to increase sustained transfer rate in direct proportion to
the number of heads.  When drives only have two to four heads, though,
the throughput increase is probably hardly considered worth the expense
of the redesign and development process.

Where SCSI's speed advantage comes in is that it supports disconnection,
so it can go off and do something else on another drive while that one
reads more data instead of having to wait for one drive at a time to
finish as on IDE, and that doing so consumes negligible resources on the
host.  On a resource consumption basis, one should think of "IDE disk"
in the same breath as "host-based printer", "winmodem", and unaccelerated
dumb framebuffers.

It's really not about doing anything any faster -- it's about doing more
of it at a time and doing it smarter, in the background, while the host
goes about its other business.

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