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From: mscritsm@isis.cs.du.edu (Milton Scritsmier)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Seagate Drives
Message-ID: <1991May25.050852.709@mnemosyne.cs.du.edu>
Date: 25 May 91 05:08:52 GMT
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In article <21894@cbmvax.commodore.com> daveh@cbmvax.commodore.com (Dave Haynie) writes:

>The trick is, the fastest and most expensive drives are also big.  So rather
>than a single or a pair of heads as in your lower density Quantums, they may
>have 6 or 8 heads, maybe even more (I don't know the typical number these days
>on a big drive).  The faster rotation gives them a little better per head
>speed and a little less rotational seeking latency, but they really get speed
>with more heads.  And the caches that most of them have.  Regardless, I don't
>think you're going to get close to saturating SCSI off-peak without multiple
>drives, no matter who's making them these days.

Actually, the number of heads has very little to do with data transfer
speeds.  A track to track seek is about 2 or 3 ms, and this is much less
than the average rotational latency of 6 to 8 ms that happens when the
drive waits for the data to come under its head after it has seeked to
the proper track.  Thus having more heads to reduce the number of seeks
doesn't buy you that much on a given command.  Plus, most SCSI drives
have 40 or more 512-byte sectors per track.  That's at least 20K on each
track, and I doubt that a lot of commands are that big.  As for caches,
there are times when it will help, but by and large the greatest factor
in drive performance is such things as how fast the drive spins and how
fast it can seek.  For example, the old standard for rotational speed
was 3600 rpm.  Now we are seeing some drives at 4500 rpm or more.  That's
at 25% increase in speed, even if the data was recorded at the same
density.  Add the increases in recording density that are in fact taking
place, and we are seeing increases from 10 megabits/sec a couple of
years ago to 24 megabits/sec or more today.