Newsgroups: comp.periphs.scsi
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!csn!arrayb!wicklund
From: wicklund@intellistor.com (Tom Wicklund)
Subject: Re: What makes a SCSI drive fast?
Message-ID: <1991Apr18.234122.28503@intellistor.com>
Keywords: SCSI drive
Organization: Intellistor
References: <14971@life.ai.mit.edu>
Date: Thu, 18 Apr 91 23:41:22 GMT

In <14971@life.ai.mit.edu> fur@ai.mit.edu (Scott Furman) writes:

>Recently I was reading some SCSI spec sheets for the Wren IV.  A few parameters
>caught my eye:

>1) Overhead time for head switch (512 byte sectors):	< 2   ms
>2) Overhead time for one-track cyclinder switch    :      6   ms typical
>3) Average rotational latency                      : 	  8.3 ms

>Two milliseconds to switch heads!?  I would have guessed that switching sense
>heads was was done purely by electrical means.  The overhead time for a track
>change also seems excessive.  Apparently these overhead times are due to the use
>of embedded servo mechanisms which are commonly found now in modern drives.
>Embedded servos, I am told, increase reliability and reduce the price per byte. 
>Does anyone have an explanation of why embedded servo are more reliable?

>Must these changes take place at the expense of performance? Are embedded servos
>particularly entrenched in the SCSI world where price/byte is a strong market
>force?  Are there any manufacturers who make make drives that don't suffer
>these sorts of performance hits?

Embedded servos are not SCSI specific.  There are versions of ESDI,
SMD, ATA, and other drive types implementing embedded servos.

Embedded servos allow higher disk density.  As the number of tracks
per inch increases, it's not possible to keep the heads aligned
exactly (temperature variations cause each arm to expand a different
amount).  So a single servo won't keep the heads aligned exactly
enough.  The solution is servo data on each platter.  There are a few
ways to implement this:

1.  Embedded servo on each drive.  Servo information is recorded on
each surface on each track.

2.  Periodic thermal calibration.  A special area on each surface is
reserved and contains servo data.  The drive periodically (every few
minutes to every few hours) performs a calibration operation, checking
the relative positions of the heads.  When a head switch occurs, it
adjusts the head position to align the head precisely.

3.  Some drive manufacturers have wanted to try method #2 but seek to
an average of all head positions (so that head switches remain
electrical only) but this method will not work as densities continue
to increase.

In summary, you can avoid the head switch time but at the expense of
smaller drive capacity.


>The 8.3 ms average rotational latency is a result of the standard 3600 RPM.  No
>surprises there.  However, I understand Micropolis and HP have introduced drives
>that operate at 5400 RPM and 4000 RPM respectively.  Has anyone used these?  Are
>any manufacturers coming out with even faster ones?

A number of manufacturers are moving to faster rotation rates.  I
believe IBM and Fujitsu have or will soon announce drives which spin
at 5400 RPM.