From: milton@teal.csn.org (Milton Scritsmier)
Subject: Re: Noise like a seek coming from new scsi hard drive.
Organization: Colorado SuperNet, Inc.
Date: Tue, 26 Jan 1993 04:43:41 GMT

In article <1330@alsys1.aecom.yu.edu> manaster@yu1.yu.edu (Chaim Manaster) writes:
>
>Most (not quite all) the bugs are out, however one thing worries me
>somewhat. Every so often on intervals on the order of 20 to 30
>minutes apart, (not too regular) there is a momentary sound from
>the hard drive that lasts for about one second (by my reckoning)
>and sound like the drive has been given a seek command. Is this
>possible? (By the way, the above reffers to periods where the
>computer has been switched on, but is otherwise idle during that
>entire period, and not being used by someoneone at the keyboard.
>The only thing happening is whatever internal loop Dos  does while
>waiting at the Dos prompt, or Windows does while wating for some
>user input.)
>

Many people have responded to say that it is done to avoid a stiction
problem. However, it is much more likely that the drive is going through
its periodic thermal calibration sweep.

The problem is that the density of data on the media has become so
great that mechanical variations within the drive and the slightly
different thermal properties of each platter causes each platter to
expand and contract differently at different temperatures. If the
drive uses a dedicated servo platter (at the top of the stack, say),
the mechanical and thermal differences between the platters actually
causes misalignments among the heads that are big enough with the
greater crowding of tracks to make it impossible to seek to a track
reliably on one platter even when the servo head is right on the
money. This was not a problem on earlier drives where the tracks
were wider and the platters not so closely spaced (newer drives
are much more likely to develop hot spots at various places
inside the drive, and have less outer cooling surface because
they're typically smaller).

The trick drive manufacturers use is to do a calibration sweep at
spin-up time to determine how far each head is from where the drive
thinks it should be on a given track.  This offset for each track is
stored and any time the drive needs to seek to a track on the platter
a kind of two stage process is used. There is the first major seek
which gets the head to about the right location, and then there is a
minor seek where the previously stored offset (plus some extra servo
sampling on the track) is used to find the precise center of the track.
Note that even when a sequential operation is being done and the drive
only needs to switch heads, the second seek must still be done (your're
on a new platter with a different offset) even though the first major
seek can be eliminated. That's why head switch times on newer drives
are measured in milliseconds now instead of the microseconds they
used to be measured in. A head switch now requires a small mini-seek.

This is the one performance area where drives have actually gotten
worse over the last few years. Often a drive manufacturer will put
things like rotational speed and seek times right up front in their
drive manual where you can't miss it, but then bury or not even
report the head switch time. This greater head switch time (which 
can be as much as a quarter or third of a revolution) really does
have an impact on performance, even when the drive is properly
formatted with the correct track skew. There is simply no way to
avoid the delay caused by the head switch mini-seek, unless you
count read-ahead and caching schemes.

I should also say here that drives that have embedded servos instead
of a dedicated servo platter also will do a calibration sweep
at power-on. They suffer from exactly the same thermal and mechanical
problems and have the same media density as drives with dedicated
servo platters.  Thus these drives must also do a mini-seek during
head switches.

The reason for periodic thermal calibration sweeps to get new offset
values is because the temperature of the drive is not constant from
power-on to running over several hours. The drive starts out relatively
cold, and warms up after it spins for awhile. Also, the computer it is
in starts to warm up as well. This changes the thermal effects for
each platter and since each platter expands differently, new offsets
must be determined by doing a calibration sweep on each head. This is
done quite often immediately after power-on, and is gradually reduced
in frequency as the drive has been on for awhile (every 20 or
30 minutes and a second or so duration sounds about right). 


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