On Jan 30, 12:07pm, Olaf Seibert wrote:
} Subject: Re: NCR Driver Problems
} Antti Miettinen <apm@vipunen.hut.fi> wrote:
} > I have a feeling that some drives might lie about the number of heads
} > and number of cylinders when they construct the imaginary geometry. If
} > this geometry is used by the disksort(), it might hurt performance.
} 
} Why should it? If sectors are sorted according to cylinder then sector,
} whatever the geometry, they will be sorted correctly according to
} their linear block number.

But accessing the blocks in block number order is not necessarily
optimum.  Let's say your disk has 50 sectors per track and you want
to write to sectors 1 and 49.  When you tell the drive to write to
sector 1, it starts seeking to this track.  If by chance sector 2
is passing under the head at the time the seek completes, the drive
must wait for nearly a complete revolution of the platters for sector
1 to pass under the heads again.  The drive then notifies the host
that the write is complete, and the host sends the command to write
sector 49.  Nearly a complete revolution later, the second write
command completes.  If you use tagged commands, you can send both
commands to the drive at once.  The drive will again miss the
opportunity to write sector 1 the first time around, but it will
be able to write sector 49, and then sector 1 shortly thereafter.
Both sectors will be written in one revolution of the platters instead
of two.  Even with 7200 RPM disk, this is a savings of 8ms to execute
these two commands.

If the two commands are writes to sectors that are closely spaced on
the same track, you may lose a revolution if it takes longer to
communicate to the host that the first command has completed and
then receive and process the second command than it does for the head
to skip the space between the sectors.  If you send both commands to
the drive, then it has a better chance of executing both in one
revolution.

If you send two commands to the drive that access sectors on different
tracks, then the drive can immediately begin seeking to the second
track after the completion of the first command.

			---  Truck