Re: disk transfer time

Mark J. Dulcey (
Sun, 8 Oct 1995 12:07:36 -0400 (EDT)

> On an good disk these days, a seek takes .000008 sec.
> a 8k transfer on a 5meg/second disk takes .0016384 sec.
> Doing an average seek between each write would only slow you down from
> 5meg to sec to 4.86meg/sec
> The seek time is eaten by the transfer time.

Wrong. You confused MILLIseconds with MICROseconds.

Disk seek times are given in milliseconds. So that seek time (average;
the maximum is typically two to three times as long) is .008 seconds, not
.000008 seconds. This is five times as long as the transfer time. So
seek time is the dominant factor in small transfers, not the transfer rate.

Of course, few disks can manage a 5MB/second transfer rate unless the
data is already in their cache (and, if the data IS in their cache, EIDE
and Fast SCSI-II drives can manage 10MB/sec. or thereabouts). 1-2MB/sec. is
more typical, so the transfer time in your example would actually be in
the neighborhood of .005 seconds -- still faster than the seek time, but
not by as big a margin.