Date: Tue, 14 Jan 92 13:49:09 +0100
   From: tthorn@daimi.aau.dk

   Which brings me to the question I've always wanted to ask:
   How do you use FFS on a SCSI disk? Given that SCSI disc's use
   logicial sectors and automatic bad sector remapping, it doesn't
   make much sence to talk of cylinder groups. Is it possible
   to avoid the logicial mapping?

The short answer is: with great difficulty, and not without a lot of
success. 

Usually, SCSI disks will generally keep logical sector whose block
numbers which are close together in the same general area of the
physical disk, but this is not guaranteed.  Depending on how automatic
bad sector remapping is done, the revectored blocks may be at the far
end of the disk, or scattered through out the disk.

So the FFS algorithms can be somewhat useful on SCSI disks by
distributing files across the entire disk, and by reducing (somewhat)
disk head activity by keeping files in the localized area wrt logical
sector numbers.  But of course it will not be as successful as when the
kernel has a much better idea of where blocks will end up on the
platters.

Another thing which comes up with SCSI disks is that the elevator sort
algorithm (which sorts the disk blocks in the request queue in an
attempt to reduce head movement) will also not work as well as
traditional disk controllers, again because there's no guarantee that
the logical sector numbering has any relationship to the physical blocks
(although there usually is some).  However, there is usually some cache
memory on the SCSI controllers on the disk drives themselves to help
this situation a bit.

Fortunately, the CPU time required to do the elevator sort and the FFS
placement of disk blocks is small enough that it gets swamped by the
time to do the disk I/O.  So even if the elevator sort and the FFS don't
help as much for SCSI disks, it shouldn't cost that much to use the
algorithms in any case.

						- Ted