A/UX® System Administrator's Reference Sections 1M, 7, and 8

mkfs(IM) mkfs(IM)
Whether or not a proto file is given, the rotational gap and the
modulus values can be specified within the mk f s command line.
The value of gap allows certain disk blocks to be treated as logically
contiguous even though they are not physically contiguous.
Specifically, those blocks that are gap blocks apart are treated as if
they are contiguous during reads and writes. By doing this, the
time delay between two consecutive reads or writes of blocks can
be accounted for and the disk media does not rotate beyond the location
of the next physical block. Rather than wait for the disk to
make a complete revolution before the missed block comes under
the read/write head once again, performance is better if alternating
disk blocks are treated as if they were contiguous.
The value of modulus is needed to help determine what blocks are
treated as logically contiguous. With each complete revolution,
some extra offset may have to be introduced besides the fixed
value gap. For example, for a gap value of 2 and a hypothetical 10
blocks per revolution, physical blocks 0, 2, 4, 6, and 8 would be
treated as contiguous. However, if this were continued throughout
the disk, odd-numbered physical blocks would never become accessible.
A modulus value of 10 corrects the mapping so that after
physical block 8 is mapped, block 1 is mapped, followed by 3, 5,
7, and 9.
Also note that modem hard disks perform a similar function internally
so that the operating system need not be encumbered with
the function of disk-block remapping. For all but a very few rare
cases (hard disks not sold by Apple and of old vintage), these
operating system facilities do not result in increased performance.
Even when optimization is possible, it cannot be achieved unless
you can determine from technical specifications for the disk what
values are needed.
The default values 1 for gap and 1 for modulus suppress the
remapping of disk blocks. The default values are used if gap and
modulus are considered illegal values or if they are omitted.
EXAMPLE
To make an 800 KB file system on a 3.5-inch floppy disk, use
mkfs /dev/rfloppyO 1600
This makes a file system on the floppy media referenced through
/dev/rfloppyO. The new file system extends for 1600 512byte
disk blocks (800 KB).
February, 1990
Revision C
5