First, consider using 'tar' instead. It doesn't need tape statistics
  or require you to unmount the filesystem you're backing up.

  In IRIX 5.3 and later, 'dump' allows you to specify tape capacity directly
  with the "C" argument (e.g. 'dump 0uCf 2m /dev/tapedevice /filesystem' for a
  2G tape), so you needn't fool with c, d and s.

  If you're using an older 'dump', or just like extra work, do like Dave
  Olson <olson@sgi.com>: When using drives with no "inter-record gaps"
  (i.e. almost every type except 9-track), use the c option, and the
  formula

	capacity in bytes = 7 * densityvalue * lengthvalue

  Round down a bit to be conservative (allowing for block rewrites,
  etc.). Keep the density under 100000 to avoid overflows in the
  capacity calculations.  Thus, for a DAT drive with a 90 meter (120
  minute) tape with 2G capacity one might use

	2*10^9 = 7 * 47619 * 6000

  or, rounding down,

	dump 0csd 6000 47000

  For older tape types, see dump(1M). Note that 1G of tape capacity is
  10^9, not 2^30, bytes. The dump(1M) manpage says the latter in IRIX
  4.0.5H, but it is wrong.