24 Oct
2004
24 Oct
'04
9:26 a.m.
Linux has the Unix-standard mt command, too, which
allows setting
the blocksize the system device uses, either explicitly or to a
variable blocksize.
Ah... I have never used that facet of 'mt' before
It doesn't even work that way: by using
'mt' to set the block size,
you only set its *reblocking* factor.
This depends on the drive hardware and the driver. Some tape drives
really do have fixed hardware block sizes. Even if setting a variable
block size just causes the drive to do reblocking of some fixed on-tape
block size, it still looks like a variable size block to the host.
(Indeed, a classic half-inch tape can be thought of as having a fixed
one-byte block size which is reblocked in hardware according to control
information on the tape, though that's not a terribly useful POV.)
where usually, a length of 0 indicates an EOF (end of
logical file,
aka "first tape mark detected") or EOT (end of tape, aka "second
consequtive tape mark detected") situation.
If you are using real half-inch tapes, detecting EOT is hard. I've run
into tapes with a dozen consecutive tape marks and data after them;
with some hardware, if you keep reading you'll eventually run off the
*physical* end of the tape - I've done it, and on the less friendly
drives it's a mess to re-thread the tape enough to rewind it onto the
original reel.
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