sdm7g@dale.acc.Virginia.EDU (Steve D. Majewski) (10/08/90)
In trying to make a copy of a tar tape with dd, the copies don't seem to
be exactly the same as the originals.
The original, I can loop thru via: "dd if=/dev/nrst0" or "tar -tvf
/dev/nrst0" and at the end, the tape is properly positioned at the start
of the next file.
Reading the copy, the tape sometimes ends up positioned at the end of the
current file, rather than the beginning of the next.
"mt status" shows:
Archive QIC-150 tape drive:
sense key(0x0)= no sense residual= 0 retries= 0
file no= 3 block no= 17506
I assume that this has something to do with padding and some magical
combination of "bs=" and "conv=block(,sync?) will fix it. Does anybody
know the right magic ? ( I assume it is padding of blocks on the tape,
because there doesn't seem to be any OTHER problems, but I'm not sure how
to verify it - I don't have room enough to diff 2 copies on disk. )
[ This is on a Sparcstation I / SUN-OS 4.03 / Archive QIC-150 tape ]
Steven D. Majewski University of Virginia Physiology Dept.
sdm7g@Virginia.EDU Box 449 Health Sciences Center
(804)-982-0831 Charlottesville, VA 22908jms@tardis.tymnet.com (Joe Smith) (11/04/90)
In article <1990Oct7.230945.3387@rice.edu> sdm7g@dale.acc.Virginia.EDU (Steve D. Majewski) writes: >In trying to make a copy of a tar tape with dd, the copies don't seem to >be exactly the same as the originals. > >The original, I can loop thru via: "dd if=/dev/nrst0" or "tar -tvf >/dev/nrst0" and at the end, the tape is properly positioned at the start >of the next file. > >Reading the copy, the tape sometimes ends up positioned at the end of the >current file, rather than the beginning of the next. That's strange. Every time I've tried reading an original tape with multiple tar saves, it always stops before the start of next file. For instance: tar cvf /dev/nrst8 dir1 tar cvf /dev/nrst8 dir2 mt -f /dev/nrst8 rew tar xvf /dev/nrst8 # restores dir1 tar xvf /dev/nrst8 # always says "unexpected EOF" tar xvf /dev/nrst8 # restores dir2 This matches the man page for tar: If there are multiple archive files on a tape, each is separated from the following one by an EOF marker. tar does not read the EOF mark on the tape after it finishes reading an archive file because tar looks for a special header to decide when it has reached the end of the archive. Now if you try to use tar to read the next archive file from the tape, tar does not know enough to skip over the EOF mark and tries to read the EOF mark as an archive instead. The result of this is an error message from tar to the effect: tar: blocksize=0 This means that to read another archive from the tape, you must skip over the EOF marker before starting another tar command. You can accomplish this using the mt(1) command, as in "mt -f /dev/nrst8 fsf 1" So, I can't explain why you luck out on your originals. Joe Smith (408)922-6220 | SMTP: jms@tardis.tymnet.com or jms@gemini.tymnet.com BT Tymnet Tech Services | UUCP: ...!{ames,pyramid}!oliveb!tymix!tardis!jms PO Box 49019, MS-C41 | BIX: smithjoe | 12 PDP-10s still running! "POPJ P," San Jose, CA 95161-9019 | humorous dislaimer: "My Amiga 3000 speaks for me."