multiple dumps on an exabyte

[Paul Allen]

In article <7069 at brazos.Rice.edu> celvin at ee.surrey.ac.uk (Chris Elvin) writes:
>X-Sun-Spots-Digest: Volume 9, Issue 138, message 3
>
[about a scheme for putting multiple dumps on an Exabyte tape with a 
 table of contents at the fron of the tape.]
>
>First experiments using dd to dump files to tape and repeatedly re-dump
>the first (fixed length) file show that simply rewinding the tape and
>writing a fixed length file at the front of the tape is not enough.
>Anybody got any suggestions????

If you just dd a file onto the front of the tape, the rest of the tape
will no longer be accessible.  One of the vendors of 8mm subsystems at the
Sun User Group meeting last year told me that their software works
something like this:  The table of contents at the front of the tape is
updated by a program that opens the tape, writes the update, and then
forward skips past the tapemark without closing the tape.  

The program I wrote to do multiple dumps on a tape uses syslog to record
the info needed for doing a restore.  My newsyslog script grep's the tape
log records out of the syslog file each day and appends them to a
permanent backup log.  Then, when a restore is needed, I simply do an
appropriate grep to yield the tape number and how far to forward skip.  In
the event that the log file is lost, each dump file begins with a
1024-byte header containing the identity of the dump.  This header must be
stripped off at restore time, but it permits reconstruction of the backup
log in an emergency.  I do unattended daily dumps of nearly 20Gb of disk,
and am fairly happy with how things are working.

Paul Allen
pallen at atc.boeing.com