Hi, Jim.

On Fri, 11 Apr 1997 12:53:58 EDT, Jim Phillips wrote:
|What I want to know is "Is the file on tape an identical copy of the one on
|disk?" not just "Is the file restoreable/readable?".  Is this what RoadRunner
|does?

RoadRunner's VALIDATE function reads the data from the tape (watching for
tape read or other errors) and processes each block read from the media to
confirm that it is a consistent and properly formatted media block in the
right place on tape for that block.  We number media blocks and check those
numbers as we read the blocks back so missing blocks or out of sequence
blocks are checked as well.  Any discrepancies are reported.

While this does not check that the data on the tape will match the data on
the disc when restored, it does read every block and confirm that the parts
of the block which RoadRunner depends on are intact.  Since the tape drive
can't tell what parts of the media blocks are user data and what are RR
data, if there are data errors not detected by the drive, then they should
(at least sometimes) hit the RR parts of the media block and generate errors
when the tape is read (either for validate or restore).

Therefore, consistently validating your backups *with no errors* implies
that the user data on the tapes on that drive are also accurate.  However,
if you regularly have errors which are detected by RoadRunner, then you
may also have errors which have gone undetected.  In general, I recommend
that at least weekly you validate a backup.  Usually it makes sense to
validate a weekly full backup if you don't have time to validate every
backup.  As long as there are no errors on the validates, then there are
probably no errors on the data on those tapes and also probably no errors on
other tapes created on that drive(s) for comparable (i.e., brand, storage,
usage, and age) tapes.  Be careful not to extrapolate that there are no
errors on daily backups (where the tapes get reused frequently) because
there are no errors on your weekly backups (where perhaps the tapes are only
reused monthly).
--
Jeff Woods
[log in to unmask] at Unison Software
[log in to unmask]   at home  [PGP key available here via finger]

"Many people would sooner die than think.  In fact they do."
      --  Bertrand Russell