Dave, et al.

I apologize for propagating mis-information about AUDIT VOLUME.  However,
in my defense, I originally got this info from a developer.

On Wed, 21 Aug 1996 14:20:42 PDT Dave Cannon said:
>Paul apparently had an experience in which certain tape defects were detected
>by a Move Data command, but went undetected by an Audit Volume.   I don't
>have an explanation for why this occurred in this specific situation.

Yes, you are correct.  Awhile ago we had several bad 8mm tapes.  One of
the tapes was able to pass an AUDIT VOLUME successfully, yet a MOVE DATA
failed repeatedly with I/O errors.  I was surprised, because I figured
the code to read the data off of the tape was the same, in some low-level
subroutine.  As an explanation of why AUDIT was working, I was told that
an AUDIT did not actually read all of the file data, it just checked for
inconsistencies between the database information and the file information
on tape.  I was surprised at this at the time, but accepted it as an
explanation for what I was seeing.

>
>In general, however, the Audit Volume command is a much better way to
>check ADSM tapes than Move Data.  The Audit Volume command reads every
>byte of every file on the volume and checks control information which the
>server imbeds in the file when it is stored.  The same code is used for
>reading and checking the file as would be used if the file were to be
>restored to a client.  In contrast, Move Data simply copies the file
>from one volume to another.

I am delighted to hear that this is the intent of AUDIT VOLUME.  Now, of
course, I am really puzzled as to why an AUDIT VOLUME failed to detect
problems that a MOVE DATA ran into repeatedly.  Naturally, I no longer
have the bad 8mm tape to experiment with!

>
>While there may be exceptional situations in which Move Data detects
>defects that go undected by Audit Volume, the latter command is
>a much more effective way of verifying the integrity of files stored
>on an ADSM storage pool volume.
>
>Dave Cannon
>ADSM Development

Thanks for the info, Dave.
..Paul

Paul Zarnowski                     Phone:   607/255-4757
Cornell Information Technologies   Fax:     607/255-6523
Cornell University                 US Mail: 315 CCC, Ithaca, NY 14853-2601