rlb@rtpark.rtp.ge.COM (Bob Boyd 8*565-3627 13-Jul-1987 0215) (07/13/87)
There have been several discussions on the net and at DECUS symposia about
the issue of whether or not to apply cpu based CRC checks to backup tapes.
The whole point of the CRC checks is that there is no other way to get
end-to-end integrity checking. ThCRC check that the tape drive is
wonderful to say the least, but it is only good for checking the data as
it came to the formatter hardware. If there is a fault in the transmission
hardware, the drive will crc bad data and it will look as if you got
a clean backup.
A backup without CRC checks or an equivalent is only as reliable as the
cpu to tape drive communication path. There are several people I have
talked with who have personally been bitten by bad connections and have
paid for not doing the extra checks.
My understanding of what the RAXCO package doesn't do and what VMS
BACKUP/NOCRC amounts to are about the same --- you don't have end to
end data integrity. This is sad but true. If you want (or MUST have)
integrity (due to corporate orgovernment regulation perhaps?) then
it is going to cost extra (NLA0: is a VERY FAST backup device -- no need
to use /CRC or /GROUP -- it is extremely reliable -- GUARANTEED results !!
8^) 8^) )
(*) (*)
> (READ: BIG SMILEY FACE!)
\__/
In other words, if you want to keep your tapes for a while and be more
certain of your ability to restore files from them, the CRC checks are worth
the extra cpu time to put them in. This is especially true as you move
into the 85xx and up cpu range. If you really want to be sure of a backup
use CRC and VERIFY -- I STRONGLY recommend this in the case of a standalone
backup of your system disk when you are about to reload the disk from tape
in order to compress the disk.
Please consider what the cost will be if you are unable to restore from
your backups. The type of computer operation you are running may have
a lot to do with your choices. A financial operation would be insane
to leave out /CRC. A personal uVAX -- might be no big deal -- just
a question of cost in time to rebuild things if the backup turns out to
be useless.
My information on this comes from discussions in public and private with
DEC tape drive and BACKUP developers.
Bob Boyd Usenet: rlb@rtpark.rtp.ge.com
DECUS VAX SIG Large Systems SIG
Commercial W/G Chair Special Projects Coordinator
GE Microelectronics Ctr. Voice: (919)549-3627
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