Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!lll-lcc!ames!ucbcad!ucbvax!rtpark.rtp.ge.COM!rlb From: rlb@rtpark.rtp.ge.COM (Bob Boyd 8*565-3627 13-Jul-1987 0215) Newsgroups: comp.os.vms Subject: RE: /NOCRC on BACKUPs -- DON'T DO IT!! Message-ID: <8707130634.AA13064@ge-rtp.GE.COM> Date: Mon, 13-Jul-87 02:34:34 EDT Article-I.D.: ge-rtp.8707130634.AA13064 Posted: Mon Jul 13 02:34:34 1987 Date-Received: Sat, 18-Jul-87 11:12:02 EDT Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 53 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 POB 13049, MS 7T3-01 GE DIALCOMM: 8*565-3627 RTP, NC 27709-3049 GE DECnet: RTPARK::RLB