Nor was I bob :-) I was creating a scenario about tapes with status 84 or 86 and the ability to recover ALL data from those tapes :-)
Regards Simon Weaver 3rd Line Technical Support Windows Domain Administrator EADS Astrium Limited, B32AA IM (DCS) Anchorage Road, Portsmouth, PO3 5PU Email: [EMAIL PROTECTED] -----Original Message----- From: bob944 [mailto:[EMAIL PROTECTED] Sent: 05 May 2006 00:53 To: veritas-bu@mailman.eng.auburn.edu; WEAVER, Simon Subject: RE: [Veritas-bu] Re: Help! All Tapes From Scratch > I agree with your comments about throwing the tape away. However, can > we assume this scenario: > > Full Backup Friday of critical Server. Completed all ok, but > during the job a status appeared (Media Write Error, or > Media Position Error) although only once. > > Sunday Server Dies > > Come in Monday, and you can ONLY restore from Fridays Backup! Are you > telling me that its VERY unlikely Netbackup will restore ANY Data from > the tapes it used? Simon, I wasn't addressing your fried-robot situation earlier, so let me clarify. Backups that were successful are restorable unless the tapes were damaged. If the backups weren't successful (0 or 1), there is no data to restore. (The rest of this note is probably too long, too pedantic and too boring for anyone without a masochistic streak. You've been warned. :-) If you were referring to "losing data" in > > Amazing. So, in your environment, it's considered more > > cost-effective to risk losing data or fail another backup than to > > throw out a $50 tape? my point was to the nature of magnetic media[1]. A successful write doesn't guarantee that you can read that block later (maybe a bit of oxide flakes off, for instance). And that's best case. Now, when a tape has already demonstrated that it has flaws (the write error you mentioned above, for instance), we _know_ it has at least one problem spot and that tape is much more likely to cost you another backup versus a tape without known problems. Or, worse, the drive's retry logic gets a successful write on the 17th automatic retry, you think the tape is now "good," and next month you're trying to recover that payroll master file and that block just isn't quite good enough to read any more. Nobody's happy. So, since an uncorrectable write error (the only ones you're going to see since the drive/driver hide the self-corrected ones) means the backup job is a failure--no data is retained, do I want to use tapes with problem histories for that payroll master server? Or anything else? Since the _only_ reason to do backups in the first place is to be able to restore data--and if we're restoring, we must _need_ that data, I think not. - bob p.s. In another lifetime, I was involved on the vendor side with a customer who cooked his mainframe. Two different times. Wound up replacing the entire room full of equipment both times--the boxes that weren't flat dead on Monday were flaky and intermittent so everything got tossed. I don't know the specifics of your weekend incident, but I wouldn't trust anything in the room, hardware or media, without at least thoroughly testing it. 1. Especially sequential media (tape versus disk). There are many mechanicals involved--head alignment, tracking, wear, drag, dirt--and these change from drive to drive. The same type of drive may be built by different companies, or with different mechanical revisions, or firmware changes. The tape can stretch, wrinkle or get its edges damaged by use, temperature/humidity and improper storage. But the oxide... there's where the variables really are. It can flake, get scratched, pick up a thickness of crud, be worn down and just plain be manufactured with flaws. Always has, which is why mag media devices/controllers/drivers all have error compensation measures, including retry logic. You can write the same data to the same tape in the same drive and get different results. With a disk (and all start conditions identical), data X gets written to sector Y, every single time. With tape... all you can guarantee is the order of the data. There are gaps between blocks and between tape marks. A common drive/ctrlr/driver response to write errors is to back up, wipe out the failed block, erase a bit of tape and try again--now everything downstream is being written to a different place than before. Gaps are not all consistent. Some drives vary the tape transport speed to match the data rate... Lots of variables. Lots of possibilities for a bad tape to pass a subsequent test--or a "good" one to fail tomorrow. Some reasons to replace flaky hardware and media at the flaky stage--not waiting until it is flat down. And good reasons to make duplicates. This email is for the intended addressee only. If you have received it in error then you must not use, retain, disseminate or otherwise deal with it. Please notify the sender by return email. The views of the author may not necessarily constitute the views of EADS Astrium Limited. Nothing in this email shall bind EADS Astrium Limited in any contract or obligation. EADS Astrium Limited, Registered in England and Wales No. 2449259 Registered Office: Gunnels Wood Road, Stevenage, Hertfordshire, SG1 2AS, England _______________________________________________ Veritas-bu maillist - Veritas-bu@mailman.eng.auburn.edu http://mailman.eng.auburn.edu/mailman/listinfo/veritas-bu