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From: OC.PEDHEM@CU20B.COLUMBIA.EDU (Guy Talbot)
Newsgroups: mod.computers.vax
Subject: Bitmap Problem
Message-ID: <12164217410.13.OC.PEDHEM@CU20B.COLUMBIA.EDU>
Date: Tue, 3-Dec-85 14:21:56 EST
Article-I.D.: CU20B.12164217410.13.OC.PEDHEM
Posted: Tue Dec  3 14:21:56 1985
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To: <INFO-VAX@sri-kl.arpa>
Reference:	28 Nov 85 02:10 +0100
		<#D14%DDATHD21.BITNET@wiscvm.wisc.edu>
Subject: Bitmap problem
We at Cooper Union recently had a bitmap problem on an RK07 without a
recent backup.  This particular pack was so badly damaged that it would
not mount even with /OVER=(LOCK). Since the data was important 
(isn't it always), we tried an interesting recovery procedure:

	1. Backup /physical the pack to tape:
$MOUNT DMA0:/FOR
$MOUNT MSA0:/FOR
$INIT MSA0: RECOVR
$BACKUP/PHY DMA0: MSA0:ADMIN.BCK/comm="The remaining blocks"
	2. Roll the Physical backup onto another blank and BAD'ded pack:
$MOUNT DMA1:/FOR
$BACKUP/PHY MSA0:ADMIN.BCK DMA1:/REW
	3. Errors about bad blocks not matching occured
		but as long as the bad blocks don't overmap
		what you need so what.
	4. MOUNT the new pack with /OVER=(id). 
	5. Backup file-by-file the new pack to tape
		PARITY errors will probably occur where bad blocks
		on the new pack overmap bad blocks on the old
		pack.
	6. If parity errors occur on any critical files repeat
		the procedure on another blank and BAD'ed pack.

This sequence allowed us to recover our critical data and recreate
the original pack.  We created the original pack from the file-by-file
backup of the new pack.  It probably best to re-BAD (or EVRAC) the pack after
after you have extracted the files as the BAD block info is probably messed
up.

					Chris Lent
					Care of OC.PEDHEM@CU20B
					CUPHOA::LENT (VAXmail)
					(203) 452-1522 (Ans. Machine)
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