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- Path: sparky!uunet!zaphod.mps.ohio-state.edu!wupost!spool.mu.edu!agate!ucbvax!lrw.com!leichter
- From: leichter@lrw.com (Jerry Leichter)
- Newsgroups: comp.os.vms
- Subject: Re: Question about RMS and MSCP-pair [correction]
- Message-ID: <9301051701.AA12189@uu3.psi.com>
- Date: 5 Jan 93 15:49:33 GMT
- Sender: daemon@ucbvax.BERKELEY.EDU
- Distribution: world
- Organization: The Internet
- Lines: 30
-
-
- I wrote:
-
- By the time you get a "forced error", the data is gone. The forced
- error indicator is set on the REPLACEMENT block, not the original one,
- and I don't believe the corrupted data, such as it was, is even copied
- into the block.
-
- It's been pointed out to me that on at least some devices, the data IS copied,
- and people have successfully recovered it in cases where it's highly redun-
- dant (such as English text, for example).
-
- I don't know for certain whether the DSA spec REQUIRES that the possibly-
- corrupt data be copied to the replacement block, or simply ALLOWS it. If
- it does NOT require it, I don't know if any existing devices fail to copy
- the data.
-
- Note that, given the size of errors that the ECC on DEC's disks can correct
- successfully, there are going to be at least some 10-12 bytes bad in the
- block - and you have no idea where, or how many bytes have been damaged. It's
- quite possible that every byte in the block contains random mush. For
- anything EXCEPT natural-language text or source code, it's going to be very
- difficult to recover much useful information.
-
- In summary: The only SAFE assumption is that if you see a "forced error",
- the data in the block is completely gone. If it is very important to you
- and you have no other copies, you MAY be able to get some of it back - but
- don't rely on it.
- -- Jerry
-
-