Re: Blockbusting news, this is important (Re: Why are bad disk sectors numbered strangely, and what happens to them?)

[Krzysztof Halasa <khc@pm.waw.pl>]

John Bradford <john@grabjohn.com> writes:

> Besides, are you positive that you always got the correct data off the
> disk?  See the discussions about hashing algorithms - maybe the drive
> simply returned data that had an additional bit flipped and wasn't
> identified as bad.

One bit? No chance. The same as with ECC RAM - one bit error will always
be detected.

>  If you are having to try over 1000 times from
> userspace, the drive is in a bad way.  You shouldn't really make
> assumptions that you do usually, (that the error correction is good
> enough to ensure bad data isn't returned as good data).  If you are
> recovering data from a spreadsheet, for example, the errors could go
> unnoticed, but have catastrophic results.

Then you have to abandon using any hard drivers. Or computers at all.
Well, mirrors (with read-and-compare) are probably good enough for you,
but it has to be done at application level.

> Of course you will - it's remapped, the data isn't overwritten!  You
> may need more advanced tools,

= in practice, it's lost. Have you seen such tools?

> but you can still seek the heads to that
> part of the platter and get data from the head-amp.  Just because you
> couldn't use your simple method anymore is real reason to argue
> against fixing the problem.

against _changing_ the problem (it doesn't go away), breaking things
which are now sane.

> This may be more sensible, but not for the reasons you are suggesting,
> and not in the way that you are suggesting.

Then note that a drive can be temporarily unable to read most of the
data - due to, say, incorrect supply voltage or very high level of
electromagnetic interferences.

Would you like to trash _all_ your data in such case automatically?

> Suspect drive?  Bin it.  Do you really not value your data enough to
> do that?

Do you really not value your data enough to mark it as inaccessible?

If it comes to non-standard recovery then you should rather go for
backups.
-- 
Krzysztof Halasa, B*FH