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Re: backup archive format saved to disk

Douglas Tutty wrote:
> The idea is that a format with built-in error-correcting would scatter
> the redundancy around the disk so that if a few blocks are bad, the data
> can still be retreived.  

Point taken.

> Even raid1 doesn't accomplish this.  With raid1 and two disks, if both
> disks have bad blocks appear, even if they are on different spots on
> each drive, as far as I can tell raid1 can't create a virtual pristine
> partition out of several damaged ones.  

Question: how likely is it that both disks develop bad blocks, while
none of them is damaged? I'm no expert on this, but I guess a better
strategy might be to rotate backups on two disks, and use (and check:
fsck and smartctl) them reguarly.

Leaving two identical disks in a bank vault for years and taking them
out, I guess the probabilty is rather high that either both work
perfectly or both are damaged and can't be read at all. There might be
cases in between, where your approach would make sense, but I doubt
there are many. Indeed I twice had a disk were one partition (or part of
the disk) was physically damaged, but it was possible to read data from
another partition, but most of the time the state of the disk is digital
(all works or nothing works). In one of the two cases a large part of
the disk (at the end of the cylinders) was damaged as far as I could
tell, so even error correction probably would not have saved data in
that region (for the second case, I can't tell).

> If par doesn't do what I need and I can't find an alternative, I'll just
> write my own, modeled first in python, then done in Fortran77 for speed.
> If I go to all this trouble, I'd probably throw in AES for good measure.
> It would make a fun project but I hate reinventing perfectly good
> wheels.  Then again, I know people who jump out of perfectly good
> airplanes.  Go figure.

As said I'm no expert on this, but I think the effort might be better
put elsewhere. Your approach gives some advantage in a special case of
failure, ie. if a disk has single points of failure, but doesn't add
security in case the whole drive goes bad.

On debian there is a program 'dvdisaster' [1] that adds error correction
data to cd and dvd images, so that potentially one could reconstruct
partially defective media. Since the program ignores the filesystem,
maybe you could use it or modify it for your needs. This approach is
based on the assumption that a cd is rather likely to be partially
damaged by scratches etc. IMHO it only makes sense, if the likelihood of
_partial_ damage is large enough to justify the effort. This might not
be the case for hard disks.


[1] http://www.dvdisaster.com/

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