Re: backup archive format saved to disk

[Mike McCarty <Mike.McCarty@sbcglobal.net>]

Ron Johnson wrote:
> > Ron Johnson wrote:
> >
> > > RAID is *not* for archives!!!
> >
> > RAID was not designed for archives. I can see no reason why
> > it wouldn't work for that. RAID 1, for example, is simply
> > making two (or more) copies of the data. Are you saying that
> > making more than one copy of a backup is not a reasonable
> > approach?
>
> [snip]

[I wrote]

> > The main advantages would be that one would essentially
> > have burst error correction of the size of the disc
> > (this being, with the FEC on the disc, if any, an
> > interleaved code in effect), which is enormous, indeed,
> > and economy in storage over using multiple copies, as
> > illustrated above.
>
>
> I'd only trust "RAID archiving" if the controller and a rescue CD
> were also stored in the "archive location" along with the hard drives.

I gave two examples of RAID archiving which required no
special controller, and which would need to special rescue CD
to use. You snipped those, and didn't answer my question.
I'll ask the question again, and then I'll supply you with
a simple software RAID implementation which I wrote in
less than 15 minutes.

RAID level 1 is simply multiple copies. Do you think that
making multiple copies of the backup is an unreasonable
way to protect data from loss?

One way of doing RAID 5 with three discs is to write data
to each of two discs, and write the bitwise XOR of the
data on the two discs to a third. This requires no special
controller, it simply requires a tiny program. Here's one
in completely portable C:

#include <stdlib.h>
#include <stdio.h>

int     main(void) {
    FILE    *Input1, *Input2, *Output;
    int     Chr1, Chr2;

    Input1 = fopen("disc1.raw","rb");
    Input2 = fopen("disc2.raw","rb");
    Output = fopen("disc3.raw","wb");
    if (!Input1 || !Input2 || !Output)
        fprintf(stderr,"unable to open files\n"), exit(EXIT_FAILURE);
    while ((Chr1 = fgetc(Input1)) != EOF && (Chr2 = fgetc(Input2)) != EOF)
        fputc(Chr1 ^ Chr2,Output);
    if (Chr1 != Chr2) {
        if (Chr1 == EOF)
            Input1 = Input2;
        while ((Chr1 = fgetc(Input1)) != EOF)
            fputc(Chr1,Output);
    }
    return EXIT_SUCCESS;
}

Code compiled but not tested. The files disc1.raw and disc2.raw
are the inputs, disc3.raw gets written. This program works both
to create the image, and to recover from a fault in one of the
discs. The shorter disc image is padded with bytes with all bits
off to fit the length of the longer. For recovery this would
need some change, of course, to truncate the output as necessary.

This program has, of course, no error checking or other fancies.
It is a proof-of-concept prototype only. But it shows just how
simple a RAID system can be.

Mike
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