[lug] recovering a tar file that spans CDs

[Bear Giles]

It's possible, but I don't know the tools you need.  (I've played around 
with archives but it was all my own code.)

Tar itself is very simple -- it's just a long sequence of 512 byte 
blocks.  One header block, then data block, then another header block, 
etc.  No per-archive headers and footers like zip files (for better or 
worse) so you could start scanning anywhere and just look for the next 
header.

Unfortunately tar doesn't directly support compression -- you just 
compress the stream.  Most people don't bother resetting the compression 
engine on every header so it's harder to recover from a problem -- you 
have to find the reset and then scan for headers.

Zip files, in contract, compresses each file individually so recovery is 
easier.

I don't know the details of how cdbackup/restore work -- it might have 
decent recover tools built into it, or an external index -- but 
hopefully this will give you a little more insight into your options.

Kenneth D Weinert wrote:
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> OK, so now that I know I've made a mistake, I'd like advice on how to
> recover (if possible) and what might have been a better option (for
> the future.)
>
> I used tar and cdbackup to create backups of images from my hard
> drive. The resulting tar file now spans 9 CDRs.
>
> Unfortunately, I consistently get an I/O error on disk 3 when I try to
> use cdrestore to grab the files back.
>
> This probably happened back in the days of spanning tapes instead of
> disks, but I'd just like to know if it's possible to recover in the
> middle of a disk set when using tar.
>
> As a separate point, anyone have a better scheme for creating backups?
> I don't mind saving to CD/DVD, but perhaps one file that spans disks
> isn't the best choice, but I'd prefer to not have to sort out ahead of
> time exactly which files will fit on each disk.
>
> Thanks for any info/pointers/FMs to R/etc.
>
> - --
> Ken Weinert
> http://quarter-flash.com
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