Need assistance with afio problem.

Anthony DeBoer adeboer at gjetor.geac.COM
Wed May 29 03:27:28 AEST 1991


In article <450 at yonder.UUCP> michael at yonder.UUCP (Michael E. Haws) writes:
>In article <29927 at hydra.gatech.EDU>, ken at dali.cc.gatech.edu (Ken Seefried iii) writes:
>> In article <447 at yonder.UUCP> michael at yonder.UUCP (Michael E. Haws) writes:
>> >
>> >It is my impression that you are suggesting that I never keep compressed
>> >data on my hard disk, since this data would be subject to the same 
>> >potential problem when trying to recover it from a tape archive.  Or have
>> >I missed something?
>
>Agreed, but the point I was trying to make was that when the compressed file
>from the hard disk is now backed up to tape the chances of recovering
>it are no better(or worse) than the file compressed during the
>backup process.  
>
>Since I was told that it was not safe to compress files during backup, I 
>assumed that already compressed files would be just as susceptible to
>non-recoverability(is that a word?) from a tape archive.

The difference is in what you mean by "compress during backup".  The easy way
to do it is "find | cpio | compress | dd", to compress the whole backup in one
pass.  An error in the middle of the tape could screw up the decompression for
the whole remainder of the tape.  Compressing each file individually, though,
would make the tape look at if you went through your hard disk beforehand and
compressed everything.  (That's probably the "easiest" way of trying to
implement this; doing it properly is nontrivial.)  An error might screw up the
remainder of the file in which it lies, but provided you can read the data off
the rest of the tape cpio could be re-syncronized on the next header and be
able to recover the rest of the files on the tape.

There's nothing inherent about compressed files that makes them less
restorable from tape than any other file, except for the factor of not being
able to decompress past a glitch.  Other files, like executables, may also be
useless if they're corrupt.  On the other hand, of course, a compressed file
IS a much smaller target for a glitch to try to hit. :-)
-- 
Anthony DeBoer NAUI#Z8800 | adeboer at gjetor.geac.com   | Programmer (n): One who
Geac Canada Ltd.          | uunet!geac!gjetor!adeboer | makes the lies the 
Toronto, Ontario          | #include <disclaimer.h>   | salesman told come true.



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