[buug] Backups [Was: Trapped in Upgrade Hell]

[Rick Moen]

Quoting James Howard (buug at multivax.net):

> Perhaps you can explain.

Sure.  Your most-important application has data files that include
fnord.1 and fnord.2.  They were in a coherent state at the time of
Friday's full backup.  During Monday's work hours, fnord.1 got changed.
The state change was duly recorded in the Monday night incremental.

During the Tuesday workday, fnord.2 also changed.  During the night,
the state change of that file gets likewise recorded -- but the prior
night's alterations to fnord.1 do not.

On Friday, you attempt the series of restores.  You restore the Friday
tape -- fine.  Monday's incremental tape fails, in whole or in part.  If
it's even possible to restore the Tuesday incremental at that point
(which would depend on your backup software), your resulting on-disk
data would include the Monday night version of fnord.1 plus, for
fnord.2, the Friday file state patched in some fashion (depending on
your backup software) with the Tuesday state changes, which may or may
not even be an individually valid file.

If the resulting fnord.2 version _is_ a valid file, it's still out of
sync with the restored version of fnord.1.  Your applicaiton may not
load at all, or it may be worthless or corrupted results.

Or you could be lucky, and the application may say "Oh, I can figure
that out.  I guess."  And, if you're very lucky, it may even be right.

But you'd better not bank on it.

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