Re: somewhat OT query on journalling

["Andreas Schäfer" <gentryx@gmx.de>]

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> Don't get too excited -- the transactions probably aren't done yet. Without those, no filesystem that claims to journal data is really any better than a filesystem which only journals metadata.  Even once 
> they are implemented (or even if they are already), applications have to support them directly.

Actually, I think transactions in a filesystem context are a bit
different from the transactions you know form databases. Generally
speaking, a transaction denotes a transition from one valid state to
another. This transition should either be performed completely or --
in case of errors -- performed not at all (a.k.a. "roll back").

Databases allow application defined transactions (i.e. the
application specifies when a valid state is being left and when one is
reached again).

IMHO for filesystems a transaction denotes the flushing of write
buffers. "metadata only" transactions/journaling mean that even after
a crash the file itself will be readable (and not pointing to
e.g. sector -4711 ...). "data _and_ metadata" now means that the
filesystem does also guarantee that the data itself is written
completely.

Please correct my if I got this totally wrong.

-Andreas


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