> Right. This is very much the guarantee that RAID (non-zero) makes,
> except "other than disk hardware failure" is replaced by "other than
> the failure of two drives". RAID gives you that (very, very
> substantial
> boost which is why it is so popular for DB servers). It doesn't give
> you power failure assurance for much the same reason that PG
> (or Oracle,
> etc) can.
As far as I know (and have tested in excess) Informix IDS does survive
any power loss without leaving the db in a corrupted state.
The basic technology is, that it only relys on writes to one "file"
(raw device in that case), the txlog, which is directly written.
All writes to the txlog are basically appends to that log. Meaning that all writes
are sync writes to the currently active (== last) page. All other IO is not a problem,
because a backup image "physical log" is kept for each page that needs to
be written. During fast recovery the content of the physical log is restored to the
originating pages (thus all pendig IO is undone) before rollforward is started.
Andreas
