I think if you have a large mail server and zero power protection,
you've got much larger problems to worry about than write-behind
caching on your disk drives... my servers have never (in my
memory) experienced a catastrophic power failure, because they're
too easy to avoid.
In the specific case of email; you want to make certain guarantees,
and having data written to non-volatile storage is one of them.
As for power-failures, given enough time and enough hardware you will
get them, even if your machines are dual or triple powered of diverse
UPSs or -48V powered; it still is possible and _will eventually
happen_ that something further down the line like the motherboard will
fry or whatever.
People who assume that a small-window is small enough and decide that
is 'good enough' are dangerous :)
--cw
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