On Sat, 25 Oct 2003, James Moe wrote:
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> On Sun, 26 Oct 2003 16:24:17 +1300, Mark Kirkwood wrote:
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> >I would conclude that it not *always* the case that power failure
> >renders the database unuseable.
> >
> >I have just noticed a similar posting from Scott were he finds the cache
> >enabled case has a dead database after power failure.
> >
> Other posts have noted that SCSI never fails under this condition. Apparently SCSI
> drives sense an impending power loss and flush the cache before power completely
> disappears. Speed *and* reliability. Hm.
Actually, it would appear that the SCSI drives simply don't lie about
fsync. I.e. when they tell the OS that they wrote the data, they wrote
the data. Some of them may have caching flushing with lying about fsync
built in, but the performance looks more like just good fsyncing to me.
It's all a guess without examining the microcode though... :-)
> Of course, anyone serious about a server would have it backed up with a UPS and
> appropriate software to shut the system down during an extended power outage. This just
> leaves people tripping over the power cords or maliciously pulling the plugs.
Or a CPU frying, or a power supply dying, or a motherboard failure, or a
kernel panic, or any number of other possibilities. Admittedly, the first
line of defense is always good backups, but it's nice knowing that if one
of my CPUs fry, I can pull it, put in the terminator / replacement, and my
whole machine will likely come back up.
But anyone serious about a server will also likely be running on SCSI as
well as on a UPS. We use a hosting center with 3 UPS and a Diesel
generator, and we still managed to lose power about a year ago when one
UPS went haywire, browned out the circuits of the other two, and the
diesel generator's switch burnt out. Millions of dollars worth of UPS /
high reliability equipment, and a $50 switch brought it all down.
