Re: Why database is corrupted after re-booting

On 26 Oct 2005, at 19:43, snacktime wrote:

>
> I remember a few months back when someone hit the emergency power
> switch to the whole floor where we host at Internap.  Subsequently
> the backup power system had a cascading failure.  Livejournal, who
> also hosts there, was up all night and into the next day restoring
> their mysql databases after a bunch of them were corrupted.  I
> believe they had write cache turned on.
>
> Of course our postgresql servers on scsi drives came right back
> up.  If it wasn't for a couple of servers that won't reboot
> automatically if the power goes out I wouldn't have even had to go
> down to the data center.
>
> Chris

I don't know about this you know. Power failures can cause seriously
random failures on most PC hardware. A few weeks ago we had a RAID 1
(fsync on, caching off, battery backed raid controller etc) system
get it's RAID partitions gets totally fried by a power failure. My
suspicion is that if the power failure isn't a particularly fast one,
(e.g. you overloaded a fuse somewhere, fuses are insanely slow to
fail compared to alternatives like MCBs) then your RAID card's RAM
will get corrupted as the voltage drops or the system memory will
resulting in bad data getting copied to the RAID controller as RAM
seems to be pretty sensitive to voltage variations in experiments
i've done on my insanely tweak-able desktop at home. I would of
though ECC probably helps, but it can only correct so much.

Of course I'm not an electrical engineer (although my friend is a
member of IEEE and he seemed to agree it was a possibility) doesn't
the possibility of this kinda make things a bit more complicated and/
or expensive to maintain data integrity during a power failure?