[Dovecot] Maintaining data integrity through proper power supplies (slightly referencing Best filesystem)
Rick Romero
rick at havokmon.com
Wed Feb 2 18:36:20 EET 2011
>> If you have a proper-sized UPS, combined with notification from the
>>> UPS to the servers to perform orderly shutdowns - including telling
> >> the application servers to shutdown prior to the storage servers,
> >> etc. - doesn't that render the (possibly more than theoretical)
> >> chances of data loss due to power interruption a moot point?
> >>
> >
> > UPSs are a great help, but they are not failure-immune. They too,
> > can fail, and will fail. They may just suddenly switch off, or they
> > may fail to provide the expected duration of service, or they may
> > fail to operate when the reticulated power does fail. We can add
> > their failure rate into the calculations. I haven't any figures for
> > them, but I'd guess at 3 years MTBF, so let's say another 0.3 events
> > per year. We could redo the calculations above, with 1.5, now,
> > instead of 1.2 - but I don't think we need to, on this list. (Of
> > course, if we don't use a UPS, we'll have a seriously high event rate
> > with every power glitch or drop wreaking havoc, so the lost message
> > calculation would be much greater.)
> >
> > Daniel, I'm delighted but not in the least surprised that you haven't
> > lost a message. But I fully expect you will sometime in your
> > operation's life unless you use
> > (a) redundant equipment (eg RAID) with
> > (b) very minimal windows of vulnerability (which, following that
> > other thread, means a filesystem that does immediately write to disk
> > when it is asked to do so and, seemingly, not all high-performance
> > filesystems do).
> >
Just to add a note about power and 'knowledge' - I built my first
OpenSolaris server with a decent size ZFS array, re-using a 'retired' case
and power supply a couple years ago. It drove me crazy at first - I
didn't even have it in production and ZFS kept failing random disks at
random intervals. I happened to stumble across a post of another user who
had the same problem and it turned out to be a 'poor' power supply. Sure
enough, a brand new power supply 'fixed' the problem. Did I lose any data
in the past? I have no idea, maybe it was temp data, maybe it culminated
in a Windows crash or odd OS error. All I know is ZFS, in a round about
way, found a problem I would have never known I had. I love ZFS, it's
snapshots are the closest thing I've found to my beloved Novel's Salvage
command ;)
Rick
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