[Dovecot] Maintaining data integrity through proper power supplies (slightly referencing Best filesystem)

[Rick Romero]

>> 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