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