John, this is a very important topic. I just finished writing a column about it for Security Technology Exesutive magazine.
One client of mine experienced such a problem earlier this year. The system was on an APC Smart-UPS 1500, and the server was running the PowerChute software set to perform an orderly shutdown on the server. However, the storage array was a separate computing box (iSCSI across a network switch) and was not set for orderly shutdown.
When the power loss exceeded the 2 hour battery time (hard drives can take a lot of power), the storage array shut down with some resulting disk corruption.
During the service call, the integrator's tech said that there wasn't a way to shut down the storage array. "We contacted the vendor and you can't do it." I contacted the same vendor, who sent me their application note on how to do it. It's a 5-minute configuration action. They have built-in support for that particular UPS.
The integrator was no longer under contract due to other deficiencies on the job, and so the new integrator will take care of it.
In another case this year, with a Genetec system, an Archiver server (recording server) drive failed and the index database got corrupted, and some of the video could not be found. Well, the video was still on the drive, in a folder structure by year, month and day, with files that have the timestamp in the filename. Genetec has a utility that finds these "orphaned" video files and brings them back into the database.
I know of several VMS systems that store the video data in separate files outside of the indexing/search/alarm correlation database.
From other experience as well, I'd say that "power loss with no backup power" and "hard drive loss" are the two primary causes of database corruption. But an experienced integrator can usually recover the data for a VMS system with a well-designed database and video file structure.
There is no excuse for losing video due to power failure. If it is important enough to record and retain the video for some period of time, its important enough to spend $900 or so on a good UPS.
COMPLEX STORAGE SYSTEMS
But if you have a video SAN, or a complex storage array, you have to make sure that all the elements will perform an orderly shutdown and orderly startup in the right sequence. Some systems will require you to shutdown the application, then SQL server (which can take a few minutes), then the operating system, then the storage array or Video SAN. Startup is usually in the reverse order--but you have to test because sometimes startup times for various elements different from the shutdown times. If on startup a raid array has to rebuild that can slow down the overall startup quite a bit, and you can think there is another problem to address, but it is likely a self-healing one.
You have to make sure that you use the UPS software to shut down at the point where you have plenty of time left for all shutdown actions. You have to have the Video SAN elements (switch included) on the UPS as well, and this can significantly shorten the battery uptime from what you might think. Accurate power calculations are important, especially when a Video SAN is involved.
All this has to be tested out before you can walk away from it.
We usually shut down the video system immediately upon power outage, using the UPS only to support an immediate orderly shutdown. Unless the network and cameras are all on emergency power, there is no sense having the server up, unless you need emergency access to recorded video, in which case you can manually start it up for a short session.
MULTIPLE OUTAGE VULNERABILITY
The charging time for a UPS battery can be quite long, and is longer the bigger the UPS battery capacity is. So if you use up most of the battery power and then shut down, you can be vulnerable if a second outage occurs before the recharge can take place. You may not have enough time for the second orderly shutdown. This is mostly a consideration with large and complex storage systems. But even small systems can have this vulnerability if their battery capacity is small.
It pays to have a good understanding of the history, nature, and duration of power outages when designing battery backup power. It causes problems with NVRs and DVRs as well as server-based recording.
I think its an issue that is ignored in many video system projects, especially small ones or NVR/DVR projects, where the customers think of the units as self-contained systems. VCRs could lose power without losing video on a tape. So it's natural for that consideration to carry over into DVR and NVR deployment.
Many integrators install VMS systems with no attention to backup power. You'd be surprised how often this is the case.