On Tue, 2007-02-27 at 09:27 +0100, Peter Kovacs wrote: > I wonder if running a machine on a UPS + 1 hot standby internal PS is > equivalent, in terms of data integrity, to using battery backed write > cache. Instinctively, I'd think that UPS + 1 hot standby internal PS > is better, since this setup also provides for the disk to actually > write out the content of the cache -- as you pointed out. It's all about the degree of safety. A battery-backed cache on a RAID controller sits below all of these points of failure: * External power * Power supply * Operating system and with proper system administration, can recover from any transient errors in the above. Keep in mind that it can only recover from transient failures: if you have a long blackout that outlasts your UPS and cache battery, you can still have data loss. Also, you need a very responsive system administrator that can make sure that data gets to disk in case of failure. Let's say you have a RAID system but you rely on the UPS to make sure the data hits disk. Well, now if you have an OS crash (caused by another piece of hardware failing, perhaps), you've lost your data. If you can afford it (in terms of dollars or performance hit) go with the safe solution. Also, put things in context. The chances of failure due to these kinds of things are fairly low. If it's more likely that someone spills coffee on your server than the UPS fails, it doesn't make sense to spend huge amounts of money on NVRAM (or something) to store your data. So identify the highest-risk scenarios and prevent those first. Also keep in mind what the cost of failure is: a few hundred bucks more on a better RAID controller is probably a good value if it prevents a day of chaos and unhappy customers. Regards, Jeff Davis
