Re: Two hard drives --- what to do with them?

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