RAID battery-backed cache - necessary? (was: Linux-PowerEdge Digest, Vol 68, Issue 35)

Adam Nielsen adam.nielsen at uq.edu.au
Wed Feb 10 18:31:38 CST 2010


This is perhaps off-topic too, but I have always wondered...

> You might also want to look at getting a hardware RAID card or 
> daughterboard like the PERC-6i - these will allow you to set up a 
> RAID-10/50/60 that will stripe all data between two drives, giving you 
> another twofold speed increase. You probably want to make sure that your 
> card has battery backup if you care about your database - otherwise a 
> power cut can lose cached data rather painfully, even if you have a UPS. 
> If you're moderately paranoid, or your data is important, you should 
> disable on-drive write caching, as these never have battery backup - but 
> this will cost you some speed. (This is a software issue, though, and 
> won't affect your purchased configuration.)

I am curious as to why this type of battery-backed cache is important. 
The OS would do a large amount of caching (Linux can have a disk cache 
of many gigabytes) which I am sure would be far more effective than the 
small caches on many RAID cards.

Given that the OS, if configured properly, should provide the best type 
of caching possible, why is it still necessary to have RAID cache and 
on-drive cache?  Surely these would provide no additional benefit?

Anyway, just something I've often wondered about :-)

Cheers,
Adam.



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