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What is the purpose of the battery on a RAID Controller Card?

L3T
L3T asked
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Last Modified: 2013-11-05
Ok, I have a new client that has a Dell Poweredge 2600 server with a Dell PERC 3/DC RAID Controller w/128MB Cache.  I noticed that his RAID card doesn't have a battery.  I did some research and found that a battery is optional on this card.  This got me to wondering.  What is the purpose of the battery on a RAID controller? And do you really need one?

I should also mention his system is attached to a very good uninterruptible power supply (UPS).
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Lee W, MVPTechnology and Business Process Advisor
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The RAID configuration is typically stored in two places - on the disk drives themselves and in NVRAM on the RAID controller.  If you don't have a battery on the controller, it typically loses the configuration.  As long as the disks hold the configuration information, you will keep running, but if you lose a disk and try to rebuild, you'll be in trouble because the NVRAM on the controller will be empty and the controller won't know how to rebuild the RAID set.
L3TNetworking Engineer

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chafer, am I correct in assuming from your answer that as long as I have power going to the server and 1 drive goes down, I can still rebuild the RAID?

Commented:
It depends upon the RAID controller.  Power to the server and your UPS are really irrelevant.  The RAID battery is a completely separate issue.  I have seen servers lose their entire RAID configuration by losing one disk if the RAID battery was dead.  I have also seen others that were able to interpret the data off the disk and rebuild.  Since the battery is optional on your card, I suspect that you will be ok if a disk dies, but I wouldn't want to bank on it.  If it were a dead battery on a card that had to have one, I'd be more concerned.

retired saggar maker
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Lee W, MVPTechnology and Business Process Advisor
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Wow... I'm quoted in the selected answer but don't share in the points...
andyalderretired saggar maker
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I tend to agree; also I haven't seen two contradictory answers selected before except in the P&R or Politics zones. The battery is either for NVRAM or it is for cache, it can't be for both so either chafer's or my answer must be wrong. Still, I don't like to upset people who have just joined.

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