Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

This is probably a dumb question. I recently purchased a couple of refurbished HP SB40C storage blade for our HP Bladesystem c3000 enclosure. Each comes with an HP Smart Array P400/256MB SAS Controller and a 4.8V NI-MH cache raid battery.

The smart array utility warns me about a low charge on the battery for each storage blade. Could it be because they probably haven't been turned on for a while just sitting around in some storage room? I mean of course batteries will drain out on their own, but I'm assuming this is the case rather than a coincidence that both batteries are dead. So just what would most people assume in this case?

Also, how would it affect my array if I use the controller without batteries?

PS: I cant be sure because my enclosure doesn't have enough power supplies (arriving soon) to turn on 4 blades and hence possibly charge the batteries. I have to make a quick decision on whether the order new batteries ASAP or count on their simply needing to recharge.

Thanks!

share|improve this question
up vote 2 down vote accepted

You're correct in that the warning is the result of the systems being offline for sometime. Power everything up and they will regain their charge.

In the case that the batteries are dead (unlikely), the impact to array performance is that the array accelerator (write cache) will be disabled. This results in low write performance. There is a mechanism in recent P4xx firmware to override this action, allowing write cache even in the event of a mission or failed battery/flash unit, but it carries consequences of data corruption if you experience total power loss on the blade or blade enclosure.

share|improve this answer

The batteries are an item that fails over time. Their purpose is to provide power to the RAID contoller in the event of a power outage, to give enough time to write data that is in the buffer to the platters.

share|improve this answer
    
thanks man, and i'll go with our guts and presume them functional – 3a2roub Feb 13 '12 at 16:38

The battery is there to preserve data in the cache in the event of a controller power failure. It allows data to be written to persistent storage (disks) rather than being lost and therefore causing corrupt/incomplete data on disk.

This battery backed cache behaviour is called 'write back' mode whereby the controller sends the system a write acknowledgment as soon as data data hits the cache (rather than waiting for the disks to write). This provides a significant write performance increase especially when dealing with small writes.

However, should the cache be in an unsafe mode such as its battery backup being dead then the controller should operate in 'write through' mode which is a direct commit to disk operation where the operating system is not notified of a successful write until the physical disks have actually written it.

If you run write back cache mode without battery and the controller does loose power, you will potentially end up with a bunch of half written files, with the other half gone for good. This = corrupt data.

share|improve this answer

Low charge on the battery of HP Smart Array P400/256 Controller will lead to Hard Disks failures, especially hot plug SAS.

share|improve this answer
    
Do you have any evidence for this contention? – MadHatter Aug 17 '15 at 13:57
    
No, but experience – Idreesf Aug 17 '15 at 14:32
1  
You are incorrect in this assumption. An empty battery / low charge may result in data loss/corruption if using the controller in write back mode when controller power fails, it will not damage your disks. – tomstephens89 Aug 17 '15 at 14:33
1  
Naw, this is wrong. – ewwhite Aug 17 '15 at 14:36
    
I'm not denying the value of experience, but you have to be careful what you learn from it, and specifically not to confuse correlation and causation; just because you had a couple of HDDs fail when batteries failed doesn't mean battery failure causes HDD failure. If you had 2000 arrays for three years, which between them experienced 450 battery undervoltages, and 90% of your HDD failures occurred in arrays in which batteries had gone undervolt in the past 7 days, that would be convincing experience (at least to me). Do you see the difference? – MadHatter Aug 17 '15 at 15:26

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.