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I noticed when I installed the Dell OpenManage that the battery on my Raid controller is missing. Is this something that is critical? What does the battery do?

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up vote 7 down vote accepted

One of two things:

  • The card will have disabled the write cache which will have a hefty impact on performance. I believe this is the case for most latter day PERC firmwares.

  • If the write cache is still enabled, then you should disable it, because in the event of sudden loss of power (power cut or hard reboot) you may lose data that hasn't yet been committed to disk.

It's advisable to purchase a Battery Backup Module for the card. You can pick up one for that particular model quite cheap - especially on Ebay, if you're that way inclined.

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It's not critical as such, basically in the even of a server power loss it retains the contents of the disk controller's write cache in an attempt to maintain file and file-system integrity. Although I tend to use BBWC (battery-backed write-cache, which is what this is) I tend to do so only as a safety net, they're not that expensive when bought at the same time as the server and could possibly get me 'out of jail' at some point. That said many file-systems are 'atomic' or journaled these days, ensuring integrity anyway, plus in my case my servers tend to all be FC SAN connected for their actual data, meaning local disks are often barely touched so not really in any danger.

If you do lots of local file write activity then I'd consider picking one up to be on the safe side but I wouldn't lose sleep over it.

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Note that the "journaling" filesystems rely on certain write characteristics of the hard drives to guarantee filesystem consistency. If these characteristics are not met (in particular if the drive reorders writes), you might still get fs errors. So I would not rely on a journaling fs alone, particularly not with RAID. – sleske Aug 18 '10 at 10:33

From the Dell Documentation:

Some RAID controllers have batteries. If the controller has a battery, Storage Management displays the battery under the controller object in the tree view.

In the event of a power outage, the controller battery preserves data that is in the nonvolatile cache memory (NVRAM) but not yet written to disk. The battery is designed to provide a minimum of 72 hours protection for DIMMs up to 64 MB and 48 hours protection for 128-MB DIMMs. The actual period of protection (or holdover time) depends on the battery's charge level.

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The battery if missing results in a significant performance hit on any of the Dell's using write cache. The batteries if purchased off of eBay or anyone selling them needs to be aware that the life cycle for any battery in a critical environment is based on past histories with the servers, storage solutions and computers in general 24 months total. Batteries as manufactured need to be charged in 12 months and that starts the clock ticking for the 24 month period. If you battery has a date code before 36 months ago it is in the OEM's opinion as well as the battery manufacturers opininon garbage and not to be trusted. Today a battery with a 2007 or earlier date code should not be trusted in any mission critical environment. I wouldn't trust a battery before 2008 later part of the year. Even if the battery takes a charge the likelyhood of it holding the cache is at best questionable and based on my own experience more than likely to fail in a critical moment. 2001, 2002 and 2004 batteries used worked for a short period but eventually had to be replaced a second time with subsequent customer dis-satisfaction!

One last thing if the server is running on the battery during a power hit for any extended period and is OLD it most likely will result in the write cache operation wrting suspect information. Again customer dis-satisfaction issue!

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