do I really need a RAID Controller Cache battery back up?

Posted on 2009-12-19
Last Modified: 2012-05-08
do I really need a RAID Controller Cache battery back up?  I have RAID 5, two redundant hot swappable power supplies and a UPS with 30 minutes backup.  the RAID Controller Cache battery back up is designed to store the RAID Controller's Cache in the event of complete power loss.  This server was sent to me by my corp/vendor team.  We took down the old one, installed this new one and all knew the RAID Controller Cache battery back up was back ordered and would arrive in the future.  Now it is here and the installation requires shutting down the server and pulling all power which will delete the RAID Controller Cache.  Does not seem to me like the Cache is all that important as it will probably regenerate on the next power up.  What say the experts?
Question by:miriggall
    LVL 14

    Accepted Solution

    The RAID Controller Cache battery back up is designed to store the RAID Controller's "Write back" Cache in the event of system crash or power outage, hence saving any data that was waiting to be saved to disk. So yes it is important to have a Cache battery back....if not, the other option is to write cache content to one or more disk drives (read info in link attached for more details)
    See below for details...

    Write cache reliability
    The disk system can acknowledge the write operation as soon as the data is in the cache, not waiting for the data to be physically written. This typically occurs in old, non-journaled systems such as FAT32, or if the Linux/Unix "writeback" option is chosen without any protections like the "soft updates" option (to promote I/O speed whilst trading-away data reliability). A power outage or system hang such as a BSOD can mean a significant loss of any data queued in such cache.
    Often a battery is protecting the write cache, mostly solving the problem. If a write fails because of power failure, the controller may complete the pending writes as soon as restarted. This solution still has potential failure cases: the battery may have worn out, the power may be off for too long, the disks could be moved to another controller, the controller itself could fail. Some disk systems provide the capability of testing the battery periodically, however this leaves the system without a fully charged battery for several hours.
    An additional concern about write cache reliability exists, specifically regarding devices equipped with a write-back cachea caching system which reports the data as written as soon as it is written to cache, as opposed to the non-volatile medium. The safer cache technique is write-through, which reports transactions as written when they are written to the non-volatile medium.
    Hope this answers your question :)


    Author Comment

    if I have two redundant hot swappable power supplies and an on line ups with 30 minutes of battery backup how great a chance of major loss of data due to power loss to raid controller cache?  maybe I should get a battery backup for the battery backup just to be sure?  
    LVL 14

    Expert Comment

    So lets say you have a power outage....

    If your client computers are not on battery backup...then your users are not writing any data and the servers 30 backup UPS minutes is plenty for cache to be written to disk. However, if your client PCs are also on UPS then you would need the RAID Controller Cache battery back up or at the end of 30 minutes data would be lost (unless you setup the server to shutdown when ony 5min power is left on UPS)
    Then again, what if the server BSOD'ed/crashed ...then there would be loss of data..

    I recommend the RAID Controller Cache battery back up , its better than haveing very upset end-users :D
    Adding the battery is a few minute operation and you lose no data...Just shut down the server....tunr off all power to tit and install the are done

    Author Comment

    I have to shut down the server, Our busiest department is out of commission, I have to pull the 60 lbs, 19 x48 x4 inch slab ouf of the rack, find a suitable flat spot to set it on, open it up, figure out which card is the Controller card, determine if it will hold the battery or if I will have to remotely attach it to the frame, make the connection, button up the server, hoist it back into the rack, plug is in and start it all up.  Not really like changing a flash light batterey.  Oh and did I mention I am doing this at 4 a.m.?  you are probably right, I just don't want to do it right now...
    LVL 14

    Expert Comment

    I hear ya ;)

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