Batery Backed Write Cache (BBWC) on RAID 1+0

Posted on 2011-05-12
Last Modified: 2012-05-11

I know that on RAID5 the BBWC does pretty good job in increasing the speed of read/write. Is it the same on RAID 1+0 ? Is it worthed to pay 350 paunds for 512MB BBWC?
Question by:goliveuk
    LVL 55

    Accepted Solution

    The benefits aren't nearly as much as with RAID 5.

    With RAID 5 the controller can sometimes store enough writes so that it can write a full width stripe, that completely offsets the RAID 5 write penalty, it doesn't have to read data and parity, xor and write them back again it just writes once to all disks, you don't get that saving with RAID 1+0 as it doesn't have to calculate parity. That means the total physical I/Os are fewer.

    The only benefit BBWC gives with RAID 1+0 is that the controller tells the OS that the data is committed as soon as it is in cache, the total physical I/Os aren't fewer though since it still has to write the data to both disks eventually.

    One exception is lock files, if a process relies on checking the state of a file repeatedly and sits there doing nothing until it's read/written then the 10ms or so it takes to do that can cripple performance if that file isn't in cache, but that's pretty rare; most I/O is buffered by the OS into a pipeline.
    LVL 30

    Expert Comment

    there is a benefit of a bbwc is even if the server crashes the cache still retains the infromation thats why its battery backed so the battery power keeps the cache consitent
    LVL 55

    Expert Comment

    Not really Ian, except on a cheap controller that cheats by enabling unprotected write cache; on a decent controller if there isn't a cache battery the write cache is disabled.

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