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

Posted on 2011-05-12
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Hello,

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?
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Question by:goliveuk
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andyalder earned 2000 total points
ID: 35744852
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.
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by:IanTh
ID: 35745459
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
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by:andyalder
ID: 35745626
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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