RE: [CentOS] Recommendations for a “real RAID" 1 card on Centos box

[Therese Trudeau]

>>> the whole point of a BBU is that you can turn on write back caching -
>>> and get a fair win in write performance on regular tasks.
>>>     
>>
>> Pardon my ignorance, what is write back caching and BBU?
>>   
> 
> Write Back Caching means the card will cache writes in its onboard 
> storage, and let the OS continue immediately
> 
> ...this is only 'safe' if the card has a 'battery backup unit' to 
> protect the cache during power failures so that the cached write data 
> can be written to the disks when the power resumes.    Some raid cards 
> even allow you to remove the battery still attached to the cache along 
> with the disks and install them on a different but similar machine in 
> case of a total server failure, this is a feature on many HP SmartArray 
> cards.
> 
> A battery backed Write Back Cache can hugely speed up random writes such 
> as from a relational database server.

Ah that makes total sense now, thanks.  Do the 3wire and the Areca cards 
allow you to remove battery/cache/disk and install into similar motherboard? Also
when you say remove battery and cache, do you mean remove the entire RAID
card with battery attached to it as complete assembly with accompaying drive
and slap them all onto a new motherboard?

>> Again pardon my ignorance, what is a hot spare?  A blank drive connected
>> in the RAID 5 setup that can be written to in case one of the other 3 drives fail?
>>
>>   
> 
> exactly.  a hot spare sits unused until one of the RAID members fails, 
> then its used to replace the failed drive by remirroring or restriping 
> the parity, once this is finished, and the original failed drive is 
> replaced it can become the new hot spare.

So if I understand correctly, RAID 5 is three active drives and one blank drive connected to a RAID 5 card, 
and if one of the three active drives fails, the fourth empty drive is automatically written to?  If correct, what happens if the drive that fails loses all it's data before the
blank drive has a chance to grab it?

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