[CentOS] OT Question about raid 5

Jerry Franz jfranz at freerun.com
Tue Apr 21 20:28:47 UTC 2009


Per Qvindesland wrote:
> Hi List
>
> I am hoping that someone here could perhaps give me a straight answer 
> on a question that someone asked me today
>
> I have always belived that if you have 5 hard drives 1 50gb second 
> 50gb third 20gb fourth 60gb firth 30gb that the largest would then be 
> the size of the smallest disk, not 80 or 100 or 120 for that matter or 
> am I wrong here
In general you are correct for simple 'out of the box' type 
configurations and for most hardware RAID controllers.

But there are advanced tricks that can be played with 'hybrid' RAID 
levels that can achieve larger sizes from smaller drives.

For your example drives of 2 x 50GB, 1 x 20GB, 1 x 60GB, and 1 x 30GB, 
using software RAID, you could use use linear mode to make one 50GB 
'drive' out of the 30GB and the 20GB and then make a RAID5 out of the 2 
X 50GB the 1 X 60GB and the 'fake' 1 x 50GB resulting in a RAID5 with 
150GB available vs a naive 'just bang them together' as a 5 x 20GB RAID5 
approach which would only give you 80GB.

-- 
Benjamin Franz


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