[CentOS] OT Question about raid 5

Per Qvindesland per at norhex.com
Tue Apr 21 20:36:25 UTC 2009


Hi All

Thanks a lot for all your answers.

So I was right but also a bit wrong :)

Agan thanks a lot.

Regards
Per Qvindesland

--- Original message follows ---
SUBJECT: Re: [CentOS] OT Question about raid 5
FROM:  Jerry Franz 
TO: "CentOS mailing list" 
DATE: 21-04-2009 21:28

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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