[CentOS] OT Question about raid 5

D Tucny d at tucny.com
Tue Apr 21 20:38:53 UTC 2009


2009/4/22 Jerry Franz <jfranz at freerun.com>

>
> 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.
>

Or, alternatively, with those disks...
You could make a raid5 array using 20GB of each disk, giving you 80GB of
usable space, a second raid5 array out of the remaining 30GB of the two 50GB
disks and 30GB of the 60GB disk giving you 60GB of space and a raid1 of the
remaining 10GB of the 60GB and 30GB disks... Total usable space also 150GB
but without having to do anything special like layering of raid on JBOD...
Performance might not be awsome though as depending on your workload the
60GB disk at least could be hit pretty hard having all 3 arrays on it...

d
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