CentOS - Apr 2009 - OT Question about raid 5

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?

Regards?
Per Qvindesland?
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>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?
Well, wrt to R5, yes. If the array is spanned across different disks,
it must use the smallest size and leave unused space for the remainder
on the larger disks..

jlc

On Tue, Apr 21, 2009 at 9:09 PM, Per Qvindesland <per at norhex.com>
wrote:
>
> 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 your example the smallest disk is 20gb, this will limit the size of
the array. This means that the size of the array would be 20 * (5-1) 80gb.

Regards,
Tim

-- 
Tim Verhoeven - tim.verhoeven.be at gmail.com - 0479 / 88 11 83

Hoping the problem  magically goes away  by ignoring it is the
"microsoft approach to programming" and should never be allowed.
(Linus Torvalds)

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

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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