raid5 or raid6 and the maximum size

raid5 or raid6 and the maximum size

[Farkas Levente]

hi,
i'd like to know the current state of the raid6 driver. we are now 
switching one of our server from 8 x 200GB to 8 x 300GB(or 400GB).
so which is the better if we use:
- 7 x 300GB in raid5 and one 300GB spare or
- 8 x 300GB in raid6?
it seems the same to me since both case we can get two disk faild and 
the same capacity. so why we choose raid6 over raid5 (when raid5 seems 
to be more stable then raid6)?
another question is there any upper limit for one raid5(6) partition? i 
read 1TB and 2TB too (although we already have a raid5 which is larger 
then 1TB). these are still real limit? or what is the current limit?
thank you for your help in advance.
yours.

-- 
   Levente                               "Si vis pacem para bellum!"

Re: raid5 or raid6 and the maximum size

[Mario 'BitKoenig' Holbe]

Farkas Levente <lfarkas@bppiac.hu> wrote:
> - 7 x 300GB in raid5 and one 300GB spare or
> - 8 x 300GB in raid6?
> it seems the same to me since both case we can get two disk faild and 

No, it's everything but the same. raid5 always gives you survival of exactly
one simultaneous disk failure. If two of your disks fail simultaneously
in a raid5, you're lost. A spare disk gets used not before any disk
failed. Then it needs time to get synched, where you have no redundancy
at all. After that your raid5 is able to survive another disk failure.


regards
   Mario
-- 
As a rule, the more bizarre a thing is, the less mysterious it proves to be.
                                    -- Sherlock Holmes by Arthur Conan Doyle

Re: raid5 or raid6 and the maximum size

[berk walker]

Farkas Levente wrote:

> hi,
> i'd like to know the current state of the raid6 driver. we are now 
> switching one of our server from 8 x 200GB to 8 x 300GB(or 400GB).
> so which is the better if we use:
> - 7 x 300GB in raid5 and one 300GB spare or
> - 8 x 300GB in raid6?
> it seems the same to me since both case we can get two disk faild and 
> the same capacity. so why we choose raid6 over raid5 (when raid5 seems 
> to be more stable then raid6)?
> another question is there any upper limit for one raid5(6) partition? 
> i read 1TB and 2TB too (although we already have a raid5 which is 
> larger then 1TB). these are still real limit? or what is the current 
> limit?
> thank you for your help in advance.
> yours.
>
With RAID5 and a spare, if another, rarely or never used area is found 
to be bad on rebuild, your spare is of no use, and without heroic 
measures, all is lost.  RAID6 can handle that scenario.

b-