nested raid possible?

nested raid possible?

[turbo]

I have a raid-5 array of six 250gb hard drives.
I just bought four 500gb hard drives.

I know I can take the six drives and make three raid-1 arrays of 500gb.
After doing that, can I make a raid-5 array consisting of the three 
raid-0 arrays, and the four 500gb hard drives?

Thanks,
Jeff


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Re: nested raid possible?

[Simon Valiquette]

turbo@weasel.com un jour écrivit:
> I have a raid-5 array of six 250gb hard drives.
> I just bought four 500gb hard drives.
> 
> I know I can take the six drives and make three raid-1 arrays of 500gb.

   Here, you probably means three RAID-0 arrays of 500GB, otherwise you 
would get three RAID-1 of 250GB.

> After doing that, can I make a raid-5 array consisting of the three 
> raid-0 arrays, and the four 500gb hard drives?

   Yes, It would works, because all you need is to get a block device. 
Software Raid takes many block devices to generate one block device, so It 
is transparent.

   That said, I don't say that It is a good idea to do It as you are 
suggesting.  You realize that It would be less reliable than a RAID-5 
array made of seven 500GB drive.

   If you really want to do than, I would feel more confident using 
RAID-6.   Another option is to use RAID-10 with 4x250GB with 2 hot spare, 
and connect everything in a RAID-5 array for a total of also 2.5TB of 
usable space.  Personnaly, considering the price of the hard drives, I 
won't ever bother and use only 500GB drive.

   That said, It is your data.  It is you that know if It is better to 
have the full 3TB, at the cost of somewhat more risk to loose your data. 
Maybe the additional risk is low enough for your needs, I don't know.

   But in a business environment, I would normaly not do It.

Simon Valiquette

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Re: nested raid possible?

[David Greaves]

Simon Valiquette wrote:
> turbo@weasel.com un jour écrivit:
>> I have a raid-5 array of six 250gb hard drives.
>> I just bought four 500gb hard drives.

Simon raised some sensible points.

Also consider a raid5 of the 4x500 drives and using LVM to join them to a raid 5 
of the 6x250s

The benefit using raid0 to join the 250s may be that you can replace each of the 
2x250 with 1x500 as time passes. In that case you may want a raid6 made of :
  500 500 500 500 250+250 250+250 250+250

One thing I'd ceertainly suggest is creating partitions of 498Gb to allow for 
geometry variations....

David
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Re: nested raid possible?

[Bill Davidsen]

turbo@weasel.com wrote:
> I have a raid-5 array of six 250gb hard drives.
> I just bought four 500gb hard drives.
>
> I know I can take the six drives and make three raid-1 arrays of 500gb.
> After doing that, can I make a raid-5 array consisting of the three 
> raid-0 arrays, and the four 500gb hard drives?
>   

If you want performance and reliability, I would suggest migrating 
everything to RAID-10, which will handle unequal size drives reasonably 
well. In the medium term you may want to upgrade the 250=>500 just so 
you can reduce your spare drive needs. And if your storage needs are 
going to grow, you might consider winding up with RAID-6 and a hot spare.

LVM offers another solution, which also adds complexity, and in my 
measurements hurts performance.

The short answer is that you can do what you asked, but it's more 
complex and less reliable than is really desirable.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979