Re: Update to Project_ideas wiki page

[Gordan Bobic <gordan@bobich.net>]

On 11/17/2010 05:56 PM, Hugo Mills wrote:
> On Wed, Nov 17, 2010 at 04:12:29PM +0100, Bart Noordervliet wrote:
>> Can I suggest we combine this new RAID level management with a
>> modernisation of the terminology for storage redundancy, as has been
>> discussed previously in the "Raid1 with 3 drives" thread of March this
>> year? I.e. abandon the burdened raid* terminology in favour of
>> something that makes more sense for a filesystem.
>
>     Well, our current RAID modes are:
>
>   * 1 Copy ("SINGLE")
>   * 2 Copies ("DUP")
>   * 2 Copies, different spindles ("RAID1")
>   * 1 Copy, 2 Stripes ("RAID0")
>   * 2 Copies, 2 Stripes [each] ("RAID10")
>
>     The forthcoming RAID5/6 code will expand on that, with
>
>   * 1 Copy, n Stripes + 1 Parity ("RAID5")
>   * 1 Copy, n Stripes + 2 Parity ("RAID6")
>
>     (I'm not certain how "n" will be selected -- it could be a config
> option, or simply selected on the basis of the number of
> spindles/devices currently in the FS).
>
>     We could further postulate a RAID50/RAID60 mode, which would be
>
>   * 2 Copies, n Stripes + 1 Parity
>   * 2 Copies, n Stripes + 2 Parity

Since BTRFS is already doing some relatively radical things, I would 
like to suggest that RAID5 and RAID6 be deemed obsolete. RAID5 isn't 
safely usable for arrays bigger than about 5TB with disks that have a 
specified error rate of 10^-14. RAID6 pushes that problem a little 
further away, but in the longer term, I would argue that RAID (n+m) 
would work best. We specify that of (n+m) disks in the array, we want n 
data disks and m redundancy disks. If this is implemented in a generic 
way, then there won't be a need to implement additional RAID modes later.

Gordan