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
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