Re: Update to Project_ideas wiki page

[Gordan Bobic <gordan@bobich.net>]

Bart Noordervliet wrote:
> On Wed, Nov 17, 2010 at 19:07, Gordan Bobic <gordan@bobich.net> wrote:
>> 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.
> 
> I presume you're talking about the uncaught read errors that makes
> many people avoid RAID5. Btrfs actually enables us to use it with
> confidence again, since using checksums it's able to detect these
> errors and prevent corruption of the array. So to the contrary, I see
> a lot of potential for parity-based redundancy in combination with
> btrfs.

No. What I'm talking about the the probability of finding an error 
during the process of rebuilding a degraded array. With a 6TB (usable) 
array and disks with 10^-14 error rate, the probability of getting an 
unrecoverable read error exceeds 50%. n+1 RAID isn't fit for use with 
the current generation of drives where n > 1-5TB depending on how 
important your data and downtime are and how good your backups are.

And I don't put much stock in the manufacturer figures, either, so 
assume that 10^-14 is optimistic of it is reported. On high capacity 
drives (especially 1TB Seagates, both 3 and 4 platter variants) I am 
certainly seeing a higher error rate than that on a significant fraction 
of the disks.

Gordan