Re: single disk reed solomon codes

[Chris Mason <chris.mason@oracle.com>]

On Sat, 2008-07-19 at 09:50 -0700, David Woodhouse wrote:
> On Sat, 2008-07-19 at 15:21 +0300, Ahmed Kamal wrote:
> > Hi,
> > Since btrfs is someday going to be the default FS for Linux, and will
> > be on so many single disk PCs and laptops, I was thinking it should be
> > a good idea to insert some redundancy in single disk deployments. Of
> > course it can help with disk failures, since it's obviously a "single"
> > disk, but it can help with bit-rot, and with hardware sector read
> > errors. To get that we'd need to implement some kind of forward error
> > correction, possibly reed solomon code. I am not sure why no
> > filesystem seems to implement such scheme, although I believe at the
> > hardware level, such schemes are being used (so the idea is
> > applicable) ?
> 
> We have implementations of such schemes in lib/reed_solomon/ in the
> kernel already.
> 
> I'm quite interested in using btrfs on flash (I mean  _real_ flash not
> SSDs where they have their own internal pseudo-fs pretending to be a
> disk). For that, we'd probably want to use precisely this kind of error
> correction. Although it's normal to do it at the block level rather than
> the filesystem object level;
> 

The long term goal is to have the checksum algorithm selectable between
a number of choices.  For metadata, you have 256 bits to use and for
data you can use anything that will fit in a btree block.

So, the way to do this for real flash would be to implement the
selectable checksum, and then store the sum + whatever error recovery
code you want in the checksum item.

-chris