On Wed, 9 Dec 2009, Michael Evans wrote:

keeps a checksum for every storage segment.  However that conflicts
with the 'zero it before creation and assume-clean works' idea.  It
also very likely has extremely poor write performance.

Generally, my experience has been that total disk failures are fairly 
rare, instead with the much larger disks today, I get single block/sector 
failures, meaning 512 bytes (or 4 k, I don't remember) can't be read. Is 
there any data to support this?

Would it make sense to add 4k to every 64k of raid chunk (non-raid1) for 
some kind of "parity" information. Since I guess all writes involves 
re-writing the whole chunk, adding 4k here shouldn't make the write 
performance be worse?

The problem I'm trying to address is the raid5 "disk failure and then 
random single block/sector error on the rest of the drives".

For arrays with few drives this would be much more efficient than going to 
raid6...?

An 8 disk raid6 with 1TB you get 6 TB of usable data, for an 8 disk raid5p 
(p for parity, I just made that up), it would be 7*64/68=6.59 TB.

For 6 disk raid6 = 4TB and raid5p makes this 5*64/68=4.71TB.

For 4 disk raid5 = 2TB and raid5p makes this 3*64=68=2.82TB.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se