[CentOS] CentOS Digest, Vol 75, Issue 5

Tue Apr 5 18:03:49 UTC 2011
Chuck Munro <chuckm at seafoam.net>

On 04/05/2011 09:00 AM, John R Pierce wrote:
> AFAIK, no standard raid modes verify parity on reads, as this would
> require reading the whole slice for every random read.  Only raid
> systems like ZFS that use block checksuming can verify data on reads.
> parity (or mirrors) are verified by doing 'scrubs'
> Further, even if a raid DID verify parity/mirroring on reads, this would
> at best create a nonrecoverable error (bad data on one of the N drives
> in the slice, no way of knowing which one is the bad one).
Thanks John, that's good information, something I didn't know.  So I 
should think of RAID-5/6 parity as a mechanism for recovering from a 
drive fault that is more space-efficient than simple mirroring.  Maybe 
RAID-10 with hot spares is more than "good enough" in most applications, 
but I do like dual parity for its ability to recover even in the face of 
a disk error popping up during the rebuild.

Am I being too paranoid?

Too bad ZFS on Linux is still up at the fuse layer.  I understand Btrfs 
is rolled into newer kernels and should be in CentOS-6, but I read 
somewhere it's not yet in stable release and has some potential issues, 
so I'm reluctant to try it.  It won't have RAID-6-like parity for a 
while.  The fact that Oracle has both ZFS and Btrfs under its wing is, 
um, "interesting".

I'm only asking for the world  :-)