[ILUG] On the topic of RAID

[David Murphy]

On Wed, 20 Oct 2004 17:29:16 +0100, John P. Looney <valen at tuatha.org> wrote:

>  Think about it. the OS does an fsck because it's not sure what's on the
> disk, and what's not. RAID systems have to be consistent also - and if the
> power goes, do you really know if the same data has been written to the
> same disk ?
> 
>  I'm sure it's possible to do journalling for RAID systems, just like it
> can be done for filesystems; as in "I'm about to write these
> blocks".."I've written those blocks, and they are no on both disks".

Yup, see 'dirty region logging' in Veritas VM. I don't recall offhand
if it actually works where the power's failed, or if it only speeds
resyncs where you've done a controlled detach of one side of a mirror
or whatever.
 
>  I think the more expensive hardware raid controllers can do it. Ones with
> battery backed up RAM for write caching.

For those, it's more like the disks aren't inconsistent in the
lost-power case, because the controller writes the cached data out
when it comes back online.

-- 
David Murphy