[ILUG] On the topic of RAID

[John P. Looney]

On Wed, Oct 20, 2004 at 05:12:37PM +0100, Bryan O'Donoghue mentioned:
> If I'm lazy and don't properly shutdown the raid box, but instead just 
> nuke it via power switch, when I boot the box again and check out 
> /proc/mdstat I notice that the *entire* 80 gb is resynching.
> 
> Is this normal ? I suspect not.

 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". 

 I think the more expensive hardware raid controllers can do it. Ones with
battery backed up RAM for write caching.

john