graid5 after-reboot problem

[Pawel Jakub Dawidek]

On Sat, May 05, 2007 at 05:31:00PM +0200, Dag-Erling Sm??rgrav wrote:
> Ivan Voras <ivoras at fer.hr> writes:
> > Is the write cache in graid5 aware of what happens on the VFS / UFS
> > layers (something like gjournal does)? Otherwise, how can you guarantee
> > consistency with write caching at the GEOM layer when there's a power
> > outage or some other system interruption?
> 
> You can't.  Google for "RAID 5 write hole" for an explanation.
> 
> The way this is handled by hardware RAID 5 controllers is that they
> keep a journal in the controller's memory (which has its own battery
> backup) and replay it when the power returns.  If the controller is
> fried, you're SOL.
> 
> ZFS uses copy-on-write, so its raidz and raidz2 do not have a "write
> hole".

RAID3 is also write-hole safe, btw.

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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