graid5 after-reboot problem

[Dag-Erling Smørgrav]

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".

DES
-- 
Dag-Erling Smørgrav - des at des.no