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From: des@des.no (Dag-Erling =?utf-8?Q?Sm=C3=B8rgrav?=)
To: Ivan Voras <ivoras@fer.hr>
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Date: Sat, 05 May 2007 17:31:00 +0200
In-Reply-To: <f1i3s4$j4n$1@sea.gmane.org> (Ivan Voras's message of "Sat, 05
	May 2007 16:20:51 +0200")
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Cc: freebsd-geom@freebsd.org
Subject: Re: graid5 after-reboot problem
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Ivan Voras <ivoras@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
--=20
Dag-Erling Sm=C3=B8rgrav - des@des.no