Date: Sat, 05 May 2007 17:31:00 +0200 From: des@des.no (Dag-Erling =?utf-8?Q?Sm=C3=B8rgrav?=) To: Ivan Voras <ivoras@fer.hr> Cc: freebsd-geom@freebsd.org Subject: Re: graid5 after-reboot problem Message-ID: <86fy6bqocr.fsf@dwp.des.no> In-Reply-To: <f1i3s4$j4n$1@sea.gmane.org> (Ivan Voras's message of "Sat, 05 May 2007 16:20:51 %2B0200") References: <171980743.20070504223126@uzvik.kiev.ua> <125507.38194.qm@web30304.mail.mud.yahoo.com> <f1i3s4$j4n$1@sea.gmane.org>
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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
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