Date: Fri, 1 Jan 2010 20:51:56 +0100 From: Bernd Walter <ticso@cicely7.cicely.de> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: freebsd-fs@freebsd.org, Danny Carroll <danny@dannysplace.net> Subject: Re: ZFS RaidZ2 with 24 drives? Message-ID: <20100101195155.GV43739@cicely7.cicely.de> In-Reply-To: <alpine.GSO.2.01.1001011050280.1586@freddy.simplesystems.org> References: <568624531.20091215163420@pyro.de> <42952D86-6B4D-49A3-8E4F-7A1A53A954C2@spry.com> <957649379.20091216005253@pyro.de> <26F8D203-A923-47D3-9935-BE4BC6DA09B7@corp.spry.com> <4B3D95AD.8050304@dannysplace.net> <alpine.GSO.2.01.1001011050280.1586@freddy.simplesystems.org>
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On Fri, Jan 01, 2010 at 10:56:21AM -0600, Bob Friesenhahn wrote: > On Fri, 1 Jan 2010, Danny Carroll wrote: > > > >You do not have this protection when ZFS has access to the raw devices. > >Even worse if the devices write cache is turned on. > > This statement does not appear to be true. ZFS will always request > that devices flush their cache. The only time there is no > "protection" is if the device ignores that flush request and the cache > is volatile. Controller battery-backed RAM is useful since the > controller can respond to the cache flush request once the data is in > battery-backed RAM, thereby dramatically improving write latencies for > small writes Which - if it is true for the controller - can be dangerous. A battery backed cache is volatile if the system is going down for a long time. Or consider the system is going down to relocate the disks to a new machine, or just to a newer controller? -- B.Walter <bernd@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
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