Date: Fri, 1 Jan 2010 21:47:53 +0100 From: Bernd Walter <ticso@cicely7.cicely.de> To: ??imun Mikecin <numisemis@yahoo.com> Cc: "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org>, "ticso@cicely.de" <ticso@cicely.de>, Danny Carroll <danny@dannysplace.net> Subject: Re: ZFS RaidZ2 with 24 drives? Message-ID: <20100101204752.GW43739@cicely7.cicely.de> In-Reply-To: <55389.88569.qm@web112405.mail.gq1.yahoo.com> References: <55389.88569.qm@web112405.mail.gq1.yahoo.com>
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On Fri, Jan 01, 2010 at 12:06:56PM -0800, ??imun Mikecin wrote: > > 1. sij. 2010., u 20:51, Bernd Walter <ticso@cicely7.cicely.de> napisao: > > 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? > > > If you are using amr driver then FreeBSD will flush cache during shutdown. Haven't tried other drivers myself, but I suppose they also do the same. > You can have a problem only on unclean shutdown after which you didn't reboot (nobody does that willingly). Everyone do this if the board dies and needs replacement. Not willingly, but it happens. And what about zfs export - relocate disks to another machine - and zfs import - without halt? It is less safe if a cache flush won't flush its cache. The real purpose to have buffered cache is to handle asyncronity in RAID systems after power failure, but RAIDZ won't have this problem by design, at least if running with CRC enabled. -- B.Walter <bernd@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
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