Date: Fri, 1 Jan 2010 23:39:08 +0100 From: Bernd Walter <ticso@cicely7.cicely.de> To: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> Cc: "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org>, "ticso@cicely.de" <ticso@cicely.de> Subject: Re: ZFS RaidZ2 with 24 drives? Message-ID: <20100101223907.GX43739@cicely7.cicely.de> In-Reply-To: <alpine.GSO.2.01.1001011538130.1586@freddy.simplesystems.org> References: <55389.88569.qm@web112405.mail.gq1.yahoo.com> <20100101204752.GW43739@cicely7.cicely.de> <alpine.GSO.2.01.1001011538130.1586@freddy.simplesystems.org>
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On Fri, Jan 01, 2010 at 03:45:19PM -0600, Bob Friesenhahn wrote: > On Fri, 1 Jan 2010, Bernd Walter wrote: > > > >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. > > A proper write-through cache should automatically commit itself (in > order) to backing store within a second or two. Other than cache > designs which are not "proper" (which we should not use) the main > concern is if the system loses power or crashes while it is producing > a significant write load so that there is uncomitted data in cache. There are many possible reasons why this won't happen. One of them is a simple write failure, which can't be reported back to the filesystem, because not even a cache flush fails. Yes - the risk might be tolerable for many people and I don't think it is very high. The main problem I see is that such controllers won't tell about their strategy, so you are left in the dark. > ZFS is not particularly more likely to lose user data, but it is much > more likely to detect and report loss since most other filesystems > don't even check, or even have a way to check. Agreed. And ZFS can win a lot from fast flushes. -- B.Walter <bernd@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.
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