Date: Thu, 17 May 2012 23:26:57 +0100 From: Anton Shterenlikht <mexas@bristol.ac.uk> To: Warren Block <wblock@wonkity.com> Cc: doc@freebsd.org Subject: Re: Handbook mirroring section Message-ID: <20120517222657.GA14370@mech-cluster241.men.bris.ac.uk> In-Reply-To: <alpine.BSF.2.00.1205171459000.61769@wonkity.com> References: <alpine.BSF.2.00.1205171459000.61769@wonkity.com>
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On Thu, May 17, 2012 at 03:17:29PM -0600, Warren Block wrote: > Summary: Handbook mirror example broken, new rewrite here, feedback > desired before commit. > > http://www.wonkity.com/~wblock/mirror/book.html > http://www.wonkity.com/~wblock/mirror/chapter.diff > > > More detail: > > The existing mirroring section in the GEOM chapter of the Handbook uses > a workaround to add another drive and make a mirror with one existing > drive without moving data. > > http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/geom-mirror.html > > That workaround creates mirrors that FreeBSD 9 and later versions of 8 > see as having invalid, overlapping areas. The more strict tests prevent > at least FreeBSD 9 from even booting from mirrors created that way: > > http://www.freebsd.org/releases/9.0R/relnotes-detailed.html#AEN1277 > > Ideally, the procedure would be able to create a single-drive mirror but > use only a limited amount of disk space on the new drive, making that > space the same size as the existing drive. (Here's a new 80G drive, but > I only want to use 40G of space for the mirror.) Of course this can be > done with partitions, but that is more complex and there may be > thrashing issues with multiple partitions. > > It turns out to be fairly difficult to do this in a situation that will > work reliably for the average user. > > I've rewritten the mirroring section to just use two new drives. > > A mirror *can* be created with two different-sized drives, provided they > are both seen at the time of creation. The smaller size is used; > exposing that size with a command-line option would be very useful. I haven't read your update thoroughly. This is just to mention that you might want to look at http://www.freebsd.org/cgi/query-pr.cgi?pr=docs/136712 It might be completely out of date by now, in which case feel free to close this PR. -- Anton Shterenlikht Room 2.6, Queen's Building Mech Eng Dept Bristol University University Walk, Bristol BS8 1TR, UK Tel: +44 (0)117 331 5944 Fax: +44 (0)117 929 4423
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