Date: Fri, 27 Jun 2008 12:45:55 -0700 From: Mark Boolootian <booloo@ucsc.edu> To: freebsd-questions@freebsd.org Subject: gmirror metadata: end of slice or end of disk? Message-ID: <20080627194555.GA42681@root.ucsc.edu>
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Hi folks, I'm trying to understand exactly where on disk gmirror is going to write its metadata, and was hoping I could elicit comments on whether I have this right or not. Assume my disk, da1, has a single slice, and I partition it as follows (i'm making up sector sizes for the sake of simplicity): length start a: 100 16 4.2BSD b: 10 116 swap c: 626 0 d: 500 126 4.2BSD If I then use 'gmirror label -v -b round-robin gm0 /dev/da1s1', my belief is that gmirror will write its metadata into sector 499 (the last sector in the slice, which falls in partition 'd'). Thus, the disk I want to mirror (da0) should have a 'd' partition that is 499 sectors in size. If it were 500, I would be asking for trouble down the road. Taking a slightly different approach, consider the recommendations here: http://lantech.geekvenue.net/chucktips/jason/chuck/1175552464/index_html where the command to establish the metadata is gmirror label -v -b round-robin gm0 /dev/da0 I believe that means the metadata will be written in the last sector of the disk. Presumably this is part of the space left when fdisk is used to set up the FreeBSD slice (assuming a single slice - and i'm assuming there is virtually always space left), so the recommendation (setting up gmirror on your live system disk) should work. Have I got that right? Assuming a single slice configuration, is there a reason to prefer one approach over the other? (i.e. metadata at end of slice versus at end of disk)? thanks in advance, mark
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