Date: Sun, 10 Jun 2012 23:15:55 +0900 (JST) From: Hiroki Sato <hrs@FreeBSD.org> To: wblock@wonkity.com Cc: doc@FreeBSD.org Subject: Re: Handbook mirroring section Message-ID: <20120610.231555.975873460722378457.hrs@allbsd.org> In-Reply-To: <alpine.BSF.2.00.1206060454200.13150@wonkity.com> References: <alpine.BSF.2.00.1206052220390.10777@wonkity.com> <20120606.185023.497714372668376681.hrs@allbsd.org> <alpine.BSF.2.00.1206060454200.13150@wonkity.com>
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----Security_Multipart(Sun_Jun_10_23_15_55_2012_960)-- Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Warren Block <wblock@wonkity.com> wrote in <alpine.BSF.2.00.1206060454200.13150@wonkity.com>: wb> On Wed, 6 Jun 2012, Hiroki Sato wrote: wb> > BTW, do you (or anyone) know the common failure pattern when trying wb> > to use GPT + gmirror of the whole disk? IIRC it was a metadata wb> > corruption but I do not remember when it happens. wb> wb> When gmirror is used to mirror two disks, metadata goes in the last wb> block of each. If GPT partitioning is used on the mirror, the GPT wb> secondary partition table overwrites the mirror metadata because, by wb> specification, GPT puts that secondary table at the end of the wb> physical drive, rather than inside the gmirror logical device. I think the interpretation that "GPT specification says the backup header must be at the last LBA of the *physical* disk" is just making things too harder. The problem is that BIOS and the loader does not support a gmirror logical volume, and the incompletely-configured volume is recognized for a short time during a boot due to it. The location of the backup header is recorded in the Backup LBA field in the primary header, so everything will work fine as long as the pre-boot programs support it and the primary header is not corrupted. I do not think this situation is against standards conformance though it is suboptimal. If we can teach GEOM to BIOS and/or UEFI it would be the best, but it is not likely. What I want to know is whether there are some practical difficulties when we apply the whole-disk mirroring procedure for MBR to a GPT-based system. Overwrite of the last LBA should not happen if FreeBSD is the only one system on the disk. One common failure scenario is when the primary header is corrupted. If there are more, I want to know it. -- Hiroki ----Security_Multipart(Sun_Jun_10_23_15_55_2012_960)-- Content-Type: application/pgp-signature Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (FreeBSD) iEYEABECAAYFAk/UrBsACgkQTyzT2CeTzy0e8ACeK1a79nR2lzkvI7sXSi9xti/H ku8Anjs0vxvWjjzSuDPwP+xvJ5N2JInL =595C -----END PGP SIGNATURE----- ----Security_Multipart(Sun_Jun_10_23_15_55_2012_960)----
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