Date: Fri, 21 Feb 2025 11:48:20 +0000 From: Frank Leonhardt <freebsd-doc@fjl.co.uk> To: questions@freebsd.org Subject: Re: GPT secondary corrupt. Is dd'ing the first 40 sectors as a backup sufficient? Message-ID: <e9266bce-67ba-4657-aa93-494e64e7c8ff@fjl.co.uk> In-Reply-To: <e26bb5cc-f0fb-4342-9448-468f0ec5a0c4@heuristicsystems.com.au> References: <e26bb5cc-f0fb-4342-9448-468f0ec5a0c4@heuristicsystems.com.au>
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On 20/02/2025 23:50, Dewayne Geraghty wrote: > I have a full disk gmirror on 12.4 using the MBR scheme. As I'm > migrating to 14.2S and would like to use GPT scheme, so to my questions: > > 1. Is there a sysctl, or flag (somewhere) to disable > " the secondary GPT table is corrupt or invalid." > notifications? And the fear-inspiring > # gpart show md0 > => 40 32688 md0 GPT (16M) [CORRUPT] > > 2. To recover a damaged primary partition, is it sufficient to restore > from a dd of the first 40 sectors of the disk, taken after disk setup > (formatting and labelling the disk)? > > 3. Is the partition header written to without my involvement? (eg is > the clean bit set/stored in the primary or a "sub-"partition (p*)?) > > I'm sorry if my language is a bit clumsy I'm used to slices and > partition. > > Cheers, Dewayne > PS Sadly ZFS doesn't support labels (re MAC_[MLS,BIBA]) Been there, done that, got the T-shirt. https://blog.frankleonhardt.com/2017/zfs-is-not-always-the-answer-bring-back-gmirror/ My conclusion (as described) is to stick with MBR or BSDlabels, but you can (could?) use gmirror to mirror a partition rather than the whole disk. In other words, use GPT on your two drives, partition them up with a "freebsd" partition inside the GPT and just mirror the partition. You can also hack geom_mirror.c (or thereabouts) - I think I did once but I forgot exactly how. In production use I decided not to mess about. MBR/gmirror/UFS2 when appropriate (e.g. for databases) and ZFS mirror for everything else. As to questions two and three I'm saying nothing until I've tested it! All I will say is when recovering a corrupt disk dd it as an image and play with the image. I'll keep an eye on this thread in case experiments are called for as I current have a "disk lab" going. Regards, Frank.
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