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From: karl@aspodata.se
To: gentoo-user@lists.gentoo.org
Subject: Re: [gentoo-user] Fun with mdadm (Software RAID)
In-reply-to: <Z2VLTVQmRBwDulyX@MAC.fritz.box>
References: <Z2VLTVQmRBwDulyX@MAC.fritz.box>
Comments: In-reply-to Alan Mackenzie <acm@muc.de>
   message dated "Fri, 20 Dec 2024 10:47:41 +0000."
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Alan Mackenzie:
...
> The cause was me booting up the machine with a rescue disk.  This
> assembled my RAID partitions /dev/md127 and /dev/md126 reversed, but
> also wrote those wrong identifiers, 126 and 127, into the "preferred
> minor" field of the partitions' super blocks.  In essence, they got
> swapped.
...
> Just for the record, all my RAID arrays have metadata version 0.90, the
> (old fashioned) one that allows auto-assembly by the kernel without the
> need of an initramfs.
> 
> The moral of the story: if your system uses software RAID, be careful
> indeed before you boot up with a rescue disk.

So, why don't you simple add "root=902 md=2,/dev/sda2,/dev/sdb2" or similar to
your boot loader kernel command line ?

///

 And... what is the need for dynamic minors now when dev_t is 32bits:

$ grep dev_t /Net/git/linux-stable/include/linux/types.h 
typedef u32 __kernel_dev_t;
typedef __kernel_dev_t          dev_t;
$

 and we have 20 bits minors:

$ grep -A1 MINORBITS /Net/git/linux-stable/include/linux/kdev_t.h 
#define MINORBITS       20
#define MINORMASK       ((1U << MINORBITS) - 1)

#define MAJOR(dev)      ((unsigned int) ((dev) >> MINORBITS))
#define MINOR(dev)      ((unsigned int) ((dev) & MINORMASK))
#define MKDEV(ma,mi)    (((ma) << MINORBITS) | (mi))

Regards,
/Karl Hammar