Les Mikesell wrote:
<snip>
'mdadm' writes a listing of the devices in the array to the md superblock and orders them by number,major,minor. You cannot add another device to the array with the same tuple.
Isn't this updated at detect time so the device minor's should always be different?
If you dd the first sector of the drive though you will duplicate the partition table and grub boot loader to the other drive. Then md-device-mapper will take care of copying the data over.
I'm curious as to why 2 complete dd'd copies don't pair at boot. One comes up running and it does work to mdadm --add the partner partitions and after the resync they do automatically pair at boot.
Gosh darn it Les your just too damn inquisitive!
Well for the sake of the truth, the whole truth, and you know the rest.
Let me dig around here, hmmm, ok, not here, ok here we go:
/usr/src/kernels/2.6.18-53.1.21.el5-x86_64/include/linux/raid/md_p.h
Ok, looking at the comments, it seems...
/* * RAID superblock. * * The RAID superblock maintains some statistics on each RAID configuration. * Each real device in the RAID set contains it near the end of the device. * Some of the ideas are copied from the ext2fs implementation. * * We currently use 4096 bytes as follows: * * word offset function * * 0 - 31 Constant generic RAID device information. * 32 - 63 Generic state information. * 64 - 127 Personality specific information. * 128 - 511 12 32-words descriptors of the disks in the raid set. * 512 - 911 Reserved. * 912 - 1023 Disk specific descriptor. */
The last part of the superblock contains a disk specific descriptor, identifier (whatever), and the middle contains a list of all the disk descriptors participating in the RAID set. (there can only be 12 disks max in a raid set? that's news to me, maybe the comment is old, if you combine the reserved area you can get 24 disks).
From this we can observe that when the disks are identified as
auto-raid and their superblocks are read it tells them what RAID they belong to and their order in the array.
What happens if a duplicate descriptor is encountered? And how does it determine which disk is the "official" disk?
I have yet to find those answers, but let me hypothesis that it ejects either a) the disk with the oldest timestamp, or b) the disk with the odd checksum out of the array.
To find out the real truth will need some detailed MD RAID docs which I am having trouble finding or the sources which I cannot be bothered/don't have time to download and audit right now.
-Ross
______________________________________________________________________ This e-mail, and any attachments thereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail, you are hereby notified that any dissemination, distribution or copying of this e-mail, and any attachments thereto, is strictly prohibited. If you have received this e-mail in error, please immediately notify the sender and permanently delete the original and any copy or printout thereof.