[ILUG] Can someone explain a little about linux-raid ?

[Conor Daly]

On Thu, Dec 05, 2002 at 09:12:15AM +0000 or so it is rumoured hereabouts, 
John P. Looney thought:
>  I got badly bitten yesterday because of my lack of knowledge of Linux
> RAID (and some assumptions that it would work like Solaris' Disksuite).
> 
>  It took about two reboots before I copped on that raidstop wasn't
> persistant across reboots, like it is on solaris. Because the partition
> types were set to "RAID Autodetect", every boot it was making an md0, and
> even when I wasn't mounting it, it was syncing the two halves of the
> mirror. Worse yet, it didn't sync from "last mounted" to "other disk", it
> was always picking the corrupted disk, and syncing that to hda5, which I
> had mounted, read-write, as root.
> 
>  Could someone with a bit more RAID knowledge than I have tell me what the
> "right way to do things" was ? I've a feeling it probably incorporates
> "Wait till both halves of the RAID mirror sync before you reboot" or some
> such...as you don't have to do this in Solaris, I didn't bother...and that
> could be what caused the massive corruption.

man raidtab says:

       persistent-superblock 0/1
              newly  created  RAID arrays should use a persistent
              superblock. A persistent superblock is a small disk
              area allocated at the end of each RAID device, this
              helps the kernel to safely detect RAID devices even
              if  disks have been moved between SCSI controllers.
              It can be used for RAID0/LINEAR arrays too, to pro­
              tect  against  accidental  disk mixups. (the kernel
              will either correctly reorder disks, or will refuse
              to  start  up an array if something has happened to
              any member disk. Of course for the 'fail-safe' RAID
              variants  (RAID1/RAID5) spares are activated if any
              disk fails.)

              Every   member    disk/partition/device    has    a
              superblock, which carries all information necessary
              to start up the whole array. (for autodetection  to
              work  all  the  'member'  RAID partitions should be
              marked type 0xfd via fdisk) The superblock  is  not
              visible  in  the  final  RAID  array  and cannot be
              destroyed accidentally  through  usage  of  the  md
              device  files,  all  RAID data content is available
              for filesystem use.

I _think_ this is what caused your "wrong way" resync.  zeroing the _end_
of /dev/hdd1 _should_ remove it whereupon it should get resynced correctly.  

Of course, to do this safely, you'd need a raidstop first.  To prevent the
automatic raid start at boot, create a "no raid" lilo entry.  Hint:

mkinitrd --omit-raid-modules /boot/no-raid.img <kernel.version>

is your friend.

Conor
-- 
Conor Daly <conor.daly at oceanfree.net>

Domestic Sysadmin :-)
---------------------
Faenor.cod.ie
 10:09am  up 29 days, 18:53,  0 users,  load average: 0.00, 0.00, 0.00
Hobbiton.cod.ie
 10:06am  up 29 days, 18:44,  3 users,  load average: 0.02, 0.02, 0.00