[linux-lvm] Adding a mirror after-the-fact

[Werner John]

Jason Tackaberry writes:
> > You could either mirror the whole disk using MD *without* persistent
> > superblocks
> 
> This sounds like the best and most transparent solution.  But
> Documentation/md.txt says:
> 
> raid level = -1 linear mode
>               0 striped mode
>   	      other modes are only supported with persistant super blocks
> 
> So it sounds like this won't work?  I'll give it a try and see what
> happens anyway.

It should work, you only need the persistent superblock, if you want to
boot from software RAID.
Here the quote from the HOWTO

>>  3.7.  The Persistent Superblock
>>
>>  Back in ``The Good Old Days'' (TM), the raidtools would read your
>>  /etc/raidtab file, and then initialize the array.  However, this would
>>  require that the filesystem on which /etc/raidtab resided was mounted.
>>  This is unfortunate if you want to boot on a RAID.
>>
>>  Also, the old approach led to complications when mounting filesystems
>>  RAID devices. They could not be put in the /etc/fstab file as usual,
>>  but would have to be mounted from the init-scripts.
>>
>>  The persistent superblocks solve these problems. When an array is
>>  initialized with the persistent-superblock option in the /etc/raidtab
>>  file, a special superblock is written in the beginning of all disks
>>  participating in the array. This allows the kernel to read the
>>  configuration of RAID devices directly from the disks involved,
>>  instead of reading from some configuration file that may not be
>>  available at all times.
>>
>>  You should however still maintain a consistent /etc/raidtab file,
>>  since you may need this file for later reconstruction of the array.
>>
>>  The persistent superblock is mandatory if you want auto-detection of
>>  your RAID devices upon system boot. This is described in the
>>  Autodetection section.

Cheers,
	Werner