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

On Tue, Aug 14, 2001 at 05:11:59PM -0400, Jason Tackaberry wrote:
> I know this isn't strictly an LVM question, but ...
> Suppose I have a volume group with 1 physical volume, say a 9GB disk.
> Now, after some time, I decide I want to mirror this VG with another 9GB
> disk.  What options do I have?  AFAICS, the only way to do this would be
> to start from scratch?  This is a pretty simple and presumably common
> thing to want to do, so I hope I'm wrong.  :)

I've done this using RAID only on ext2fs partitions, no LVM...  it's not
too tough.  I recommend *using* persistent superblocks (they come in handy
sometimes), though it's a little fun to add them to a live file system.

Fact 1:  with or without persistent superblocks, adding raid to a device
doesn't change the initial offset (ie the underlying filesystem begins at
the 0th block regardless of raid or non raid).

Fact 2:  persistent superblock adds a superblock at the end of the device,
of variying size (not sure why).

You can "ext2resize /dev/blah numblocks" where numblocks is the size of
the device minus 5 to 50mb (enough for the superblock, yes it's overkill).  
Once you mkraid on the device, just "ext2resize /dev/blah" to get back
whatever space you over-reserved.  

If you *KNOW* the filesystem isn't using blocks near the end of the
device, you can skip the initial resize, do the mkraid, and "e2fsck
/dev/blah" (read off the actual block size, then abort the e2fsck) and
"ext2resize /dev/blah newnumblocks" 

If you're not using ext2, then do some tests with your filesystems resize
utility to make sure it's bulletproof and go at it.  reiserfs_resize was
buggy in this regard a few months ago when I was testing it...  I've
not tested recently, so maybe it's been fixed.  (it grows just fine, but
introduces errors when shrinking).

And finally, since this is a LVM list, I've no idea what changing device
layers under LVM will do... I know the raid won't do anything but clip
some blocks off the end of the device, but I don't know LVM's handling or
block resize options.

Ted Deppner

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