System Recovery

[VirginSnow at vfemail.net]

> Date: Fri, 8 Aug 2008 13:49:02 -0400
> From: "Labitt, Bruce" <labittb1 at tycoelectronics.com>

> > I LVM'd the bigger disk (300GB) onto the base system disk (80GB)
> 
> >>Uh-ohh.  As I said, my LVM-fu is poor but if the result of
> doing that was to create a block device whose aggregated size
> was ~380Gb and you then built an ext3 filesystem in that device
> from which all but the first 80Gb are now MIA, I suspect you'll
> not be able to recover very much...  :-/

I'm going to have to second that gloomy prediction... with a few quid
pro qui:

  (1) Are you sure that you actually LVM'd the two disks into the same
      logical volume?  When installing to the 80GB drive, you probably
      created a LV from the entire drive.  Did the / fs initially
      appear to be 80GB right after you installed?  When you added the
      300GB drive, you could have either added the 800GB drive to the
      same volume group as / or (maybe, hopefully) created a second
      volume group, a second logical volume, and mounted the 300GB
      drive on its own mount point.  Did the 300GB drive appear 300GB
      big when you ran df?  Or did you see a / 380GB in size?

  (2) How much of the 300GB disk, if any, was ACTUALLY USED to hold
      data when you took it offline to reinstall?  Do you remember
      running df just prior to doing this?  Was usage below or above
      80GB?  By how much?  This is important to know because whatever
      "overflowed" onto the 800GB disk may now be lost.  Hopefully, it
      was nothing big, important, or both.

  (3) How did you take the 300GB disk offline?  If you used lvreduce
      et al., you might not have lost any data at all.

Assuming you haven't yet fdisked the 80GB drive, and both drives were
part of the same volume group, the 80GB drive will have a copy of the
VGDA (volume group descriptor area?).  That's a start.  It contains
info about how your LVM array used to be configured.  You might try
bringing the LV online and seeing what LVM complains is missing.
Note: Trying to mount the PV as a filesystem of type ext3 will NOT
work.  It's "formatted" for LVM.  You have to mount the LV that
contains the ext3 filesystem.

Since you haven't overwritten the 80GB drive yet, at least the first
80GB of the ext3fs should still be intact.  Remember, too, that ext3fs
puts copies of the superblock at (ir)regular intervals within the fs.
So if a straight "mount" of the LV fails, you can try using the "-o
sb=..." option to mount.

Perhaps removing disks like this and expecting them to retain their
data is a common LVM mistake.  When you install to a hard drive, most
installers issue loud warnings before letting you repartition/reformat
anything.  But there's no such warning issued when you try to
dismember a VG incorrectly.  Perhaps our installers should be updated
to provide advance warning of potential problems to those installing
to volume groups!