[linux-lvm] VG defragmenting and backing up the extent map

Brian J. Murrell brian at interlinx.bc.ca
Thu May 4 01:39:36 UTC 2006

It has often occurred to me that the fact that a logical volume is not
contiguous will make recovery in the face of a partial disaster more

For example, if I happen to lose the logical->physical extent map in the
LVM superblock (the single only place this information is kept AFAIK)
then I am pretty screwed with respect to getting my filesystems back off
the disk.

Compared with just losing the partition table from a traditionally
partitioned disk, I could use a tool like findsuper and losetup the
whole disk with offsets (losetup -o <offset> to retrieve the individual

This technique would even work with LVM given that I had lost the part
of the disk with the extent mapping except that my filesystems are not
contiguous if I have extended any of them.

So, the question is, has anyone developed a tool to "defragment" a
Volume Group so that all of the logical volumes in it are contiguous?

On a somewhat related note, what provisions can I take to mitigate the
loss of the extent map?  I am correct in that only one copy of it exists
at the head of a Physical Volume?  Can I make a backup of it in any way
so that it could be used at a later date to reconstruct logical volumes?

As you might have guessed, I have lost a small portion of a 160GB disk,
the portion which has the extent mapping for the whole PV and I want to
mitigate this problem in the future.


My other computer is your Microsoft Windows server.

Brian J. Murrell
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