RE: [Xen-users] LVM and resize2fs

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Jonathan Tripathy wrote:

> Would resize2fs allow you to "over-commit" ? I'd imagine that if the 
> LV size was smaller than expected, wouldn't it throw an error?
Indeed it does :

# lvm lvcreate -n test -L 1G vg0
  Logical volume "test" created
# mkfs -t ext3 /dev/vg0/test
...
# fsck -f /dev/vg0/test
...
# resize2fs /dev/vg0/test 2G
resize2fs 1.41.3 (12-Oct-2008)
The containing partition (or device) is only 262144 (4k) blocks.
You requested a new size of 524288 blocks.

So there's no problem resizing the filesystem.



What that won't help with is if you resize the underlying volume to 
be too small for the filesystem :
# lvm lvcreate -n test -L 3G vg0
  Logical volume "test" created
# mkfs -t ext3 /dev/vg0/test
...
# fsck -f /dev/vg0/test
...
# resize2fs /dev/vg0/test 2G
resize2fs 1.41.3 (12-Oct-2008)
Resizing the filesystem on /dev/vg0/test to 524288 (4k) blocks.
The filesystem on /dev/vg0/test is now 524288 blocks long.

# lvm lvresize -L 1G /dev/vg0/test
  WARNING: Reducing active logical volume to 1.00 GB
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce test? [y/n]: y
  Reducing logical volume test to 1.00 GB
  Logical volume test successfully resized
# fsck -f /dev/vg0/test
fsck 1.41.3 (12-Oct-2008)
e2fsck 1.41.3 (12-Oct-2008)
Superblock has an invalid ext3 journal (inode 8).
Clear<y>? no

fsck.ext3: Illegal inode number while checking ext3 journal for /dev/vg0/test



To prevent that, LVM would need to know about what might be in the 
volume and prompt you accordingly - but then it would lose that 
decoupling between storage volumes and filesystems.
Ie, if LVM "knew about" ext2/3 (and all the other) filesystems, then 
it could check the contents of the volume and ask you if you were 
sure you wanted to screw up your data. It would significantly 
increase the complexity of LVM to be able to do that though.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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