Thorsten Gunkel

LVM - Logical volume management

LVM Howto

Overview of the elements of an LVM system

Prepare PV (discs or partitions)

Create new VG with the prepared PV

Remove VG

Add new PV to VG

Remove PV from VG

(e.g., when want to remove an old disc) Is there enough space even without the PV?

If not, first add a new disc like described above:

• Prepare discs / partitions
• Add new PV to VG

Then do a backup! Note that the following will require that you enabled the kernel option

Move everything from the old discs to the other PV

Or specify to which PV the data should be moved

Now remove the PV

pvmove

The command pvmove obviously requires, that you have enough space left on your VG which has not yet been occupied by a LV. But even when you have enough free space in total, pvmove will need your help if the amount of space is not available on a single PV. Even you try to move for example this sdb1

but there is no other PV which can carry it, this will fail

Just help it with some manual intervention. Use pvdisplay to find the PV with the largest value for Free PE for your LV. In this example it is sda6

Now you can ask pvmove to move 33267 PE on this PV

Check if the destination is now full, if not, just repeat the last command with an increased upper limit. When it is finally full just move the next area

to the next PV with some free space. If you specify an area where parts have already been moved, this parts will be automatically ignored. So this one

would be equivalent in our case, as we moved the part at the beginning already. However, this method might leads to LV which are fragmented. A better way is, to initiate a backup of the LVM structure

change into this directory

and read the information about your LVM. Let's assume you have the following output

The source device and the start point can be found in the stripes section

This shows how long it is

Now we only have to find the mapping for pv2 to a real device in the file. In this example we assume it is sda6. Now the segment can be moved completely to another device

If you move the segments of a LVM consecutively to the same device they will merge automatically.

Create LV on a VG

How much space is left?

Create LV with 1.5GB size, linear

Create LV with 1.5GB size, stripped

Create LV with 1.5GB size, linear and use only /dev/hdb5

Now create a filesystem on it

Remove LV

Extend LV

How much space is left?

Extend +1 GB (negativ numbers are discussed below, dangerous!)

Now extend the filesystem (ext2 / ext3)

Extend the filesystem (reiserfs, unmount may be optional)

Shrink LV

Use lvm to shrink lvm and filesystem in one step

Do it by yourself:

First reduce the filesystems size, then the LV! This will reduce the filesystem to a size of 20GB

Then shrink the LV to a size of 21GB (for paranoia a bit larger)

If you get this error

you probably tried to use the /dev/mapper/... device instead of the normal device.

Now you can extend the filesystem up to the maximum size which is possible with your LV, removing the gap we introduced for paranoia.

Forgot to issue vgreduce

When you moved all the data from a PV with pvmove away, but forgot to use vgreduce to remove it from the VG, the next LVM start will fail with this error

If you are sure that there is no more data on the PV, you can skip and remove it

Move VG to another system

unmount everything Disable VG

Export

On the other system

mount again

Partition table has been deleted

LVM fails to start with:

In this case the partition table of /dev/sdb had be deleted.

Try to guess the old partition table (while take some time ...)

If it succeeds: