Resizing a LVM partition

by Etienne Millon on May 13, 2014

Tagged as: linux, lvm.

I like to have /home on a separate partition. But sometimes it can backfire. If the root partition is full, you don’t have a lot of solutions. In particular, I found that debian-installer’s “automatic partitioning” sometimes creates very small root partitions. If you want to install big packages (ghc, eclipse, libreoffice, …), a 16GiB root partition is not enough.

In the past, filesystems were sitting on directly on top of partitions. It means that it was very difficult to change their size.

Modern systems (post-1998) can use LVM, which is a layer between filesystems and partitions. One of its advantages is that you can resize logical volumes (a LV is the virtual device node where the filesystem sits) after they have been created.

To resize the / and /home filesystems, it is necessary to change the change the size of both the filesystems and the LVs. But it is not possible to do it in any order: at any time, the filesystem must be smaller than its LV. So, the correct order of operations is:

• shrink home filesystem
• shrink home LV
• expand root LV
• expand root filesystem

Wait a second before you start reaching for your favorite live CD: all these operations can be done online. Actually, the two first ones need /home to be unmounted, so it has to be done in single user mode. Online expansion of the root file system is fairly new (it’s from Linux 3.3, 2012) but it works like a charm.

Manipulating partition and volume sizes are always a bit tricky. Sometimes you have to give sizes in blocks, sometimes in bytes. Sometimes it’s multiples of 1000 and sometimes it’s multiples of 1024. I would not feel comfortable after typing 4 commands with 4 sizes. Fortunately, LVM tools are wonderful and can “talk” to the underlying filesystem (using fsadm). And LVM knows how much space is free, so it can expand a partition to fill completely the disk (or more precisely the volume group).

In a nutshell, this complex operation can be done in two commands (in single user mode):

lvresize -r -L 800G /dev/mapper/machine-home
lvresize -r -l '+100%FREE' /dev/mapper/machine-root

The -r switch enables fsadm. -L indicates the new size in terms of bytes, and -l in terms of LVM units (+100%FREE means: increase by the whole free space, ie fill the volume group).

That was almost too easy!