LVM root partition only uses half the volume size

Question

I have an Ubuntu server 20.04 with an encrypted 50GB LVM root partition and I just realized the filesystem itself only shows 25GB

The install was default (apart from the encryption bit) and I don't understand why it didn't use all the space for the root partition?

How do I expand the root filesystem?

 PV                     VG        Fmt  Attr PSize  PFree
  /dev/mapper/dm_crypt-0 ubuntu-vg lvm2 a--  48.48g <24.24g

 VG        #PV #LV #SN Attr   VSize  VFree
  ubuntu-vg   1   1   0 wz--n- 48.48g <24.24g

 LV        VG        Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  ubuntu-lv ubuntu-vg -wi-ao---- 24.24g

Answer 1

why it didn't use all the space for the root partition?

When the logical volume was created, only 24.24 GB was allocated for it. That can actually be a good thing: the remainder can be used to create another logical volume if you find you need one for some reason, or you can use the free space to extend an existing logical volume, even while its filesystem is mounted and in use.

Having some unallocated space held in reserve can be a good thing, as it allows you to react to unexpected future requirements easily:

• A filesystem needs more space than expected? No problem, you can extend it on-line. (Extending a filesystem is usually much easier than shrinking one, so lowballing the expected requirements and then extending as needed can be a good strategy.)
• You need a small filesystem with special mount options for a chroot jail? Just create a new LV for it using some of the unallocated space.

How do I expand the root filesystem?

For example, to extend it by 5 GB:

sudo lvextend --resizefs -L +5G ubuntu-vg/ubuntu-lv

Or if you want to use all the remaining unallocated capacity to extend the root filesystem:

sudo lvextend --resizefs -l +100%FREE ubuntu-vg/ubuntu-lv

If you don't use the --resizefs option, then the command will just extend the logical volume but not the filesystem inside it. Then you must use another command to tell the filesystem to take advantage of the extension: either fsadm resize /dev/mapper/ubuntu--vg-ubuntu--lv or a filesystem-specific command like resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv or xfs_growfs /.

(The --resizefs option of lvextend will actually just run the fsadm resize ... command for you once the LV is successfully extended.)

Answer 2

CONTEXT

There should be (sort of) multiple file systems on a LVM block. This is the way a physical file system is represented in LVM (at least as far as LVM and LVM2 are concerned).

If you look at the fs with Gparted GUI, it will look different than other partitions. Whereas most partitions will be represented by a bunch of blocks laid out in a row which represents a drive, the LVM partition will be 2 blocks, one stacked on top of the other, nested within the row.

This is because LVM consists of multiple volumes, which work together to store, and give you access to, your data. Sort of like RAID, although here is where their similarities end.

And this is where it may get a bit confusing. Technically, these two drives are representing the same data, but in different ways. If you are familiar with archiving, the same abstraction can be made to work for LVM (although the terms are not interchangeable.)

LVM has logical volumes (LVs), physical volumes (PVs), and volume groups (VGs). A single volume group can hold multiple physical volumes. It will also contain a logical volume, which holds metadata for the physical volumes. In other words, it is said that the logical volume in a VG 'maps' the VG's physical volumes. This metadata is why I compared earlier to the concept of an archive.

This mapping functionality gives LVM some advantages over other file systems. For one thing, since its physical portion can be split into portions, the LVM partition, itself, can be split across more than one drive. This is not possible with, for example, Ext2,3,4 volumes.

The main disadvantage of LVM (at least in my experience) occurs in the 'applied' realm. Especially where Ubuntu is concerned.

For example, if you obtained your LVM via the "repair drive" option on the Ubuntu Live media (i.e. you used a USB boot stick to repair a broken partition, and got back a LVM2 / LUKs partition), then their is a high likelihood that it will not work.

Disclaimer: This is intended to be a very high level explanation, not a deep dive. In that regard, some of the statements are in a "more or less" format, so as to make the concepts understandable. In some places, I actually started to explain more deeply, but found the result to be hard to follow, so I changed them. For a deeper look, see here.

TO ANSWER THE QUESTION ITSELF

The filesystem itself is only a piece of your lvm partition. In each instance I have seen so far, in a LVM partition created by Ubuntu Live, there are 2 PVs. This may be editable. That said, I have worked with 5 separate blocks, all created by different users at different times and on different machines, and in my experience, 2 has always been the magic number. Take that however you want.

The best way to see the concept for yourself (on 20.04; not sure about the Gparted version) is to inspect your drive with Gparted GUI. You will see the abstraction I've laid out first hand.