Resizing Partitions on Linux

Sometimes, you need to resize your partitions on Linux. And this may not be as easy as you think...

When I installed Debian on my work computer a few months ago, the wizard suggested to create 2 separate partitions: one for root (with the OS itself), another for home (with my personal data). Alas! the suggested sizes (that I naively accepted) were highly unsuitable and a month later, the root partition (only 40 Go) was full. I had to resize them. It turns out that now, 5 months later, the home is full (I really generate a lot of data for my current project). It's time to resize my partitions again but this time I will keep track of the entire procedure in this article. Who knows when I will have to resize them one more time...

Current State and Goal

My computer as a single encrypted SSD disk:

 % lsblk
NAME                    MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINTS
nvme0n1                 259:0    0  3.6T  0 disk 
├─nvme0n1p1             259:1    0  512M  0 part  /boot/efi
├─nvme0n1p2             259:2    0  488M  0 part  /boot
└─nvme0n1p3             259:3    0  3.6T  0 part 
  └─nvme0n1p3_crypt     253:0    0  3.6T  0 crypt 
    ├─debian--vg-root   253:1    0  2.6T  0 lvm   /
    ├─debian--vg-swap_1 253:2    0  976M  0 lvm   [SWAP]
    └─debian--vg-home   253:3    0    1T  0 lvm   /home

fdisk -l shows similar information.

As I said, my home partition is full. However, the root partition is almost empty:

 % df -h | grep debian
Filesystem                   Size  Used Avail Use% Mounted on
/dev/mapper/debian--vg-root  2.6T  202G  2.3T   8% /
/dev/mapper/debian--vg-home 1007G  937G   19G  99% /home

My goal is to resize the partitions so that root is 1T and home occupies the rest of the disk. This will be a 2-step process: first, shrink root ; second, extend home.

Apparently, I can't do this from the graphical disk utility (note "Edit Partition..." and "Resize..." being greyed out):

It's time to open a terminal and input some weird commands!

Shrinking Root

Because I want the resize the root partition, it seems that I can't directly work from my installed Debian. I put my hands back on my bootable USB drive (the one I used to install Debian in the first place) and I reboot my computer on it. It doesn't provide a live Debian mode so I have to use the rescue mode to get a shell:

Yes, I took pictures on my laptop's screen.

Once I have configured my language and keyboard layout, and skipped the network configuration, I can finally enter the rescue mode for real:

I have to enter my passphrase because my disk is encrypted and I now have a shell. Hooray!

Resizing a "partition" is not a single-step operation. Indeed, there are in fact a logical volume and a filesystem on top of it. Both need to be resized in the correct order.

First, reduce the filesystem with the resize2fs command. In this first step, I use an temporary size, not the desired size. It must be big enough to hold all the files but smaller that the desired size. The output df -h above shows that 202G are used on root. The desired size is 1T. Hence, 300G seems fine.

resize2fs /dev/mapper/debian--vg-root 300G

I am asked to check the filesystem first with e2fsck -f /dev/mapper/debian--vg-home. Once the check and possible fixes are done, I can call resize2fs again.

Second, reduce the volume to the desired size (1T) with the lvreduce command:

lvreduce -L 1T /dev/vgroot/lvroot

Third (and last), extend the filesystem to use the entire volume. This is done by passing only the volume to resize2fs because if the "size parameter is not specified, it will default to the size of the partition".

resize2fs /dev/vgroot/lvroot

This is how it looks like in real life:

Note that step 3 may be performed directly in step 2, because lvreduce has an option that seems to do it:

-r, --resizefs
Resize underlying filesystem together with the logical volume using fsadm.

To be honnest, I wasn't really keen to try on my work computer, I can't afford any data loss. So I sticked a procedure that I knew to work but I encourage you to test it 😅

Checking Root

Now, I shutdown my computer, I remove the USB drive, I reboot to my actual system, and I'm glad to see that it still works 😄

I can check the state of my disk:

 % df -h | grep mapper
/dev/mapper/debian--vg-root 1008G  202G  756G  22% /
/dev/mapper/debian--vg-home 1007G  937G   19G  99% /home

 % lsblk
NAME                    MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINTS
nvme0n1                 259:0    0  3.6T  0 disk  
├─nvme0n1p1             259:1    0  512M  0 part  /boot/efi
├─nvme0n1p2             259:2    0  488M  0 part  /boot
└─nvme0n1p3             259:3    0  3.6T  0 part  
  └─nvme0n1p3_crypt     253:0    0  3.6T  0 crypt 
    ├─debian--vg-root   253:1    0    1T  0 lvm   /
    ├─debian--vg-swap_1 253:2    0  976M  0 lvm   [SWAP]
    └─debian--vg-home   253:3    0    1T  0 lvm   /home

The root partition is fine. Of course, the home partition is unchanged. Let's resize it now.

Extending Home

This time, I don't need to boot on my USB drive.

First, extend the volume with the lvextend command. I don't have to calculate the remaining size, I just ask to add 100% of the free space of the disk to the volume:

 % sudo lvextend -l +100%FREE /dev/mapper/debian--vg-home      
  Size of logical volume debian-vg/home changed from 1.00 TiB (262144 extents) to <2.64 TiB (691219 extents).
  Logical volume debian-vg/home successfully resized.

Note that the option is -l (lowercase) not -L (uppercase), unlike with lvreduce.

Second (and last), resize the filesystem to use the entire volume (same as last operation for root):

 % sudo resize2fs /dev/mapper/debian--vg-home
resize2fs 1.47.0 (5-Feb-2023)
Filesystem at /dev/mapper/debian--vg-home is mounted on /home; on-line resizing required
old_desc_blocks = 128, new_desc_blocks = 338
The filesystem on /dev/mapper/debian--vg-home is now 707808256 (4k) blocks long.

Checking Home

Finally, I have reached my goal:

 % df -h | grep mapper                       
/dev/mapper/debian--vg-root 1008G  202G  756G  22% /
/dev/mapper/debian--vg-home  2.6T  937G  1.6T  37% /home

 % lsblk                                           
NAME                    MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINTS
nvme0n1                 259:0    0  3.6T  0 disk  
├─nvme0n1p1             259:1    0  512M  0 part  /boot/efi
├─nvme0n1p2             259:2    0  488M  0 part  /boot
└─nvme0n1p3             259:3    0  3.6T  0 part  
  └─nvme0n1p3_crypt     253:0    0  3.6T  0 crypt 
    ├─debian--vg-root   253:1    0    1T  0 lvm   /
    ├─debian--vg-swap_1 253:2    0  976M  0 lvm   [SWAP]
    └─debian--vg-home   253:3    0  2.6T  0 lvm   /home

Conclusion

I have to be honnest, it was quite scary to resize partitions 😱 Of course, I backed up my important data, but I can't really back up a full system with all the installed tools and settings I have as a sofware developper on my machine.

At the end of the day, I left this unconfortable situation:

Before:

 % df -h | grep mapper       
Filesystem                   Size  Used Avail Use% Mounted on
/dev/mapper/debian--vg-root  2.6T  202G  2.3T   8% /
/dev/mapper/debian--vg-home 1007G  937G   19G  99% /home

And I have reached with much more cosy one:

 % df -h | grep mapper                       
/dev/mapper/debian--vg-root 1008G  202G  756G  22% /
/dev/mapper/debian--vg-home  2.6T  937G  1.6T  37% /home

I hope this was the last time I had to resize my partitions on this computer! 😁