Enlarge the system partition of my Debian VM

Long story short, my docker containers on the Debian VM is taking up too much space. Instead of cleaning things up, I decided to give it more space. The thing is that increasing the disk size from the Proxmox side will not directly result in a large partition in the VM, because it will not automatically allocate the space. Here is what we have to do manually.

Note that even though this is for a KVM, it should work for bare-metal as well.

Also note if you regret assigning more disk space to the VM, you are (probably) out of luck with a zfspool (exactly my case). If you are having an .img or .qcow2 virtual disk, you can use qemu-img to shrink the disk.

Procedure

  1. Turn off swap with sudo swapoff -a and you will know why we do this in one second.

  2. Run sudo fdisk /dev/sda. /dev/sda should be the drive you want to work on. You will see the following output

    Welcome to fdisk (util-linux 2.38.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
   
This disk is currently in use - repartitioning is probably a bad idea.
It's recommended to umount all file systems, and swapoff all swap
partitions on this disk.
   
   
Command (m for help):

  3. Print all partitions with command p

    Command (m for help): p

Disk /dev/sda: 70 GiB, 75161927680 bytes, 146800640 sectors
Disk model: QEMU HARDDISK
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xf9687e2f

Device     Boot     Start       End   Sectors  Size Id Type
/dev/sda1  *         2048 102856703 102854656   49G 83 Linux
/dev/sda2       102858750 104855551   1996802  975M  5 Extended
/dev/sda5       102858752 104855551   1996800  975M 82 Linux swap / Solaris

    Print the free space with F

    Command (m for help): F
Unpartitioned space /dev/sda: 20.95 GiB, 22499295232 bytes, 43943936 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes

    Start       End  Sectors Size
104855552 146800639 41947134  20G

    Based on the output, the disk partitions are like this. /dev/sda1 is the OS partition, followed by an extended partition /dev/sda2 with one logical partition /dev/sda5. The 20 GB free space is behind the the extended partition.

    Without manipulating /dev/sda2, we cannot merge the free space with the OS partition. Luckily, dev/sda2 has only one partition, /dev/sda5, which is the swap partition, and we have turned swap off already, so we can safely remove /dev/sda2.

  4. Remove the swap partition with commend d

    
Command (m for help): d
Partition number (1,2,5, default 5): 2

Partition 2 has been deleted.

Command (m for help): p
Disk /dev/sda: 70 GiB, 75161927680 bytes, 146800640 sectors
Disk model: QEMU HARDDISK
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xf9687e2f

Device     Boot Start       End   Sectors Size Id Type
/dev/sda1  *     2048 102856703 102854656  49G 83 Linux

Command (m for help): F
Unpartitioned space /dev/sda: 20.95 GiB, 22499295232 bytes, 43943936 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes

    Start       End  Sectors Size
102856704 146800639 43943936  21G

  5. Now we only have /dev/sda1. To increase its size, we should delete the partition first and recreate it with 20 GB extra size.

    Command (m for help): d
Selected partition 1
Partition 1 has been deleted.


Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-146800639, default 2048):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-146800639, default 146800639): 144803837

Created a new partition 1 of type 'Linux' and of size 69 GiB.
Partition #1 contains a ext4 signature.

Do you want to remove the signature? [Y]es/[N]o: n

    Note that fdisk does not support a human-readable unit or simply specifying an increment in size. You have to work out the numbers in sectors yourself.

  6. Then we will recrate the extended partition, swap partition, and change its type.

    Command (m for help): n
Partition type
   p   primary (1 primary, 0 extended, 3 free)
   e   extended (container for logical partitions)
Select (default p): e
Partition number (2-4, default 2): 2
First sector (144803838-146800639, default 144803840):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (144803840-146800639, default 146800639):

Created a new partition 2 of type 'Extended' and of size 975 MiB.

Command (m for help): n
All space for primary partitions is in use.
Adding logical partition 5
First sector (144805888-146800639, default 144805888):
Last sector, +/-sectors or +/-size{K,M,G,T,P} (144805888-146800639, default 146800639):

Created a new partition 5 of type 'Linux' and of size 974 MiB.

Command (m for help): t
Partition number (1,2,5, default 5):
Hex code or alias (type L to list all): 82

Changed type of partition 'Linux' to 'Linux swap / Solaris'.


Command (m for help): p
Disk /dev/sda: 70 GiB, 75161927680 bytes, 146800640 sectors
Disk model: QEMU HARDDISK
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xf9687e2f

Device     Boot     Start       End   Sectors  Size Id Type
/dev/sda1            2048 144803837 144801790   69G 83 Linux
/dev/sda2       144803840 146800639   1996800  975M  5 Extended
/dev/sda5       144805888 146800639   1994752  974M 82 Linux swap / Solaris

    Take a look at the partition table to make sure everything is right. Next, we will save the table which will exit fdisk.

    Command (m for help): w
The partition table has been altered.
Syncing disks.

  7. The partition table is changed, but we are not done yet. As we have deleted and recreated the swap partition, it does not have a valid UUID yet. After a new one is generated, it will surely be different from the previous, so that the OS and kernel do not know where the new swap is.

    sudo partprobe /dev/sda
sudo mkswap /dev/sda5

Setting up swapspace version 1, size = 974 MiB (1021308928 bytes)
no label, UUID=6e88d9a6-bcc4-423b-9d18-bd81c370ef3b

    Above comments will make the new partition as the swap partition. Next we need copy this new UUID to replace the existing one in /etc/initramfs-tools/conf.d/resume.

    sudo echo "RESUME=UUID=6e88d9a6-bcc4-423b-9d18-bd81c370ef3b" > /etc/initramfs-tools/conf.d/resume

    Then rebuild the kernel and reenable swap.

    sudo update-initramfs -u
sudo swapon -a

    After this, you should be all set.

Be very careful that this may result in a data loss. Since I am doing all of this in a VM, the whole VM has been backed up and I can easily restore everything in seconds, so I was not afraid of a data loss. If you are doing this on a bare-metal installation, be extremely careful.

Originally published on March 10, 2022.
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