Return to Answer

    /dev/mapper/ExistingExt4 is active.
      type:    LUKS1
      cipher:  aes-cbc-essiv:sha256
      keysize: 256 bits
      device:  /dev/sda2
      offset:  2056 sectors
      size:    156049348 sectors
      mode:    read/write

One sector is always 512 bytes (mentioned in cryptsetup manual page). Thus, to subtract 15 GiB, use a sector size of 156049348 - 15 * 1024 * 1024 * 2 = 124592068:

WARNING: the following commands has destructed data. Do not copy it without understanding what is happening. The sector dimensions must be changed, otherwise you WILL destroy your partition(s). I am in no way responsible for your stupidness, BACKUP BACKUP BACKUP your data to a second storage medium before risking your data.

Purpose of the cryptsetup resize command

    /dev/mapper/ExistingExt4 is active.
      type:    LUKS1
      cipher:  aes-cbc-essiv:sha256
      keysize: 256 bits
      device:  /dev/sda2
      sector size:  512
      offset:  2056 sectors
      size:    156049348 sectors
      mode:    read/write

(As of cryptsetup 2.0.0 (December 2017), the sector size may be larger than 512 bytes: see the cryptsetup(8) manpage and the --sector-size option.)

Thus, to subtract 15 GiB, use a sector size of 156049348 - 15 * 1024 * 1024 * 2 = 124592068:

Resizing the partition with parted

WARNING: the following commands has destroyed data. Do not copy it without understanding what is happening. The sector dimensions must be changed, otherwise you WILL destroy your partition(s). I am in no way responsible for your stupidness, BACKUP BACKUP BACKUP your data to a second storage medium before risking your data.

As 15 GiB has to be shaved off, the new end becomes 156301438 - 15 * 1024 * 1024 * 2 = 124844158. Since I want to change partition 2, I first have to remove it and then recreate it with the label "Everything" (this could be changd if you like):

(parted) rm 2
(parted) mkpart Everything 250035s 124844158s
Warning: The resulting partition is not properly aligned for best performance.
Ignore/Cancel? ignore
(parted) p
Model: ATA INTEL SSDSA2CW08 (scsi)
Disk /dev/sda: 156301488s
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start    End         Size        File system  Name    Flags
 1      34s      2082s       2049s                    Boot    bios_grub
 3      2083s    250034s     247952s     ext2         RootBoot
 2      250035s  124844158s  124594124s               Everything
(parted) quit

As 15 GiB has to be shaved off, the new end becomes 156301438 - 15 * 1024 * 1024 * 2 = 124844158. Since I want to change partition 2, I first have to remove it and then recreate it with the label "Everything" (this could be changed if you like). Note: this disk has a GPT layout. For MBR, you should replace Everything by primary or extended (untested, resizing a partition on MBR has not been tested and is not recommended because it is untested).

WARNING: the following commands has destructed data. Do not copy it without understanding what is happening. The sector dimensions must be changed, otherwise you WILL destroy your partition(s). I am in no way responsible for your stupidness, BACKUP BACKUP BACKUP your data to a second storage medium before risking your data.

(parted) rm 2
(parted) mkpart Everything 250035s 124844158s
Warning: The resulting partition is not properly aligned for best performance.
Ignore/Cancel? ignore
(parted) p
Model: ATA INTEL SSDSA2CW08 (scsi)
Disk /dev/sda: 156301488s
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start    End         Size        File system  Name    Flags
 1      34s      2082s       2049s                    Boot    bios_grub
 3      2083s    250034s     247952s     ext2         RootBoot
 2      250035s  124844158s  124594124s               Everything
(parted) quit

Between 1 and 2, run fsck to ensure that the filesystem is healthy:

After backing up (step 1) and unmounting (between 2 and 3), run fsck to ensure that the filesystem is healthy: