Resizing a LUKS encrypted volume

I have a 500GiB ext4 filesystem on top of LUKS on top of an LVM LV. I want to resize the LV to 100GiB. I know how to resize ext4 on top of an LVM LV, but how do I deal with the LUKS volume?

mgorven@moab:~% sudo lvdisplay /dev/moab/backup
  --- Logical volume --- 
  LV Name                /dev/moab/backup
  VG Name                moab
  LV UUID                nQ3z1J-Pemd-uTEB-fazN-yEux-nOxP-QQair5                                                                      
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                500.00 GiB                                                                                                           
  Current LE             128000
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     2048 
  Block device           252:3
mgorven@moab:~% sudo cryptsetup status backup
/dev/mapper/backup is active and is in use.
  type:    LUKS1
  cipher:  aes-cbc-essiv:sha256
  keysize: 256 bits
  device:  /dev/mapper/moab-backup
  offset:  3072 sectors
  size:    1048572928 sectors
  mode:    read/write
mgorven@moab:~% sudo tune2fs -l /dev/mapper/backup
tune2fs 1.42 (29-Nov-2011)
Filesystem volume name:   backup
Last mounted on:          /srv/backup
Filesystem UUID:          63877e0e-0549-4c73-8535-b7a81eb363ed
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags:         signed_directory_hash 
Default mount options:    (none)
Filesystem state:         clean with errors
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              32768000
Block count:              131071616
Reserved block count:     0
Free blocks:              112894078
Free inodes:              32044830
First block:              0
Block size:               4096
Fragment size:            4096
Reserved GDT blocks:      992
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
RAID stride:              128
RAID stripe width:        128
Flex block group size:    16
Filesystem created:       Sun Mar 11 19:24:53 2012
Last mount time:          Sat May 19 13:29:27 2012
Last write time:          Fri Jun  1 11:07:22 2012
Mount count:              0
Maximum mount count:      100
Last checked:             Fri Jun  1 11:03:50 2012
Check interval:           31104000 (12 months)
Next check after:         Mon May 27 11:03:50 2013
Lifetime writes:          118 GB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               256
Required extra isize:     28
Desired extra isize:      28
Journal inode:            8
Default directory hash:   half_md4
Directory Hash Seed:      383bcbc5-fde9-4720-b98e-2d6224713ecf
Journal backup:           inode blocks

LUKS doesn't actually store the size of the device -- it simply discovers it when the volume is opened. It therefore only comes into play if the volume is not closed and reopened during the process (e.g. doing an online grow). In this case the size of the open volume needs to be rediscovered.

To shrink your volume, use the following process:

1. Unmount the filesystem with umount
2. Resize the filesystem with resize2fs
3. Close the LUKS volume with cryptsetup luksClose
4. Resize the LV with lvreduce or lvresize
5. Open the LUKS volume with cryptsetup luksOpen
6. Mount the filesystem with mount

You could also omit the luksClose and luksOpen steps, and use cryptsetup resize after resizing the LV. Also remember that LUKS uses some extra space to store metadata, so the LV needs to be slightly bigger than the filesystem. I usually resize the filesystem significantly smaller, and then grow it again after resizing the LV.

If you were growing the filesystem and wanted to do it online, you would use the following process:

1. Resize the LV with lvextend or lvresize
2. Update the size of the open LUKS volume with cryptsetup resize
3. Grow the filesystem with resize2fs