One of my oldest servers has been through many upgrades, from Debian Wheezy (7) all the way through Debian Bookworm (12). Along the way the original 256MiB /boot partition got a little tight and eventually it got to the point where I could no longer upgrade the kernel.

The system has two boot drives, each with 2 partitions in Linux-MD mirrors, the first partitions for the boot mirror and the 2nd partitions for the OS drive Crypto/LVM. 

$ fdisk -l /dev/sda
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 499711 497664 243M fd Linux raid autodetect
/dev/sda2 499712 125044735 124545024 59.4G fd Linux raid autodetect

$ lsblk /dev/sda
NAME                        MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
sda                           8:0    0  59.6G  0 disk  
├─sda1                        8:1    0   243M  0 part  
│ └─md0                       9:0    0 242.8M  0 raid1 /boot
└─sda2                        8:2    0  59.4G  0 part  
  └─md1                       9:1    0  58.4G  0 raid1 
    └─md1_crypt             253:14   0  58.4G  0 crypt 
      ├─stora_vg-root_lv    253:15   0  14.3G  0 lvm   /
      └─stora_vg-swap       253:16   0  11.2G  0 lvm   [SWAP]

$ mdadm --detail /dev/md1
/dev/md1:
           Version : 1.2
     Creation Time : Thu Mar  7 08:29:14 2019
        Raid Level : raid1
        Array Size : 61262848 (58.42 GiB 62.73 GB)
     Used Dev Size : 61262848 (58.42 GiB 62.73 GB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

       Update Time : Mon Jul 10 18:33:08 2023
             State : clean
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0 
     Spare Devices : 0

Consistency Policy : resync

              Name : server:1
              UUID : 175fc546:57d222bf:b7e667c4:2c489708
            Events : 1666

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       1       8       18        1      active sync   /dev/sdb2

$ pvs
  PV                    VG       Fmt  Attr PSize  PFree 
  /dev/mapper/md1_crypt server_vg lvm2 a--  58.42g 32.93g

$ lvs
  LV      VG       Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  root_lv server_vg -wi-ao----  14.31g                                                    
  swap    server_vg -wi-ao---- <11.18g

$ cryptsetup status  md1_crypt
/dev/mapper/md1_crypt is active and is in use.
  type:    LUKS1
  cipher:  aes-xts-plain64
  keysize: 512 bits
  key location: dm-crypt
  device:  /dev/md1
  sector size:  512
  offset:  4096 sectors
  size:    122521600 sectors
  mode:    read/write
  flags:   discards 

The end goal of today project was to decrease sda2 and increase sda1, to get there however the all children of sda2 had to be shrunk to allow it to be resized: the crypt container has to be shrunk, then the md has to be shrunk and then the partitions can be recreated. We have enough unused space so we will be subtracting 15GiB or about 31457280(15 * 1024 * 1024 * 2) 512-byte sectors reducing 122521600 to 91064320 512-byte sectors.

The linux-md uses 1024-byte sectors and would  be resized from 61262848 down to 45534208.

$ cryptsetup resize md1_crypt -b 91064320
$ pvresize /dev/mapper/md1_crypt
$ mdadm --grow --size=45534208 /dev/md1

Now we will remove the drive partitions from linux-md, re-partition the drives and re-add them.

$ mdadm --fail /dev/md1 /dev/sda2
$ mdadm --fail /dev/md0 /dev/sda1
$ mdadm --remove /dev/md1 /dev/sda2
$ mdadm --remove /dev/md0 /dev/sda1
$ wipefs -a /dev/sda
$ fdisk /dev/sda
$ fdisk -l /dev/sda
Device     Boot   Start       End   Sectors  Size Id Type
/dev/sda1          2048   4196351   4194304    2G fd Linux raid autodetect
/dev/sda2       4196352 125045423 120849072 57.6G fd Linux raid autodetect
$ mdadm --add /dev/md0 /dev/sda1
$ mdadm --add /dev/md1 /dev/sda2

Once re-added and and the linux-md is synced we repeat the process with sdb. Once sdb had the same treatment we can resize the linux-md arrays and children to utilize all the space again

$ mdadm --grow /dev/md0 -z max
$ mdadm --grow /dev/md1 -z max
$ resize2fs /dev/md0
$ pvresize /dev/mapper/md1_crypt
$ cryptsetup resize md1_crypt

Finally we need reinstall the boot loader that may have gotten clobbered by the repartitioning

$ grub-install /dev/sdb
$ grub-install /dev/sda