# Move the root filesystem from single disk to a RAID 1 without downtime

root$  df -h
Filesystem           Size  Used Avail Use% Mounted on
devtmpfs             7.7G     0  7.7G   0% /dev
tmpfs                7.7G     0  7.7G   0% /dev/shm
tmpfs                7.7G  8.8M  7.7G   1% /run
tmpfs                7.7G     0  7.7G   0% /sys/fs/cgroup
/dev/mapper/vg-root  5.2G  1.4G  3.5G  29% /
/dev/sdb1            922M  143M  732M  17% /boot
tmpfs                1.6G     0  1.6G   0% /run/user/0


# Create RAID 1:   /dev/md0 = (/dev/sda, /dev/sdc)

root$ sudo mdadm --create /dev/md0 --auto md --level=1 --raid-devices=2 /dev/sda /dev/sdc
mdadm: Note: this array has metadata at the start and
    may not be suitable as a boot device.  If you plan to
    store '/boot' on this device please ensure that
    your boot-loader understands md/v1.x metadata, or use
    --metadata=0.90
Continue creating array? y
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.


root$ cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdc[1] sda[0]
      2930134464 blocks super 1.2 [2/2] [UU]
      [>....................]  resync =  0.2% (6527104/2930134464) finish=395.1min speed=123312K/sec
      bitmap: 22/22 pages [88KB], 65536KB chunk

unused devices: <none>


root$ mdadm --examine /dev/sd[ac]
/dev/sda:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x1
     Array UUID : 812f8b4b:eec3b59b:c5f0f680:c525b1aa
           Name : n0s0:0  (local to host n0s0)
  Creation Time : Thu May 16 01:10:45 2019
     Raid Level : raid1
   Raid Devices : 2

 Avail Dev Size : 5860268976 (2794.39 GiB 3000.46 GB)
     Array Size : 2930134464 (2794.39 GiB 3000.46 GB)
  Used Dev Size : 5860268928 (2794.39 GiB 3000.46 GB)
    Data Offset : 264192 sectors
   Super Offset : 8 sectors
   Unused Space : before=264112 sectors, after=48 sectors
          State : active
    Device UUID : 3222008a:8b67b467:5d7a89d0:000b55dc

Internal Bitmap : 8 sectors from superblock
    Update Time : Thu May 16 01:12:50 2019
  Bad Block Log : 512 entries available at offset 24 sectors
       Checksum : ff7a421c - correct
         Events : 25


   Device Role : Active device 0
   Array State : AA ('A' == active, '.' == missing, 'R' == replacing)
/dev/sdc:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x1
     Array UUID : 812f8b4b:eec3b59b:c5f0f680:c525b1aa
           Name : n0s0:0  (local to host n0s0)
  Creation Time : Thu May 16 01:10:45 2019
     Raid Level : raid1
   Raid Devices : 2

 Avail Dev Size : 5860268976 (2794.39 GiB 3000.46 GB)
     Array Size : 2930134464 (2794.39 GiB 3000.46 GB)
  Used Dev Size : 5860268928 (2794.39 GiB 3000.46 GB)
    Data Offset : 264192 sectors
   Super Offset : 8 sectors
   Unused Space : before=264112 sectors, after=48 sectors
          State : active
    Device UUID : f42f92c1:8e8c6347:53fee391:8fd6cfa1

Internal Bitmap : 8 sectors from superblock
    Update Time : Thu May 16 01:12:50 2019
  Bad Block Log : 512 entries available at offset 24 sectors
       Checksum : 9d90c467 - correct
         Events : 25


   Device Role : Active device 1
   Array State : AA ('A' == active, '.' == missing, 'R' == replacing)



root$ mdadm --detail /dev/md0
/dev/md0:
           Version : 1.2
     Creation Time : Thu May 16 01:10:45 2019
        Raid Level : raid1
        Array Size : 2930134464 (2794.39 GiB 3000.46 GB)
     Used Dev Size : 2930134464 (2794.39 GiB 3000.46 GB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Thu May 16 01:13:35 2019
             State : clean, resyncing
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : bitmap

     Resync Status : 0% complete

              Name : n0s0:0  (local to host n0s0)
              UUID : 812f8b4b:eec3b59b:c5f0f680:c525b1aa
            Events : 34

    Number   Major   Minor   RaidDevice State
       0       8        0        0      active sync   /dev/sda
       1       8       32        1      active sync   /dev/sdc




# Add Raid 1 to the volume group
root$  pvcreate /dev/md0
  Physical volume "/dev/md0" successfully created.

root$ pvs
  PV         VG Fmt  Attr PSize   PFree
  /dev/md0      lvm2 ---   <2.73t <2.73t
  /dev/sdb2  vg lvm2 a--  <13.39g     0

root$ vgextend vg /dev/md0
  Volume group "vg" successfully extended


root$ lvextend -r -L 200GiB vg/root
  Size of logical volume vg/root changed from <5.39 GiB (1379 extents) to 200.00 GiB (51200 extents).
  Logical volume vg/root successfully resized.
resize2fs 1.42.9 (28-Dec-2013)
Filesystem at /dev/mapper/vg-root is mounted on /; on-line resizing required
old_desc_blocks = 1, new_desc_blocks = 25
The filesystem on /dev/mapper/vg-root is now 52428800 blocks long.

root$ lvs
  LV   VG Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  root vg -wi-ao---- 200.00g
  swap vg -wi-ao----   8.00g

root$ df -h
Filesystem           Size  Used Avail Use% Mounted on
devtmpfs             7.7G     0  7.7G   0% /dev
tmpfs                7.7G     0  7.7G   0% /dev/shm
tmpfs                7.7G  8.8M  7.7G   1% /run
tmpfs                7.7G     0  7.7G   0% /sys/fs/cgroup
/dev/mapper/vg-root  197G  1.5G  188G   1% /
/dev/sdb1            922M  143M  732M  17% /boot
tmpfs                1.6G     0  1.6G   0% /run/user/0


# Moves the extends from the old device to the new RAID 1
root$  pvmove /dev/sdb2 /dev/md0
  /dev/sdb2: Moved: 0.00%
  /dev/sdb2: Moved: 8.02%
  /dev/sdb2: Moved: 43.95%
  /dev/sdb2: Moved: 75.05%
  /dev/sdb2: Moved: 100.00%


root$  pvs
  PV         VG Fmt  Attr PSize   PFree
  /dev/md0   vg lvm2 a--   <2.73t  <2.53t
  /dev/sdb2  vg lvm2 a--  <13.39g <13.39g


# Remove the old device

root$  vgreduce vg /dev/sdb2
  Removed "/dev/sdb2" from volume group "vg"


root$  pvremove /dev/sdb2
  Labels on physical volume "/dev/sdb2" successfully wiped.


# Save the configuration
root$ mdadm --detail --scan --verbose >> /etc/mdadm.conf