Adding a device to a raid with mdadm

Reordering partitions to make room

So my current hard drives are:

# lsscsi
[0:0:0:0]    disk    ATA      Samsung SSD 860  1B6Q  /dev/sda 
[4:0:0:0]    disk    ATA      WDC WD4003FZEX-0 1A01  /dev/sdb 
[5:0:0:0]    disk    ATA      WDC WD5000AAKS-0 3B01  /dev/sdc 
[5:0:1:0]    disk    ATA      ST2000DM001-1CH1 CC29  /dev/sdd 
[10:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sde 
[11:0:0:0]   disk    WD       Ext HDD 1021     2021  /dev/sdf 

I had a lot of unused space on /dev/sdb, and thought I could rearrange the partitions and use it as an additional device in my (software) RAID.

I used Parted¹, but GParted² is also very nice. GParted can also scan my /dev/md127, which is a raid device.

Of course there's the general problem nowadays of looking what kind of MBR/GPT or whatever boot schema you use.

So:

Disk /dev/sdb: 3.64 TiB, 4000787030016 bytes, 7814037168 sectors
Disk model: WDC WD4003FZEX-0
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: D344E388-FA66-4B30-BA00-4D3B12A07B79

Device          Start        End    Sectors  Size Type
/dev/sdb1        2048       4095       2048    1M BIOS boot
/dev/sdb2        4096    1028095    1024000  500M Linux filesystem
/dev/sdb3     1028096 5273437500 5272409405  2.5T Linux filesystem
/dev/sdb4  5273438208 7420921855 2147483648    1T Linux filesystem

So it's a 4 TB hard drive, and it turns out that I only use /dev/sdb3. (The other partitions are from an old Linux install before I got a nice SDD and reinstalled Linux/Windows on that.)

So I could first rearrange it: resize /dev/sdb3 to 2T and move it to /dev/sdb1.

That worked fine using Parted. It just took some time to actually move all the data.

And then create a new /dev/sdb2 of 2T for my raid with gdisk³ (the GPT version of fdisk).

I had to look up the appropriate partition type in GPT⁴ for mdadm.

# gdisk /dev/sdb
GPT fdisk (gdisk) version 1.0.9

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Command (? for help): n
Partition number (2-128, default 2): 
First sector (34-7814037134, default = 3907006464) or {+-}size{KMGTP}: 
Last sector (3907006464-7814037134, default = 7814035455) or {+-}size{KMGTP}: 
Current type is 8300 (Linux filesystem)


Hex code or GUID (L to show codes, Enter = 8300): fd00
Changed type of partition to 'Linux RAID'

Command (? for help): p
Disk /dev/sdb: 7814037168 sectors, 3.6 TiB
Model: WDC WD4003FZEX-0
Sector size (logical/physical): 512/4096 bytes
Disk identifier (GUID): D344E388-FA66-4B30-BA00-4D3B12A07B79
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 7814037134
Partitions will be aligned on 2048-sector boundaries
Total free space is 3693 sectors (1.8 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048      3907006463   1.8 TiB     8300  
   2      3907006464      7814035455   1.8 TiB     FD00  Linux RAID

Adding the device

Current raid setup:

# mdadm --assemble /dev/md127 /dev/sdd1 /dev/sde1 /dev/sdf1
mdadm: /dev/md127 has been started with 3 drives.

# mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Wed Mar  6 22:16:05 2013
        Raid Level : raid1
        Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
      Raid Devices : 3
     Total Devices : 3
       Persistence : Superblock is persistent

       Update Time : Wed Aug 16 09:08:07 2023
             State : clean 
    Active Devices : 3
   Working Devices : 3
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : resync

              Name : micemouse:0
              UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
            Events : 7285

    Number   Major   Minor   RaidDevice State
       4       8       65        0      active sync   /dev/sde1
       5       8       81        1      active sync   /dev/sdf1
       3       8       49        2      active sync   /dev/sdd1

# cat /proc/mdstat
Personalities : [raid1] 
md127 : active raid1 sde1[4] sdd1[3] sdf1[5]
      1953380160 blocks super 1.2 [3/3] [UUU]
      
unused devices: <none>

Adding a device:

mdadm --add /dev/md127 /dev/sdb2
mdadm: added /dev/sdb2
  
md127 : active raid1 sdb2[6](S) sde1[4] sdd1[3] sdf1[5]
      1953380160 blocks super 1.2 [3/3] [UUU]
      
unused devices: <none>
 mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Wed Mar  6 22:16:05 2013
        Raid Level : raid1
        Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
      Raid Devices : 3
     Total Devices : 4
       Persistence : Superblock is persistent

       Update Time : Sat Aug 19 16:47:58 2023
             State : clean 
    Active Devices : 3
   Working Devices : 4
    Failed Devices : 0
     Spare Devices : 1

Consistency Policy : resync

              Name : micemouse:0
              UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
            Events : 7286

    Number   Major   Minor   RaidDevice State
       4       8       65        0      active sync   /dev/sde1
       5       8       81        1      active sync   /dev/sdf1
       3       8       49        2      active sync   /dev/sdd1

       6       8       18        -      spare   /dev/sdb2

[

root@localhost ~]# mdadm --grow /dev/md127 --raid-devices=4
raid_disks for /dev/md127 set to 4

  # mdadm --detail /dev/md127
/dev/md127:
           Version : 1.2
     Creation Time : Wed Mar  6 22:16:05 2013
        Raid Level : raid1
        Array Size : 1953380160 (1862.89 GiB 2000.26 GB)
     Used Dev Size : 1953380160 (1862.89 GiB 2000.26 GB)
      Raid Devices : 4
     Total Devices : 4
       Persistence : Superblock is persistent

       Update Time : Sat Aug 19 16:49:32 2023
             State : clean, degraded, recovering 
    Active Devices : 3
   Working Devices : 4
    Failed Devices : 0
     Spare Devices : 1

Consistency Policy : resync

    Rebuild Status : 0% complete

              Name : micemouse:0
              UUID : ed4531c4:59c132b2:a6bfc3d1:6da3b928
            Events : 7290

    Number   Major   Minor   RaidDevice State
       4       8       65        0      active sync   /dev/sde1
       5       8       81        1      active sync   /dev/sdf1
       3       8       49        2      active sync   /dev/sdd1
       6       8       18        3      spare rebuilding   /dev/sdb2

And now we wait.

# cat /proc/mdstat
Personalities : [raid1] 
md127 : active raid1 sdb2[6] sde1[4] sdd1[3] sdf1[5]
      1953380160 blocks super 1.2 [4/3] [UUU_]
      [>....................]  recovery =  0.1% (2956032/1953380160) finish=1156.0min speed=28118K/sec
      
unused devices: <none>

Ready in about 12 hours.

References

[1] Parted User’s Manual

https://www.gnu.org/software/parted/manual/parted.html

[2] GParted is a free partition editor for graphically managing your disk partitions.

https://gparted.org/

[3] gdisk(8) - Linux man page

https://linux.die.net/man/8/gdisk/

[4] Raid Wiki Kernel Org - Partition Types

https://raid.wiki.kernel.org/index.php/Partition_Types

WP Guru - How to add a drive to software RAID with mdadm

https://wpguru.co.uk/2021/01/expand-software-raid-mdadm/