What does delta devices of the mdadm--detail output mean?

Question

Here is an output of mdadm --detail:

$ mdadm -D /dev/md2
/dev/md2:
        Version : 1.2
  Creation Time : Wed Jun 24 16:41:17 2020
     Raid Level : raid5
     Array Size : 7809195456 (7447.43 GiB 7996.62 GB)
  Used Dev Size : 7809195456 (7447.43 GiB 7996.62 GB)
   Raid Devices : 3
  Total Devices : 3
    Persistence : Superblock is persistent

    Update Time : Sun Jun 28 09:45:28 2020
          State : clean, reshaping
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

 Reshape Status : 38% complete
  Delta Devices : 1, (2->3)

           Name : XXXXXXXXXXX  (local to host XXXXXXXXXXXXXXXX)
           UUID : XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
         Events : 9425

    Number   Major   Minor   RaidDevice State
       0       8        5        0      active sync   /dev/sda5
       1       8       21        1      active sync   /dev/sdb5
       2       8       37        2      active sync   /dev/sdc5

What does the line

Delta Devices : 1, (2->3)

mean?

Answer 1

This means you have increased your device number by one recently. Currently the array is reshaping from the status of having two devices to having three (38% complete). The first number represents the amount of added devices (one) and the numbers between the arrow show the absolut amount of devices before(two)->after(three).