从RAID1降级一个盘并重建成RAID4,双盘RAID4相当于RAID0,然后RAID4降级到RAID0,下面是流程记录.

root@ns******:~# cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md2 : active raid1 sda2[1] sdb2[0]
      1952855040 blocks super 1.2 [2/2] [UU]
      bitmap: 1/15 pages [4KB], 65536KB chunk

unused devices: <none>
root@ns******:~# mdadm /dev/md2 --fail /dev/sdb2
mdadm: set /dev/sdb2 faulty in /dev/md2
root@ns******:~# mdadm /dev/md2 --remove  /dev/sdb2
mdadm: hot removed /dev/sdb2 from /dev/md2
root@ns******:~# wipefs -a /dev/sdb2
/dev/sdb2: 4 bytes were erased at offset 0x00001000 (linux_raid_member): fc 4e 2b a9
root@ns******:~# cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md2 : active raid1 sda2[1]
      1952855040 blocks super 1.2 [2/1] [_U]
      bitmap: 3/15 pages [12KB], 65536KB chunk

unused devices: <none>
root@ns******:~# mdadm --detail /dev/md2
/dev/md2:
           Version : 1.2
     Creation Time : Mon Nov 22 15:19:28 2021
        Raid Level : raid1
        Array Size : 1952855040 (1862.39 GiB 1999.72 GB)
     Used Dev Size : 1952855040 (1862.39 GiB 1999.72 GB)
      Raid Devices : 2
     Total Devices : 1
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Tue Nov 23 01:46:30 2021
             State : clean, degraded 
    Active Devices : 1
   Working Devices : 1
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : bitmap

              Name : md2
              UUID : 6849a02d:97f671af:786a5197:14d8f4a8
            Events : 20

    Number   Major   Minor   RaidDevice State
       -       0        0        0      removed
       1       8        2        1      active sync   /dev/sda2
root@ns******:~# mdadm --grow /dev/md2 --level=0 
mdadm: level of /dev/md2 changed to raid0
root@ns******:~# mdadm --misc --detail /dev/md2
/dev/md2:
           Version : 1.2
     Creation Time : Mon Nov 22 15:19:28 2021
        Raid Level : raid0
        Array Size : 1952855040 (1862.39 GiB 1999.72 GB)
      Raid Devices : 1
     Total Devices : 1
       Persistence : Superblock is persistent

       Update Time : Tue Nov 23 01:47:35 2021
             State : clean 
    Active Devices : 1
   Working Devices : 1
    Failed Devices : 0
     Spare Devices : 0

        Chunk Size : 64K

Consistency Policy : none

              Name : md2
              UUID : 6849a02d:97f671af:786a5197:14d8f4a8
            Events : 24

    Number   Major   Minor   RaidDevice State
       1       8        2        0      active sync   /dev/sda2
root@ns******:~# mdadm --add /dev/md2 /dev/sdb2
mdadm: level of /dev/md2 changed to raid4
mdadm: added /dev/sdb2
root@ns******:~# mdadm --misc --detail /dev/md2
/dev/md2:
           Version : 1.2
     Creation Time : Mon Nov 22 15:19:28 2021
        Raid Level : raid4
        Array Size : 1952855040 (1862.39 GiB 1999.72 GB)
     Used Dev Size : 1952855040 (1862.39 GiB 1999.72 GB)
      Raid Devices : 3
     Total Devices : 2
       Persistence : Superblock is persistent

       Update Time : Tue Nov 23 01:48:38 2021
             State : active, FAILED, reshaping 
    Active Devices : 1
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 1

        Chunk Size : 64K

Consistency Policy : resync

    Reshape Status : 0% complete
     Delta Devices : 1, (2->3)

              Name : md2
              UUID : 6849a02d:97f671af:786a5197:14d8f4a8
            Events : 45

    Number   Major   Minor   RaidDevice State
       1       8        2        0      active sync   /dev/sda2
       2       8       18        1      spare rebuilding   /dev/sdb2
       -       0        0        2      removed
root@ns******:~# watch cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md2 : active raid4 sdb2[2] sda2[1]
      1952855040 blocks super 1.2 level 4, 64k chunk, algorithm 5 [3/2] [U__]
      [>....................]  reshape =  0.9% (19050868/1952855040) finish=750.7min speed=42927K/sec
      
unused devices: <none>

正在努力Reshape,然后漫长时间后重启,就Raid0了.

root@ns******:~# mdadm --misc --detail /dev/md2
/dev/md2:
           Version : 1.2
     Creation Time : Tue Nov 23 01:45:45 2021
        Raid Level : raid0
        Array Size : 3905710080 (3724.78 GiB 3999.45 GB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

       Update Time : Tue Nov 23 11:31:20 2021
             State : clean 
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

        Chunk Size : 64K

Consistency Policy : none

              Name : md2
              UUID : 305efdee:76ec5b85:d66afca1:002a4f1f
            Events : 20170

    Number   Major   Minor   RaidDevice State
       1       8        2        0      active sync   /dev/sda2
       2       8       18        1      active sync   /dev/sdb2
root@ns******:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev             16G     0   16G   0% /dev
tmpfs           3.2G  900K  3.2G   1% /run
/dev/md2        1.8T  1.2G  1.7T   1% /
tmpfs            16G     0   16G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           3.2G     0  3.2G   0% /run/user/0
      
root@ns******:~# resize2fs /dev/md2
resize2fs 1.46.2 (28-Feb-2021)
Filesystem at /dev/md2 is mounted on /; on-line resizing required
old_desc_blocks = 117, new_desc_blocks = 233
The filesystem on /dev/md2 is now 976427520 (4k) blocks long.

root@ns******:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev             16G     0   16G   0% /dev
tmpfs           3.2G  900K  3.2G   1% /run
/dev/md2        3.6T  1.2G  3.5T   1% /
tmpfs            16G     0   16G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           3.2G     0  3.2G   0% /run/user/0
root@ns******:~#