RAID 10

[root@linuxprobe ~]# mdadm -Cv /dev/md0 -a yes -n 4 -l 10 /dev/sdb /dev/sdc /dev/sdd /dev/sde
mdadm: layout defaults to n2
mdadm: layout defaults to n2
mdadm: chunk size defaults to 512K
mdadm: size set to 20954624K
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

[root@linuxprobe ~]# mkfs.ext4 /dev/md0
mke2fs 1.42.9 (28-Dec-2013)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=128 blocks, Stripe width=256 blocks
2621440 inodes, 10477312 blocks
523865 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=2157969408
320 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks: 
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 
    4096000, 7962624

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done   

[root@linuxprobe ~]# mkdir /RAID10
[root@linuxprobe ~]# mount /dev/md0 /RAID10
[root@linuxprobe ~]# df -h
Filesystem               Size  Used Avail Use% Mounted on
/dev/mapper/centos-root   18G  3.5G   15G  20% /
devtmpfs                 905M     0  905M   0% /dev
tmpfs                    914M  140K  914M   1% /dev/shm
tmpfs                    914M  8.8M  905M   1% /run
tmpfs                    914M     0  914M   0% /sys/fs/cgroup
/dev/sr0                 6.6G  6.6G     0 100% /media/cdrom
/dev/sda1                497M  119M  379M  24% /boot
/dev/md0                  40G   49M   38G   1% /RAID10

[root@linuxprobe ~]# mdadm -D /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Thu Jan 16 19:30:24 2020
     Raid Level : raid10
     Array Size : 41909248 (39.97 GiB 42.92 GB)
  Used Dev Size : 20954624 (19.98 GiB 21.46 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Thu Jan 16 19:48:47 2020
          State : clean 
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

         Layout : near=2
     Chunk Size : 512K

           Name : linuxprobe:0  (local to host linuxprobe)
           UUID : 3c045189:f5f903db:5728adf6:fa3baac1
         Events : 19

    Number   Major   Minor   RaidDevice State
       0       8       16        0      active sync   /dev/sdb
       1       8       32        1      active sync   /dev/sdc
       2       8       48        2      active sync   /dev/sdd
       3       8       64        3      active sync   /dev/sde

[root@linuxprobe ~]# mdadm /dev/md0 -f /dev/sdb
mdadm: set /dev/sdb faulty in /dev/md0
[root@linuxprobe ~]# mdadm -D /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Thu Jan 16 19:30:24 2020
     Raid Level : raid10
     Array Size : 41909248 (39.97 GiB 42.92 GB)
  Used Dev Size : 20954624 (19.98 GiB 21.46 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Thu Jan 16 19:52:27 2020
          State : clean, degraded 
 Active Devices : 3
Working Devices : 3
 Failed Devices : 1
  Spare Devices : 0

         Layout : near=2
     Chunk Size : 512K

           Name : linuxprobe:0  (local to host linuxprobe)
           UUID : 3c045189:f5f903db:5728adf6:fa3baac1
         Events : 99

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1       8       32        1      active sync   /dev/sdc
       2       8       48        2      active sync   /dev/sdd
       3       8       64        3      active sync   /dev/sde

       0       8       16        -      faulty   /dev/sdb

[root@linuxprobe ~]# mdadm /dev/md0 -a /dev/sdb
mdadm: Cannot open /dev/sdb: Device or resource busy
[root@linuxprobe ~]# mdadm /dev/md0 -a /dev/sdf
mdadm: added /dev/sdf
[root@linuxprobe ~]# mdadm -D /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Thu Jan 16 19:30:24 2020
     Raid Level : raid10
     Array Size : 41909248 (39.97 GiB 42.92 GB)
  Used Dev Size : 20954624 (19.98 GiB 21.46 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Thu Jan 16 20:03:18 2020
          State : clean 
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

         Layout : near=2
     Chunk Size : 512K

           Name : linuxprobe:0  (local to host linuxprobe)
           UUID : 3c045189:f5f903db:5728adf6:fa3baac1
         Events : 339

    Number   Major   Minor   RaidDevice State
       4       8       80        0      active sync   /dev/sdf
       1       8       32        1      active sync   /dev/sdc
       2       8       48        2      active sync   /dev/sdd
       3       8       64        3      active sync   /dev/sde