RAID 5+1

[root@linuxprobe ~]# mdadm -Cv /dev/md0 -n 3 -l 5 -x 1 /dev/sdb /dev/sdc /dev/sdf /dev/sdg
mdadm: layout defaults to left-symmetric
mdadm: layout defaults to left-symmetric
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 ~]# mdadm -D /dev/md0
/dev/md0:
        Version : 1.2
  Creation Time : Thu Jan 16 20:56:10 2020
     Raid Level : raid5
     Array Size : 41909248 (39.97 GiB 42.92 GB)
  Used Dev Size : 20954624 (19.98 GiB 21.46 GB)
   Raid Devices : 3
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Thu Jan 16 20:57:56 2020
          State : clean, degraded, recovering 
 Active Devices : 2
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 2

         Layout : left-symmetric
     Chunk Size : 512K

 Rebuild Status : 23% complete

           Name : linuxprobe:0  (local to host linuxprobe)
           UUID : 7a86eb15:2da27ed6:af887a2f:29a3cac5
         Events : 4

    Number   Major   Minor   RaidDevice State
       0       8       16        0      active sync   /dev/sdb
       1       8       32        1      active sync   /dev/sdc
       4       8       80        2      spare rebuilding   /dev/sdf

       3       8       96        -      spare   /dev/sdg

[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 ~]# echo "/dev/md0 /RAID5 ext4 defaults 0 0" >> /etc/fstab
[root@linuxprobe ~]# mkdir /RAID5
[root@linuxprobe ~]# mount -a

[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 20:56:10 2020
     Raid Level : raid5
     Array Size : 41909248 (39.97 GiB 42.92 GB)
  Used Dev Size : 20954624 (19.98 GiB 21.46 GB)
   Raid Devices : 3
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Thu Jan 16 21:09:43 2020
          State : clean, degraded, recovering 
 Active Devices : 2
Working Devices : 3
 Failed Devices : 1
  Spare Devices : 1

         Layout : left-symmetric
     Chunk Size : 512K

 Rebuild Status : 1% complete

           Name : linuxprobe:0  (local to host linuxprobe)
           UUID : 7a86eb15:2da27ed6:af887a2f:29a3cac5
         Events : 114

    Number   Major   Minor   RaidDevice State
       3       8       96        0      spare rebuilding   /dev/sdg
       1       8       32        1      active sync   /dev/sdc
       4       8       80        2      active sync   /dev/sdf

       0       8       16        -      faulty   /dev/sdb