mdadm: RAID-5

RAID 5 (block-level striping with distributed parity) distributes parity along
with the data and requires all drives but one to be present to operate; drive
failure requires replacement, but the array is not destroyed by a single drive
failure. Upon drive failure, any subsequent reads can be calculated from the
distributed parity such that the drive failure is masked from the end user. The
array will have data loss in the event of a second drive failure and is
vulnerable until the data that was on the failed drive is rebuilt onto a
replacement drive. A single drive failure in the set will result in reduced
performance of the entire set until the failed drive has been replaced and

install mdadm
fdisk /dev/sdb
create partition with type: fd
fdisk /dev/sdc
create partition with type: fd
fdisk /dev/sdd
create partition with type: fd
create raid:
mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1
create filesystem:
mkfs.ext4 /dev/md0
monitoring of the process of constructing a raid:
cat /proc/mdstat
write config:
mdadm --detail --scan --verbose | awk '/ARRAY/ {print}' >> /etc/mdadm/mdadm.conf
create mount point:
mkdir /raid5
edit fstab:
/dev/md0      /raid5     ext4    defaults    1 2
and after the construction of the raid, you can use it (see: cat /proc/mdstat)

If the disk fell out of the array you can do next steps:
remove disk from raid:
mdadm /dev/md0 -r /dev/sdX1
add disk to raid:
mdadm /dev/md0 -a /dev/sdX1
and extend your space:
mdadm -G /dev/md0 --raid-devices=4
e2fsck -f /dev/md0
resize2fs -p /dev/md0