RAID : Like RAID 4, RAID 5 requires three or more disks
RAID 6 literally goes one better than RAID 5. In other words, while it requires four or more disks,
RAID mirrors RAID1: requires 2 disks
 
RAID 1 or RAID 1+0, which yield redundancy, give only s / 2 storage capacity, where s is the sum of the capacities of n drives used
 
Raid 5 useable capacity ( N - 1) * Smin where N is the total number of drives in the array and Smin is the capacity of the smallest drive in the array.
Raid 6 useable capacity (N -2 ) * Smin where N is the total number of drives in the array and Smin is the capacity of the smallest drive in the array.


ｍｄａｄｍ　－Ｃ　／ｄｅｖ／ｍｄ０　－ｌ　４　－ｎ　３　－ｘ　／ｄｅｖ／ｓｄａ５　／ｄｅｖ／ｓｄａ６　／ｄｅｖ／ｓｄａ７　／ｄｅｖ／ｓｄａ８

ｃａｔ　／ｐｒｏｃ／ｍｄｓｔａｔ　ｔｏ　ｃｈｅｃｋ　ｔｈｅ　ｓｔａｔｕｓ

ｍｄａｄｍ　／ｄｅｖ／ｍｄ０　－ｆ　／ｄｅｖ／ｓｄａ５

ｍｄａｄｍ　／ｄｅｖ／ｍｄ０　－ｒ　／ｄｅｖ／ｓｄａ５

ｍｄａｄｍ　／ｄｅｖ／ｍｄ０　－ａ　／ｄｅｖ／ｓｄａ５

ｍｋｎｏｄ　／ｄｅｖ／ｍｄ１　ｂ　９　１　ｔｏ　ｍａｋｅ　ａ　ｎｅｗ　ｄｅｖｉｃｅ　

ｍｋｆｓ　－ｔ　ｅｘｔ３　／ｄｅｖ／ｍｄ０
ｍｏｕｎｔ　／ｄｅｖ／ｍｄ０　／ｍｎｔ／ｒａｉｄ

to release the partition taken by raid

mdadm /dev/md0 -f /dev/sdb1 /dev/sdb2 /dev/sdb3
mdadm /dev/md0 -r /dev/sdb1 /dev/sdb2 /dev/sdb3



extend Lv
extend vol and bring online

lvresize -L 550M /dev/vg0/lv0
resize2fs /dev/vg0/lv0   

check online: df -h

reduce LV
umount /dev/vg0/lv0
fsck -f /dev/vg0/lv0
resize2fs /dev/vg0/lv0  200M
lvreduce /dev/vg0/lv0 -L 200M
mount /dev/vg0/lv0 /vol
df -h


#/boot can not put into LV as well.
# before make LV , ensure hard disk raided ,raid and add into VG
----------
sequence
linux block device ( partition or hard disk)--pvcreate--vgcreate--lvcreate
# create LV datalv  on vg0
lvcreate -L 5G -n datalv vg0
------------
suggest use GUI to reduce LV
------
#vgextend vg0 /dev/sda11  ---add one block to vg0
#pvmove  /dev/sda11 
#vgreduce  vg0 /dev/sda12