What exactly does “Used Dev Size” mean under mdadm -D /dev/mdX?

  linux, question

For example, if there are 3 2G disk groups raid 5, then the total space is 6G, the available space is 4G, and the check data takes up 2G.

Next:

[root@testcentos65x64-1 ~]# mdadm -D /dev/md1
 /dev/md1:
 Version : 1.1
 Creation Time : Sat Dec 19 07:49:09 2015
 Raid Level : raid5
 Array Size : 4189184 (4.00 GiB 4.29 GB)
 Used Dev Size : 2094592 (2045.84 MiB 2144.86 MB)
 Raid Devices : 3
 Total Devices : 3
 Persistence : Superblock is persistent
 
 Intent Bitmap : Internal
 
 Update Time : Tue Mar  1 10:33:54 2016
 State : clean
 Active Devices : 3
 Working Devices : 3
 Failed Devices : 0
 Spare Devices : 0
 
 Layout : left-symmetric
 Chunk Size : 512K
 
 Name : 1
 UUID : 623f2e4d:aa69449e:31952d5a:4771d752
 Events : 80
 
 Number   Major   Minor   RaidDevice State
 0       8       65        0      active sync   /dev/sde1
 1       8       81        1      active sync   /dev/sdf1
 3       8       97        2      active sync   /dev/sdg1

Used Dev Size is displayed as 2G. What does this mean? I searched:

Information 1: Bird Brother’s Private Cuisine “http://vbird.dic.ksu.edu.tw/linux_basic/0420quota_2.php], Ctrl+F to [Used Dev Size], his explanation is [usable capacity of each device], this is obviously wrong.

Information 2: Linux Questions “http://www.linuxquestions.org/questions/red-hat-31/what-does-used-dev-size-mean-mdadm-detail-767177/], need to climb over the wall, but I paste the last answer:

Hmm, yes, on a second thought, I think I understand the real meaning of "Used Dev Size". It refers to "disk size that are not for data".
 
 For RAID0, "Used Dev Size" does not exist.
 For RAID1, "Used Dev Size" equals to half of total disks' size.
 For RAID5, "Used Dev Size" equals to size that are used for parity.
 
 Thanks for replying, Mr-Bisquit.

This answer is consistent with the above data.

Data 3: serverfault “http://serverfault.com/questions/432963/raid10-in-mdadm-reports-incorrect-used-dev-size[Used Dev Size] refers to the capacity of the smallest disk in the disks that make up the array.

The above information, each kind of information to its interpretation is different, so:

What exactly does “Used Dev Size” mean under mdadm -D /dev/mdX?

Brother Bird’s private dish is right.
Raid5, 3 hard disks, 2 data, 1 check backup, then each data disk is 2G, the total data is 4G