[CentOS] CentOS 7: software RAID 5 array with 4 disks and no spares?

Wed Feb 18 07:09:13 UTC 2015
Niki Kovacs <info at microlinux.fr>

Hi,

I just replaced Slackware64 14.1 running on my office's HP Proliant 
Microserver with a fresh installation of CentOS 7.

The server has 4 x 250 GB disks.

Every disk is configured like this :

	* 200 MB /dev/sdX1 for /boot
	* 4 GB /dev/sdX2 for swap
	* 248 GB /dev/sdX3 for /

There are supposed to be no spare devices.

/boot and swap are all supposed to be assembled in RAID level 1 across 4 
disks.

The / partition is supposed to be assembled in RAID level 5 across 4 disks.

With Slackware I created the arrays manually like this:

   # mdadm --create /dev/md1 --level=1 --raid-devices=4 --metadata=0.90
     /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
   # mdadm --create /dev/md2 --level=1 --raid-devices=4 --metadata=0.90
     /dev/sda2 /dev/sdb2 /dev/sdc2 /dev/sdd2
   # mdadm --create /dev/md3 --level=5 --raid-devices=4 --metadata=0.90
     /dev/sda3 /dev/sdb3 /dev/sdc3 /dev/sdd3

Using this setup, I had 650 MB of disk space on /dev/md3.

Now I tried to do the same thing with CentOS 7. Everything seemed to 
work at first, but here's what I got now:

[root at nestor:~] # df -h
Sys. de fichiers Taille Utilisé Dispo Uti% Monté sur
/dev/md127         226G    1,1G  213G   1% /
devtmpfs           1,4G       0  1,4G   0% /dev
tmpfs              1,4G       0  1,4G   0% /dev/shm
tmpfs              1,4G    8,5M  1,4G   1% /run
tmpfs              1,4G       0  1,4G   0% /sys/fs/cgroup
/dev/md125         194M     80M  101M  45% /boot
/dev/sde1          917G     88M  871G   1% /mnt

The root partition (/dev/md127) only shows 226 G of space. So where has 
everything gone?

[root at nestor:~] # cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md125 : active raid1 sdc2[2] sdd2[3] sdb2[1] sda2[0]
       204736 blocks super 1.0 [4/4] [UUUU]

md126 : active raid1 sdd1[3] sdc1[2] sdb1[1] sda1[0]
       4095936 blocks super 1.2 [4/4] [UUUU]

md127 : active raid5 sdc3[2] sdb3[1] sdd3[4] sda3[0]
       240087552 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4] 
[UUUU]
       bitmap: 0/1 pages [0KB], 65536KB chunk

unused devices: <none>

[root at nestor:~] # mdadm -D /dev/md127
/dev/md127:
         Version : 1.2
   Creation Time : Wed Feb 18 06:49:01 2015
      Raid Level : raid5
      Array Size : 240087552 (228.97 GiB 245.85 GB)
   Used Dev Size : 80029184 (76.32 GiB 81.95 GB)
    Raid Devices : 4
   Total Devices : 4
     Persistence : Superblock is persistent

   Intent Bitmap : Internal

     Update Time : Wed Feb 18 08:04:26 2015
           State : active
  Active Devices : 4
Working Devices : 4
  Failed Devices : 0
   Spare Devices : 0

          Layout : left-symmetric
      Chunk Size : 512K

            Name : localhost:root
            UUID : cfc13fe9:8fa811d8:85649402:58c4846e
          Events : 4703

     Number   Major   Minor   RaidDevice State
        0       8        3        0      active sync   /dev/sda3
        1       8       19        1      active sync   /dev/sdb3
        2       8       35        2      active sync   /dev/sdc3
        4       8       51        3      active sync   /dev/sdd3

Apparently no spare devices have been created. So why do I only have 226 
GB of disk space under CentOS, when I had roughly 650 GB under Slackware?

I'm a bit lost here. Any suggestions?

Cheers,

Niki

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