9 Testing

Now let's simulate a hard drive failure. It doesn't matter if you select /dev/sda or /dev/sdb here. In this example I assume that /dev/sdb has failed.

To simulate the hard drive failure, you can either shut down the system and remove /dev/sdb from the system, or you (soft-)remove it like this:

mdadm --manage /dev/md0 --fail /dev/sdb1
mdadm --manage /dev/md1 --fail /dev/sdb2
mdadm --manage /dev/md2 --fail /dev/sdb3

mdadm --manage /dev/md0 --remove /dev/sdb1
mdadm --manage /dev/md1 --remove /dev/sdb2
mdadm --manage /dev/md2 --remove /dev/sdb3

Shut down the system:

shutdown -h now

Then put in a new /dev/sdb drive (if you simulate a failure of /dev/sda, you should now put /dev/sdb in /dev/sda's place and connect the new HDD as /dev/sdb!) and boot the system. It should still start without problems.

Now run

cat /proc/mdstat

and you should see that we have a degraded array:

[root@server1 ~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sda1[0]
      200704 blocks [2/1] [U_]

md1 : active raid1 sda2[0]
      522048 blocks [2/1] [U_]

md2 : active raid1 sda3[0]
      9759360 blocks [2/1] [U_]

unused devices: <none>
[root@server1 ~]#

The output of

fdisk -l

should look as follows:

[root@server1 ~]# fdisk -l

Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          25      200781   fd  Linux raid autodetect
/dev/sda2              26          90      522112+  fd  Linux raid autodetect
/dev/sda3              91        1305     9759487+  fd  Linux raid autodetect

Disk /dev/sdb: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/sdb doesn't contain a valid partition table

Disk /dev/md2: 9993 MB, 9993584640 bytes
2 heads, 4 sectors/track, 2439840 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md2 doesn't contain a valid partition table

Disk /dev/md1: 534 MB, 534577152 bytes
2 heads, 4 sectors/track, 130512 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md1 doesn't contain a valid partition table

Disk /dev/md0: 205 MB, 205520896 bytes
2 heads, 4 sectors/track, 50176 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md0 doesn't contain a valid partition table
[root@server1 ~]#

Now we copy the partition table of /dev/sda to /dev/sdb:

sfdisk -d /dev/sda | sfdisk /dev/sdb

(If you get an error, you can try the --force option:

sfdisk -d /dev/sda | sfdisk --force /dev/sdb

)

[root@server1 ~]# sfdisk -d /dev/sda | sfdisk /dev/sdb
Checking that no-one is using this disk right now ...
OK

Disk /dev/sdb: 1305 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/sdb: unrecognized partition table type
Old situation:
No partitions found
New situation:
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
/dev/sdb1   *        63    401624     401562  fd  Linux raid autodetect
/dev/sdb2        401625   1445849    1044225  fd  Linux raid autodetect
/dev/sdb3       1445850  20964824   19518975  fd  Linux raid autodetect
/dev/sdb4             0         -          0   0  Empty
Successfully wrote the new partition table

Re-reading the partition table ...

If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)
[root@server1 ~]#

Afterwards we remove any remains of a previous RAID array from /dev/sdb...

mdadm --zero-superblock /dev/sdb1
mdadm --zero-superblock /dev/sdb2
mdadm --zero-superblock /dev/sdb3

... and add /dev/sdb to the RAID array:

mdadm -a /dev/md0 /dev/sdb1
mdadm -a /dev/md1 /dev/sdb2
mdadm -a /dev/md2 /dev/sdb3

Now take a look at

cat /proc/mdstat

[root@server1 ~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
      200704 blocks [2/2] [UU]

md1 : active raid1 sdb2[1] sda2[0]
      522048 blocks [2/2] [UU]

md2 : active raid1 sdb3[2] sda3[0]
      9759360 blocks [2/1] [U_]
      [=======>.............]  recovery = 39.4% (3846400/9759360) finish=1.7min speed=55890K/sec

unused devices: <none>
[root@server1 ~]#

Wait until the synchronization has finished:

[root@server1 ~]# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
      200704 blocks [2/2] [UU]

md1 : active raid1 sdb2[1] sda2[0]
      522048 blocks [2/2] [UU]

md2 : active raid1 sdb3[1] sda3[0]
      9759360 blocks [2/2] [UU]

unused devices: <none>
[root@server1 ~]#

Then run

grub

and install the bootloader on both HDDs:

root (hd0,0)
setup (hd0)
root (hd1,0)
setup (hd1)
quit

That's it. You've just replaced a failed hard drive in your RAID1 array.

10 Links

• The Software-RAID Howto: http://tldp.org/HOWTO/Software-RAID-HOWTO.html
• CentOS: http://www.centos.org/