How To Set Up Software RAID1 On A Running System (Incl. GRUB Configuration) (Mandriva 2008.0) - Page 4

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  1. 9 Testing
  2. 10 Links

9 Testing

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

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

mdadm --manage /dev/md0 --fail /dev/hdb1
mdadm --manage /dev/md1 --fail /dev/hdb5
mdadm --manage /dev/md2 --fail /dev/hdb6

mdadm --manage /dev/md0 --remove /dev/hdb1
mdadm --manage /dev/md1 --remove /dev/hdb5
mdadm --manage /dev/md2 --remove /dev/hdb6

Shut down the system:

shutdown -h now

Then put in a new /dev/hdb drive (if you simulate a failure of /dev/hda, you should now put /dev/hdb in /dev/hda's place and connect the new HDD as /dev/hdb!) 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:

[[email protected] ~]# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 hda5[0]
      417536 blocks [2/1] [U_]

md0 : active raid1 hda1[0]
      176576 blocks [2/1] [U_]

md2 : active raid1 hda6[0]
      4642688 blocks [2/1] [U_]

unused devices: <none>
[[email protected] ~]#

The output of

fdisk -l

should look as follows:

[[email protected] ~]# fdisk -l

Disk /dev/hda: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/hda1   *           1          22      176683+  fd  Linux raid autodetect
/dev/hda2              23         652     5060475    5  Extended
/dev/hda5              23          74      417658+  fd  Linux raid autodetect
/dev/hda6              75         652     4642753+  fd  Linux raid autodetect

Disk /dev/hdb: 5368 MB, 5368709120 bytes
16 heads, 63 sectors/track, 10402 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes
Disk identifier: 0x00000000

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

Disk /dev/md2: 4754 MB, 4754112512 bytes
2 heads, 4 sectors/track, 1160672 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Disk identifier: 0x00000000

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

Disk /dev/md0: 180 MB, 180813824 bytes
2 heads, 4 sectors/track, 44144 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Disk identifier: 0x00000000

Disk /dev/md0 doesn't contain a valid partition table

Disk /dev/md1: 427 MB, 427556864 bytes
2 heads, 4 sectors/track, 104384 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Disk identifier: 0x00000000

Disk /dev/md1 doesn't contain a valid partition table
[[email protected] ~]#

Now we copy the partition table of /dev/hda to /dev/hdb:

sfdisk -d /dev/hda | sfdisk --force /dev/hdb

[[email protected] ~]# sfdisk -d /dev/hda | sfdisk --force /dev/hdb
Warning: extended partition does not start at a cylinder boundary.
DOS and Linux will interpret the contents differently.
Checking that no-one is using this disk right now ...

Disk /dev/hdb: 10402 cylinders, 16 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/hdb: 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/hdb1   *        63    353429     353367  fd  Linux raid autodetect
/dev/hdb2        353430  10474379   10120950   5  Extended
/dev/hdb3             0         -          0   0  Empty
/dev/hdb4             0         -          0   0  Empty
/dev/hdb5        353493   1188809     835317  fd  Linux raid autodetect
/dev/hdb6       1188873  10474379    9285507  fd  Linux raid autodetect
Warning: partition 1 does not end at a cylinder boundary
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).)
[[email protected] ~]#

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

mdadm --zero-superblock /dev/hdb1
mdadm --zero-superblock /dev/hdb5
mdadm --zero-superblock /dev/hdb6

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

mdadm -a /dev/md0 /dev/hdb1
mdadm -a /dev/md1 /dev/hdb5
mdadm -a /dev/md2 /dev/hdb6

Now take a look at

cat /proc/mdstat

[[email protected] ~]# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 hdb5[2] hda5[0]
      417536 blocks [2/1] [U_]

md0 : active raid1 hdb1[1] hda1[0]
      176576 blocks [2/2] [UU]

md2 : active raid1 hdb6[2] hda6[0]
      4642688 blocks [2/1] [U_]
      [===========>.........]  recovery = 59.9% (2784512/4642688) finish=7.5min speed=4076K/sec

unused devices: <none>
[[email protected] ~]#

Wait until the synchronization has finished:

[[email protected] ~]# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 hdb5[1] hda5[0]
      417536 blocks [2/2] [UU]

md0 : active raid1 hdb1[1] hda1[0]
      176576 blocks [2/2] [UU]

md2 : active raid1 hdb6[1] hda6[0]
      4642688 blocks [2/2] [UU]

unused devices: <none>
[[email protected] ~]#

Then run


and install the bootloader on both HDDs:

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

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


Falko Timme

About Falko Timme

Falko Timme is an experienced Linux administrator and founder of Timme Hosting, a leading nginx business hosting company in Germany. He is one of the most active authors on HowtoForge since 2005 and one of the core developers of ISPConfig since 2000. He has also contributed to the O'Reilly book "Linux System Administration".

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By: Scott Grayban

Now how do you reverse this and go back to a normal drives ?