February 2004 Archives

Dell's developers have provided an mrtg config sample file for monitoring temperatue of a poweredge server. Available here

This document describes how to setup software RAID in linear mode on a RedHat Linux  system. Linear mode combines the capacity of multiple disk partitions into a single large partition with no redundancy.

Step 1: Organise RAID partitions

All data on the constituent partitions will be erased!

Decide which partitions will be used to form the RAID device. The system ID on these partitions should be changed to "fd" (Linux Raid Auto) so that the RAID device can be automatically restarted after a reboot (and mkraid sometimes complains if they're not of type "fd").  To change a partition's system ID, start fdisk with the name of the physical drive device as an argument. When prompte, type "t" for "change a partition's system Id" and then select "fd".
 

Step 2: Create a raidtab configuration file

If /etc/raidtab does not exist, create one using the sample template at /usr/share/doc/raidtools-0.90/raidtab.sample.

cp /usr/share/doc/raidtools-0.90/raidtab.sample /etc/raidtab

Note: If an error occurs due to the RAID configuration and the machine won't boot correctly as a result, rename or move the raidtab file to prevent the init processes from attempting to start the RAID drivers. The machine can then be rebooted successfully and further configuration can be done.

Step 3: Raidtab settings

For linear raid configuration, edit /etc/raidtab to resemble the following settings:

#
# linear RAID setup, with no spare disks:
#
raiddev /dev/md0
    raid-level                linear
    nr-raid-disks             2
    persistent-superblock     1
    chunk-size                32

    device                    /dev/hda7
    raid-disk                 0
    device                    /dev/hdb7
    raid-disk                 1
 

To add another device to the RAID, increment the nr-raid-disks parameter and add another set of device and raid-disk parameters.

The persistent-superblock option has to be switched on (set to 1) to enable the system to auto-detect the raid device after a reboot.

The chunk-size option is meaningless for a linear RAID configuration so this can have any value.

Step 4: Initialise the RAID device

Prior to initialising the RAID device, the consistuent partitions must be unmounted:

umount /dev/hda7
umount /dev/hdb7

Use the mkraid command to initialise the partition:

mkraid /dev/md0

The /proc/mdstat virtual file can be used to check the status of the new RAID device.

[root@test big]# cat /proc/mdstat
Personalities : [linear]
read_ahead 1024 sectors
md0 : active linear hdb7[1] hda7[0]
      47664640 blocks 32k rounding

unused devices: <none>
 

Step 5: Create the filesystem

A RAID device does not rely on having a particular type of filesystem. To create an ext2 filesystem on the new RAID device use the mke2fs command:

mke2fs /dev/md0

To convert this filesystem to ext3, run the command:

tune2fs -j /dev/md0
 

Step 6: Mount the RAID partition

If the constituent devices are still listed in /etc/fstab they should be removed so that they are not mounted during a reboot.

Add a new entry to /etc/fstab for the RAID device as follows:

/dev/md0         /big   ext3   defaults 1 2

Mount the RAID device as follows:

mount /big

Finally, check the size of the RAID device using  df -h:

Filesystem            Size  Used Avail Use% Mounted on
/dev/hda1             251M   91M  146M  39% /
/dev/hda6             121M  6.9M  108M   6% /home
none                  250M     0  250M   0% /dev/shm
/dev/hda2             2.0G  354M  1.5G  19% /usr
/dev/hda5             121M   21M   95M  18% /var
/dev/md0             45G   20k   42G   1%  /big