Build a Linux Software RAID from Scratch - Page 2

Part Two: Last week we considered Linux software RAID's advantages, this week we show you how to do it.

By Carla Schroder | Posted May 17, 2005
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Continued From Page 1

Creating The Array and Filesystem
The next steps are starting up the array and creating filesystems. First create an /etc/raidtab file. This is your master RAID configuration file. Let's say our new data partitions are /dev/hdb1 and /dev/hdc1:

# /etc/raidtab
raiddev /dev/md0
raid-level              0
nr-raid-disks           2
persistent-superblock   1
chunk-size              32
device                  /dev/hdb1
raid-disk                0
device                  /dev/hdc1
raid-disk               1

Now initialize the array:

# mkraid /dev/md0

Monitor progress:

# watch /proc/mdstat

Stop the watch command with CTRL+C. When the array is built you may create whatever filesystem you like on the data partitions: Ext2/3, JFS, ReiserFS, XFS, VFAT- anything that Linux supports. Use the utilities specific to the filesystems to create them. For example, this creates Ext3:

# mke2fs -j /dev/md0

This creates a Reiser filesystem:

# mkreiserfs /dev/md0

Be sure to follow the documentation for your chosen filesystem. Then create and initialize the swap partitions:

# mkswap /dev/hdb2
# mkswap /dev/hdc2
# swapon /dev/hdb2
# swapon /dev/hdc2

Start It Up
Now it's time to fire up your shiny new RAID 0 array:

# raidstart /dev/md0

Create mountpoints and mount the array:

# mkdir /mnt/raid0
# mount /dev/md0 /mnt/raid0

Now you can read and write to /mnt/raid0 just like any other directory.

Starting RAID at Boot
You probably want your nice new RAID 0 array to be automatically mounted at boot, so you must add a line to your /etc/fstab file:

/dev/md0   /mnt/raid0  reiserfs  defaults  0 1

Of course you may specify whatever mount options you like. Finally, there is one more kewl performance-enhancing trick you can do with the swap partitions. This is not a RAID function, but a kernel function- set up swapping in parallel in /etc/fstab:

/dev/hdb2   none   swap   sw,pri=0     0 0
/dev/hdc2   none   swap   sw,pri=0     0 0

This is a good time to reboot and verify that everything works. cat /proc/mdstat tells the status of the array.

RAID 1 and RAID 5
Now that you know the basics of setting up RAID, it's simple to create other RAID levels in /etc/raidtab. RAID 1 is just the same as our RAID 0 example, except for this line:

raid-level        1

Adding a three-disk RAID 5 array requires but three changes to /etc/raidtab:

raid-level       5
nr-raid-disks    3
parity-algorithm     left-symmetric

Continued on page 3: Standby Spare Disk


Updated: The examples in this article included a mistake in the commands used to create and prepare swap partitions for use. We've updated the examples. Thanks to the readers who notified us of the error.

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