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Setting up your RAID-1 devices.
(C) 1997 Ingo Molnar, Gadi Oxman, Miguel de Icaza.
Send comments and suggestions to:
linux-raid@vger.rutgers.edu
First of all, make sure you read the INSTALL file for details on how
to install the userland utilties for managing the Linux/RAID code.
1a. Setting up a 2 way mirrored RAID-1 array:
(you may setup an n-way mirrored array by just adding more devices
to the MD device).
a) Get two partitions of equal size, say /dev/hda1 and /dev/hdb1.
If they have different sizes, the RAID-1 code won't be able to use
the difference, but besides wasting that space, there is nothing
bad with this. If you have two disks with different sizes, you
may probably want to partition the bigger one to have one
partition with the same size as the small disk and use the other
partition to put other information.
b) Copy the configuration from raid1.conf.sample into
/etc/raid1.conf. Edit the file according to your system.
c) run:
$ mkraid /etc/raid1.conf
This creates the RAID-1 superblocks and *ERASES* both paritions.
Might take a long time for big arrays.
Be careful, this will *wipe* all the information on your composing
devices.
1b. Setting up a software RAID-4/RAID-5:
a) Get n-partitions of equal size, say /dev/sda1, /dev/sdb1,
/dev/sdc1, /dev/sdd1.
If the partitions are of different size, the driver will not be
able to use the difference in disk space.
b) Copy the configuration from raid4.conf.sample or raid5.conf.sample
into /etc/raid4.conf or /etc/raid5.conf. Edit the file according
to your setup.
The configurable things you want to change in this file:
nr-raid-disks: the number of disks in your disk array
(this includes the parity disk).
the device/raid-disk pairs.
2. Creating your MD device:
run:
# mdadd /dev/md0 /dev/hda1 /dev/hdb1
Those two devices should be the same devices that are specified in
your raidX.conf file.
3. Activating your MD device:
Activate your disk array, choose one according to the setup you selected:
For RAID-1 setups:
# mdrun -p1 /dev/md0
For RAID-4 setups:
# mdrun -p4 /dev/md0
For RAID-5 setups:
# mdrun -p5 /dev/md0
With this instruction you tell the kernel that you want to use the
/dev/md0 device as a specific RAID personality.
4. Using your MD device:
Now you can mount /dev/md0 as if it was a normal disk
partition. You can do:
# mke2fs /dev/md0
# mount /dev/md0 /mnt
[ work, work, work ]
# umount /mnt
5. Shutting down:
Before shutdown (but after umount) you should run:
# mdstop /dev/md0
6. Checking the disk array:
If mdrun reports that the array couldn't be activated because the
mirrors were out of sync, you should run:
For a RAID-1 setup:
# ckraid /etc/raid1.conf
For a RAID-4 setup:
# ckraid /etc/raid4.conf
For a RAID-5 setup:
# ckraid /etc/raid5.conf
This checks your disk array and marks it 'clean'. It might take
longer time for big arrays as it has to check every block on the
mirrors.
7. Mirroring over stripping:
You might desire to run the mirroring personality on top of the
stripping personality or the linear personality for various
reasons:
a) too improve the performance of your disk array. The idea here
is that you want to have a mirror, but have the speed benefits of the
stripping personality, or
b) you want to concatenate some smaller devices as one side of the
mirror.
For example: you have two 100meg disks, and one 200meg disk, so
you concatenate the two 100meg disks with the linear
personality and then you create a mirror from this new composed
device and the 200meg disk.
Sample setup mirror over stripping setup:
(/dev/sda + /dev/sdb) mirrored with (/dev/sdc + /dev/sdd).
the '$' is the shell prompt, comments are indented.
$ mdadd /dev/md0 /dev/sda /dev/sdb
This adds devices /dev/sda and /dev/sdb to /dev/md0
$ mdadd /dev/md1 /dev/sdc /dev/sdd
This adds devices /dev/sdc and /dev/sdd to /dev/md1
$ mdrun -p0 /dev/md0
This tells the kernel that you want to use /dev/md0 in
stripping mode (RAID-0).
$ mdrun -p0 /dev/md1
This tells the kernel that you want to use /dev/md1 in
stripping mode (RAID-0).
Now, create the configuration file for this setup, very
simple, we just changed the devices to be the md0 and md1
devices:
$ cat > stripped-raid1.conf << EOF
# Sample raid-1 on top of linear/stripped devices configuration
raiddev /dev/md2
raid-level 1
nr-raid-disks 2
nr-spare-disks 0
device /dev/md0
raid-disk 0
device /dev/md1
raid-disk 1
EOF
$ mkraid stripped-raid1.conf
This creates the superblocks on /dev/md0 and /dev/md1 and
*wipes* the contents of it. It might take a long time to run.
$ mdadd /dev/md2 /dev/md0 /dev/md1
This adds the stripped devices to the mirroring device
$ mdrun -p1 /dev/md2
Now we actuall tell the kernel to use /dev/md2 as a mirroring
device. Note that you can't use stripping over stripping.
Ready! Now you have a mirror over stripping device in /dev/md2
At shutdown time, don't forget to mdstop your devices:
$ mdstop /dev/md2
This stops the mirroring personality
$ mdstop /dev/md0
This stops the first stripping component
$ mdstop /dev/md1
This stops the second stripping component
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