RAID0 Implementation Under Linux

Now you're ready to actually create a RAID0 device. The compilation created several tools for the task: mdadd, mdrun and mdstop. mdadd is used to add block devices to an md device. If you want to use sda1, sdb1 and sdc1, you issue the command:

/sbin/mdadd /dev/md0 /dev/sda1 /dev/sdb1 \
        /dev/sdc1

This command adds sda1, sdb1 and sdc1 to md0. This same result can also be accomplished by giving these commands:

/sbin/mdadd /dev/md0 /dev/sda1
/sbin/mdadd /dev/md0 /dev/sdb1
/sbin/mdadd /dev/md0 /dev/sdc1

Remember that the order in which the devices are added is significant. If you change the order, any data previously written will be lost. I recommend adding the devices in what seems like a logical order and then sticking to it.

Now we must start the device. mdrun has the following command syntax:

/sbin/mdrun -p

where x indicates the mode: -l for linear, 0 for RAID0 and 1 for RAID1. To start the device we just made, the command would be:

/sbin/mdrun -p0 /dev/md0

When using RAID devices, another option you can use is -cnk to specify chunk size, where n is the chunk size in KB (n must be a power of two). For example, -c6k indicates a 6KB chunk size. The default value is the value of your PAGE_SIZE. The best value for chunk size would be the average request size, so chances are two requests will write to different physical disks. If you plan to use the md for swap space, stick with the default.

Once the device is running, you can create a file system and mount it. For example:

/sbin/mkfs.ext2 /dev/md0
mount /dev/md0 /var/spool/news

This will create an ext2 file system and then mount it as the news spool. Your RAID0 device is now ready for data. To check its status, type:

cat /proc/mdstat

and receive the following output:

Personalities : [2 raid0]
read_ahead 120 sectors
md0 : active raid0 sda1 sdb1 sdc1 168588 blocks 4k chunks
md1 : inactive
md2 : inactive
md3 : inactive

This report tells you which modes are supported, the current read_ahead value, the state of each md device, its mode, physical parts, total size and chunk size.

At this point we have our RAID device running and mounted; as soon as the machine is rebooted, we will have to rerun mdadd, mdrun and mount. All of this can easily be added to your rc.local file, but there is a better way. mdcreate automatically creates an /etc/mdtab file. The mdtab file serves a function similar to the /etc/fstab file, informing the system of the component devices, modes and mount points. The syntax is:

mdcreate [-cxk] mode md_dev dev0 dev1 ...

To create an mdtab file for our example device we would use:

/sbin/mdcreate raid0 /dev/md0 /dev/sda1\
        /dev/sdb1 /dev/sdc1
cat /etc/mdtab
# mdtab entry for /dev/md0:
# /dev/md0  raid0,4k,0,fe8a9ffb  /dev/sda1 /dev/sdb1 /dev/sdc1

With this file in place, we can reduce the mdadd command to mdadd -a or mdadd -ar to automatically add the devices and run them. This also ensures that the devices will always be added in the correct order.

If there is ever a need to stop the device, first unmount it and then use mdstop. mdstop will free the physical devices and flush the buffers. For our example device, we would first stop the news server if it was running with the command:

/sbin/mdstop /dev/md0

Then, we could unmount it using:

umount /var/spool/news

md0 is now inactive, and the physical partitions can be used elsewhere. Remember, if the device is stopped, none of the data that was written to the md device is accessible.

With md, the implementation and management of RAID devices is made easy. As development continues, we will see RAID1 and the tools necessary for mirror management and recovery. To stay current on the development process, join the Linux-raid mailing list. To subscribe send an email to Majordomo@vger.rutgers.edu with a one line body that says:

subscribe linux-raid <

Be sure to look at the documentation that comes with the md package. It's tools like this one that are helping Linux find a place in the business world.

MD at Work