IDE devices have file devices under Linux named /dev/hd..., to SCSI devices correspond /dev/sd..., and to metadisks there will be /dev/md.. after compiling the kernel with the options specified later. Four such devices should be present:

brw-rw----   1 root     disk       9,   0 may 28  1997 md0
brw-rw----   1 root     disk       9,   1 may 28  1997 md1
brw-rw----   1 root     disk       9,   2 may 28  1997 md2
brw-rw----   1 root     disk       9,   3 may 28  1997 md3

Types of RAID

RAID0 (Stripping mode)

In this mode, all the disk devices are organized alternatively so that blocks are taken equally from all disks, alternatively, in order to reach higher efficiency. Since the probability of finding a block of a file is identical for all disks, there are force to work simultaneously thus making the performance of the metadisk almost N times that of a single disk.

RAID1

In this mode, the goal is to reach the highest security of the data. Blocks of data are duplicated in all physical disks (each block of the `virtual' disk has a duplicate in each of the physical disks). This configuration provides N times the reading performance of a single device, but it degrades writing operations. Read operations can be organized to read N blocks simultaneoulsly, one from each device at a time. Similarly when writing 1 block it has to be duplicated N times, one for each physical device. There is no advantage in this configuration regarding storage capacity.

RAID4

(Note: the RAID2 and RAID3 types are obsolete).

In this mode the ultimate goal is to balance the advantages of the type RAID0 and RAID1. Data is organized mixing both methods. The physical 1 to N-1 are organized in striping mode (RAID0) and the Nth stores the parity of the individual bits corresponding to blocks 1 to N-1. If any of the disks fails, it is possible to recover by using the parity information on the Nth hard disk. Efficiency during read operations is N-1 and during write operations is 1/2 (because writing a data block now involves writing also to the parity disk). In order to restore a broken hard disk, one only has to re-read the information and re-write it (it reads from the parity disk but it writes to the newly install hard disk).

RAID5

This type is similar to RAID4, except that now the information of the parity disk is spread over all the hard disks (no parity disk exists). It allows to reduce the work load of the parity disk, that in RAID4 it had to be accessed for every write operation (now the disk where parity information for a track is stored differs for every track)

If the reader cannot use identical disks take into account that RAID systems always work with identical blocks of information. It is possible that the slow hard disks will be forced to work harder, but in any case the RAID configuration will still yield a better performance. The increase of performance on a RAID system that is properly configure is truly spectacular. It is almost true to say that the performance increases linearly with the number of hard disks in the RAID.

How to Install a RAID0

RAID0 has no redundancy but consider that to have redundancy it is advised a large number of disks in order not to waste too much disk capacity. Wasting a whole disk when we only have three is a waste. Furthermore, it does not cover all the possible cases of information lost but only those due to physical deterioration of the hard disks, a very uncommon event. If 10 hard disks were available, then using one for parity control is not so much of a waste. On a RAID0 having a disk failure on any of the disks means losing all the information stored in all the physical disks, consequently we recommend an appropriate policy of backups.

The first step to take is adding the appropriate drivers to the kernel. For Linux 2.0.xx RAID the options are:

   Multiple devices driver support (CONFIG_BLK_DEV_MD) [Y/n/?] Y
      Linear (append) mode (CONFIG_MD_LINEAR) [Y/m/n/?] Y
      RAID-0 (striping) mode (CONFIG_MD_STRIPED) [Y/m/n/?] Y

        -mdadd
        -mdrun
        -mdstop
        -mdop

For kernels 2.1.62 and higher there is a different package called 'RAIDtools' that permits to use a RAID0, RAID4 or RAID5.

In the following example we illustrate how to define a RAID0 metadisk that uses two hard disks, more specifically /dev/sdb1 and /dev/sdc1.
 

Once the metadisk is formatted it should not be altered under any cricunstance or all the information in it would be lost.

mdadd -a
mdrun -a

mke2fs /dev/md0

mkdir /mount/md0
mount /dev/md0 /mount/md0

For Example:

HD 4.2Gb SCSI

HD 2Gb SCSI

#######</etc/fstab>################################################
# <file system> <mount point>  <type>  <options>     <dump>  <pass>
/dev/hda1       /               ext2    defaults       0       1
/dev/hda2       /mnt/hda2       ext2    defaults       0       2
/dev/md0        /mnt/md0        ext2    defaults       0       2
proc            /proc           proc    defaults       0       2
/dev/hda3        none           swap    sw,pri=10 
/dev/sdb1        none           swap    sw,pri=10 
/dev/sda3        none           swap    sw,pri=10

#########</etc/mdtab>####################################### 
# <meta-device> <RAID-mode> <DskPart1> <DskPart1> <DskPart1> 
/dev/md0         RAID0,8k    /dev/hda4  /dev/sda4 /dev/sdb2

We should try to make all disks (partitions) in the RAID of approximately the same size because large differences will decrease the RAID performance. Small differences are not significant. We use all the space available so that all the data from the disks that can be entangled is and the remaining data remains free.

Mounting several IDE disks on a single RAID is not very efficient, but mounting an IDE with various SCSI works very well. IDE disks do not allow concurrent access, while SCSI disks do.

For more information:

• mdutils COmes with documentation
• mini-howto Multiple-Disk
• The Multiple Disk Layout mini-HOWTO Homepage www.nyx.net/~sgjoen/disk.html