If you have more than one harddrive To be honest, you can construct MD device even from partitions residing on single physical drive, but that won't bring you anything useful. in your computer, you can use mdcfg to setup your drives for increased performance and/or better reliability of your data. The result is called Multidisk Device (or after its most famous variant software RAID). MD is basically a bunch of partitions located on different disks and combined together to form a logical device. This device can then be used like an ordinary partition (i.e. in partman you can format it, assign a mountpoint, etc.). The benefit you gain depends on a type of a MD device you are creating. Currently supported are: RAID0 Is mainly aimed at performance. RAID0 splits all incoming data into stripes and distributes them equally over each disk in the array. This can increase the speed of read/write operations, but when one of the disks fails, you will loose everything (part of the information is still on the healthy disk(s), the other part was on the failed disk). The typical use for RAID0 is a partition for video editing. RAID1 Is suitable for setups where reliability is the first concern. It consists of several (usually two) equally sized partitions where every partition contains exactly the same data. This essentially means three things. First, if one of your disks fails, you still have the data mirrored on the remaining disks. Second, you can use only a fraction of the available capacity (more precisely, it is the size of the smallest partition in the RAID). Third, file reads are load balanced among the disks, which can improve performance on a server, such as a file server, that tends to be loaded with more disk reads than writes. Optionally you can have a spare disk in the array which will take the place of the failed disk in the case of failure. RAID5 Is a good compromise between speed, reliability and data redundancy. RAID5 splits all incomming data into stripes and distributes them equally on all but one disks (similar to RAID0). Unlike RAID0, RAID5 also computes parity information, which gets written on the remaining disk. The parity disk is not static (that would be called RAID4), but is changing periodically, so the parity information is distributed equally on all disks. When one of the disks fails, the missing part of information can be computed from remaining data and its parity. RAID5 must consist of at least three active partitions. Optionally you can have a spare disk in the array which will take the place of the failed disk in the case of failure. As you can see, RAID5 has similar degree of reliability like RAID1 while achieving less redundancy. On the other hand it might be a bit slower on write operation than RAID0 due to computation of parity information. To sum it up: Type Minimum Devices Spare Device Survives disk failure? Available Space RAID0 2 no no Size of the smallest partition multiplied by number of devices in RAID RAID1 2 optional yes Size of the smallest partition in RAID RAID5 3 optional yes Size of the smallest partition multiplied by (number of devices in RAID minus one) If you want to know the whole truth about Software RAID, have a look at Software RAID HOWTO. To create a MD device, you need to have the desired partitions it should consist of marked for use in a RAID. (This is done in partman in the Partition settings menu where you should select Use as: physical volume for RAID .) Support for MD is a relatively new addition to the installer. You may experience problems for some RAID levels and in combination with some bootloaders if you try to use MD for the root (/) filesystem. For experienced users, it may be possible to work around some of these problems by executing some configuration or installation steps manually from a shell. Next, you should choose Configure software RAID from the main partman menu. (The menu will only appear after you mark at least one partition for use as physical volume for RAID.) On the first screen of mdcfg simply select Create MD device. You will be presented with a list of supported types of MD devices, from which you should choose one (e.g. RAID1). What follows depends on the type of MD you selected. RAID0 is simple — you will be issued with the list of available RAID partitions and your only task is to select the partitions which will form the MD. RAID1 is a bit more tricky. First, you will be asked to enter the number of active devices and the number of spare devices which will form the MD. Next, you need to select from the list of available RAID partitions those that will be active and then those that will be spare. The count of selected partitions must be equal to the number provided few seconds ago. Don't worry. If you make a mistake and select different number of partitions, the &d-i; won't let you continue until you correct the issue. RAID5 has similar setup procedure as RAID1 with the exception that you need to use at least three active partitions. It is perfectly possible to have several types of MD at once. For example if you have three 200 GB hard drives dedicated to MD, each containing two 100 GB partitions, you can combine first partitions on all three disk into the RAID0 (fast 300 GB video editing partition) and use the other three partitions (2 active and 1 spare) for RAID1 (quite reliable 100 GB partition for /home). After you setup MD devices to your liking, you can Finish mdcfg to return back to the partman to create filesystems on your new MD devices and assign them the usual attributes like mountpoints.