The Debian Administrator's Handbook

Virtual memory allows the Linux kernel, when lacking sufficient memory (RAM), to free a bit of storage by storing the parts of the RAM that have been inactive for some time on the swap partition of the hard disk.

To simulate the additional memory, Windows uses a swap file that is directly contained in a filesystem. Conversely, Linux uses a partition dedicated to this purpose, hence the term “swap partition”.

When choosing a partition, you can indicate the manner in which you are going to use it:

format it and include it in the file tree by choosing a mount point;

use it as a swap partition;

make it into a “physical volume for encryption” (to protect the confidentiality of data on certain partitions, see below);

make it a “physical volume for LVM” (this concept is discussed in greater detail later in this chapter);

use it as a RAID device (see later in this chapter);

or the choice not to use it, and therefore leave it unchanged.

4.2.13.3. Configuring Multidisk Devices (Software RAID)

Some types of RAID allow the duplication of information stored on hard drives to prevent data loss in the event of a hardware problem affecting one of them. Level 1 RAID keeps a simple, identical copy (mirror) of a hard drive on another drive, while level 5 RAID splits redundant data over several disks, thus allowing the complete reconstruction of a failing drive.

We will only describe level 1 RAID, which is the simplest to implement. The first step involves creating two partitions of identical size located on two different hard drives, and to label them “physical volume for RAID”.

You must then choose “Configure software RAID” in the partitioning tool to combine these two partitions into a new virtual disk and select “Create MD device” in the configuration screen. You then need to answer a series of questions about this new device. The first question asks about the RAID level to use, which in our case will be “RAID1”. The second question asks about the number of active devices — two in our case, which is the number of partitions that needs to be included in this MD device. The third question is about the number of spare devices — 0; we have not planned any additional disk to take over for a possible defective disk. The last question requires you to choose the partitions for the RAID peripheral — these would be the two that we have set aside for this purpose (make sure you only select the partitions that explicitly mention “raid”).

Back to the main menu, a new virtual “RAID” disk appears. This disk is presented with a single partition which can not be deleted, but whose use we can choose (just like for any other partition).

For further details on RAID functions, please refer to Section 12.1.1, “Software RAID”.

4.2.13.4. Configuring the Logical Volume Manager (LVM)

LVM allows you to create “virtual” partitions that span over several disks. The benefits are twofold: the size of the partitions are no longer limited by individual disks but by their cumulative volume, and you can at any time increase the size of an existing partition by adding an additional disk when needed.

LVM uses a particular terminology: a virtual partition is a “logical volume”, which is part of a “volume group”, or an association of several “physical volumes”. Each of these terms in fact corresponds to a “real” partition (or a software RAID device).

This technique works in a very simple way: each volume, whether physical or logical, is split into blocks of the same size, which are made to correspond by LVM. The addition of a new disk will cause the creation of a new physical volume, and these new blocks can be associated to any volume group. All of the partitions in the volume group that is thus expanded will have additional space into which they can extend.

The partitioning tool configures LVM in several steps. First you must create on the existing disks the partitions that will be “physical volumes for LVM”. To activate LVM, you need to choose “Configure the Logical Volume Manager (LVM)”, then on the same configuration screen “Create a volume group”, to which you will associate the existing physical volumes. Finally, you can create logical volumes within this volume group. Note that the automatic partitioning system is able to do all of this implementation.

In the partitioning menu, each physical volume will appear as a disk with a single partition which can not be deleted, but that you can use as desired.

The usage of LVM is described in further detail in Section 12.1.2, “LVM”.

4.2.13.5. Setting Up Encrypted Partitions