Red Hat ENTERPRISE LINUX 4 - LVM ADMINISTRATOR Guide d'installation télécharger pdf (Page 22)

Chapter 2. LVM Components

Figure 2.1. Physical Volume layout

2.1.2. Multiple Partitions on a Disk

LVM allows you to create physical volumes out of disk partitions. It is generally recommended that you

create a single partition that covers the whole disk to label as an LVM physical volume for the following

reasons:

• Administrative convenience

It is easier to keep track of the hardware in a system if each real disk only appears once. This

becomes particularly true if a disk fails. In addition, multiple physical volumes on a single disk may

cause a kernel warning about unknown partition types at boot-up.

• Striping performance

LVM cannot tell that two physical volumes are on the same physical disk. If you create a striped

logical volume when two physical volumes are on the same physical disk, the stripes could be on

different partitions on the same disk. This would result in a decrease in performance rather than an

increase.

Although it is not recommended, there may be specific circumstances when you will need to divide

a disk into separate LVM physical volumes. For example, on a system with few disks it may be

necessary to move data around partitions when you are migrating an existing system to LVM

volumes. Additionally, if you have a very large disk and want to have more than one volume group for

administrative purposes then it is necessary to partition the disk. If you do have a disk with more than

one partition and both of those partitions are in the same volume group, take care to specify which

partitions are to be included in a logical volume when creating striped volumes.

2.2. Volume Groups

Physical volumes are combined into volume groups (VGs). This creates a pool of disk space out of

which logical volumes can be allocated.

Within a volume group, the disk space available for allocation is divided into units of a fixed-size

called extents. An extent is the smallest unit of space that can be allocated. Within a physical volume,

extents are referred to as physical extents.

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Red Hat ENTERPRISE LINUX 4 - LVM ADMINISTRATOR Guide d'installation Page 22