Lvm Components; Physical Volumes; Lvm Physical Volume Layout - Red Hat LVM ADMINISTRATOR 5.0 Administrator's Manual

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Chapter 2.

LVM Components

This chapter describes the components of an LVM Logical volume.

The underlying physical storage unit of an LVM logical volume is a block device such as a

partition or whole disk. To use the device for an LVM logical volume the device must be

initialized as a physical volume (PV). Initializing a block device as a physical volume places a

label near the start of the device.

By default, the LVM label is placed in the second 512-byte sector. You can overwrite this default

by placing the label on any of the first 4 sectors. This allows LVM volumes to co-exist with other

users of these sectors, if necessary.

An LVM label provides correct identification and device ordering for a physical device, since

devices can come up in any order when the system is booted. An LVM label remains persistent

across reboots and throughout a cluster.

The LVM label identifies the device as an LVM physical volume. It contains a random unique

identifier (the UUID) for the physical volume. It also stores the size of the block device in bytes,

and it records where the LVM metadata will be stored on the device.

The LVM metadata contains the configuration details of the LVM volume groups on your

system. By default, an identical copy of the metadata is maintained in every metadata area in

every physical volume within the volume group. LVM metadata is small and stored as ASCII.

Currently LVM allows you to store 0, 1 or 2 identical copies of its metadata on each physical

volume. The default is 1 copy. Once you configure the number of metadata copies on the

physical volume, you cannot change that number at a later time. The first copy is stored at the

start of the device, shortly after the label. If there is a second copy, it is placed at the end of the

device. If you accidentally overwrite the area at the beginning of your disk by writing to a

different disk than you intend, a second copy of the metadata at the end of the device will allow

you to recover the metadata.

For detailed information about the LVM metadata and changing the metadata parameters, see

Appendix D, LVM Volume Group

Metadata.

Figure 2.1, "Physical Volume layout"

shows the layout of an LVM physical volume. The LVM

label is on the second sector, followed by the metadata area, followed by the usable space on

the device.

Note

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Configuration and administration 5.0