Lvm Components; Physical Volumes; Lvm Physical Volume Layout - Red Hat ENTERPRISE LINUX 5 - LOGICAL VOLUME MANAGER ADMINISTRATION Manual

Chapter 2.

LVM Components

This chapter describes the components of an LVM Logical volume.

2.1. Physical Volumes

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

2.1.1. LVM Physical Volume Layout

Figure 2.1, "Physical Volume layout"
the second sector, followed by the metadata area, followed by the usable space on the device.
Note
In the Linux kernel (and throughout this document), sectors are considered to be 512
bytes in size.
Metadata.
shows the layout of an LVM physical volume. The LVM label is on
7