Lvm Volume Group Metadata; The Physical Volume Label; Metadata Contents - Red Hat ENTERPRISE LINUX 5 - LOGICAL VOLUME MANAGER ADMINISTRATION Manual

Appendix D. LVM Volume Group
Metadata
The configuration details of a volume group are referred to as the metadata. By default, an identical
copy of the metadata is maintained in every metadata area in every physical volume within the volume
group. LVM volume group metadata is small and stored as ASCII.
If a volume group contains many physical volumes, having many redundant copies of the metadata
is inefficient. It is possible to create a physical volume without any metadata copies by using the --
metadatacopies 0 option of the pvcreate command. Once you have selected the number of
metadata copies the physical volume will contain, you cannot change that at a later point. Selecting
0 copies can result in faster updates on configuration changes. Note, however, that at all times every
volume group must contain at least one physical volume with a metadata area (unless you are using
the advanced configuration settings that allow you to store volume group metadata in a file system).
If you intend to split the volume group in the future, every volume group needs at least one metadata
copy.
The core metadata is stored in ASCII. A metadata area is a circular buffer. New metadata is appended
to the old metadata and then the pointer to the start of it is updated.
You can specify the size of metadata area with the --metadatasize. option of the pvcreate
command. The default size is too small for volume groups with many logical volumes or physical
volumes.
D.1. The Physical Volume Label
By default, the pvcreate command places the physical volume label in the 2nd 512-byte sector.
This label can optionally be placed in any of the first four sectors, since the LVM tools that scan for
a physical volume label check the first 4 sectors. The physical volume label begins with the string
LABELONE.
The physical volume label Contains:
• Physical volume UUID
• Size of block device in bytes
• NULL-terminated list of data area locations
• NULL-terminated lists of metadata area locations
Metadata locations are stored as offset and size (in bytes). There is room in the label for about 15
locations, but the LVM tools currently use 3: a single data area plus up to two metadata areas.
D.2. Metadata Contents
The volume group metadata contains:
• Information about how and when it was created
• Information about the volume group:
The volume group information contains:
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