Red Hat GLOBAL FILE SYSTEM 5.0, Руководство администратора

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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.
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.
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