Red Hat ENTERPRISE LINUX 5 - LOGICAL VOLUME MANAGER ADMINISTRATION Instalační příručka Strana 15
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Chapter 2. LVM Components
This chapter describes the components of an LVM Logical volume.
1. Physical Volumes
The underlying physical storage unit of an LVM logical volume is a block device such as a parti-
tion 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 sys-
tem. 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 phys-
ical 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 differ-
ent 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.
1.1. LVM Physical Volume Layout
Figure 2.1, “Physical Volume layout” shows the layout of an LVM physical volume. The LVM la-
bel is on 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
6
- Table of Contents 4
- 1. About This Guide 7
- 2. Audience 7
- 3. Software Versions 7
- 4. Related Documentation 7
- 5. Document Conventions 8
- Important 9
- 1. Logical Volumes 10
- 2. LVM Architecture Overview 11
- 3. Running LVM in a Cluster 12
- 4. Document Overview 13
- Chapter 2. LVM Components 15
- 2. Volume Groups 17
- 3. LVM Logical Volumes 17
- Figure 2.2. Extent Mapping 18
- 3.2. Striped Logical Volumes 20
- 3.3. Mirrored Logical Volumes 21
- 3.4. Snapshot Volumes 22
- Chapter 3. LVM Administration 24
- Overview 24
- 4. Logical Volume Backup 25
- 5. Logging 25
- • external log function 26
- LVM Configuration Files 26
- CLI Commands 27
- 3.1. Creating Volume Groups 31
- 3.3. Displaying Volume Groups 32
- 3.8. Removing Volume Groups 35
- 3.9. Splitting a Volume Group 35
- 3.10. Combining Volume Groups 35
- 3.12. Renaming a Volume Group 36
- 4.1. Creating Logical Volumes 37
- 4.3. Resizing Logical Volumes 41
- 4.5. Renaming Logical Volumes 42
- 4.6. Removing Logical Volumes 42
- 4.8. Growing Logical Volumes 43
- 5. Creating Snapshot Volumes 46
- 7. Online Data Relocation 48
- Nodes in a Cluster 48
- 9.1. Format Control 49
- 9.2. Object Selection 51
- Table 4.1. pvs Display Fields 52
- The vgs Command 53
- The lvs Command 54
- Table 4.3. lvs Display Fields 56
- 9.3. Sorting LVM Reports 57
- 9.4. Specifying Units 58
- Chapter 5. LVM Configuration 60
- Examples 60
- 1.4. Creating the File System 61
- 3. Splitting a Volume Group 62
- 3.1. Determining Free Space 63
- 3.2. Moving the Data 63
- 4.2.3. Moving the Data 66
- Volume Group 73
- Appendix A. The Device Mapper 76
- Appendix B. The LVM 77
- Configuration Files 77
- 2. Sample lvm.conf File 78
- Appendix C. LVM Object Tags 84
- Appendix D. LVM Volume Group 86
- Metadata 86
- 3. Sample Metadata 87
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