Linux Logical Volume Manager (LVM) format

The Linux Logical Volume Manager (LVM) format is used by the Logical Volume Manager (LVM) on Linux.

Overview

A Linux LVM consist of:

• Volume group
• Physical volumes
• Logical volumes

Characteristics

According to “RHEL 5 - Logical Volume Manager Administration” the number of bytes per sector is 512.

Checksums use a “weak” CRC-32, which is a CRC-32 without the initial and final XOR with 0xffffffff, using the polynominal 0xedb88320 and initial value 0xf597a6cf.

Terminology

Physical volume

A physical volume consist of:

• Empty sector
• The physical volume label header
  • The physical volume header
    • data area descriptor list
    • metadata area descriptor list
• The metadata area
• Data area (or data extents)

Physical volume label

The physical volume label is stored in the second sector of the physical volume. The physical volume label is currently 512 bytes in size and consists of:

• physical volume label header
• physical volume header

Note that according to “RHEL 5 - Logical Volume Manager Administration” the physical volume label can be stored in any of the first 4 sectors.

Physical volume label header

The physical volume label header (struct label_header) is 32 bytes in size and consist of:

Physical volume header

The physical volume header (struct pv_header) is of variable size and consist of:

The physical volume identifier can be used to uniquely identify a physical volume. The physical volume identifier is stored as “9LBcEB7PQTGIlLI0KxrtzrynjuSL983W” but is equivalent to its formatted variant “9LBcEB-7PQT-GIlL-I0Kx-rtzr-ynju-SL983W”, which is used in the metadata.

Note that the data area size can be 0, and currently it is assumed that this represents all remaining (available) space within the physical volume.

Data area descriptor

The metadata area

The metadata area consist of:

• Metadata area header
• Metadata

According to “RHEL 5 - Logical Volume Manager Administration” the 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. The metadata area, therefore, can contain copies of older versions of the metadata.

Metadata area header

The metadata area header (struct mda_header) is 512 bytes in size and consist of:

Raw location descriptor

The raw location descriptor (struct raw_locn) is 24 bytes in size and consist of:

Note that the data area size can be 0. It is assumed it represents the remaining available data.

Raw location descriptor flags

Metadata

The metadata consist of:

• Volume group main section
  • Physical volumes object
    • Physical volume object
  • Logical volumes object
    • Logical volume object
      • Segment object
• Top level properties

According to “RHEL 5 - Logical Volume Manager Administration” by default, an identical copy of the metadata is maintained in every metadata area in every physical volume within the volume group. The metadata is stored as ASCII.

The metadata can also be stored in a stand-alone file.

Example

# Generated by LVM2: Tue Jan 30 16:28:15 2007

contents = "Text Format Volume Group"
version = 1

description = "Created *before* executing 'lvextend -L+5G /dev/myvg/mylv /dev/sdc'"

creation_host = "tng3-1"  # Linux tng3-1 2.6.18-8.el5 #1 SMP Fri Jan 26 14:15:21 EST 2007 i686
creation_time = 1170196095  # Tue Jan 30 16:28:15 2007

myvg {
  id = "0zd3UT-wbYT-lDHq-lMPs-EjoE-0o18-wL28X4"
  seqno = 3
  status = ["RESIZEABLE", "READ", "WRITE"]
  extent_size = 8192    # 4 Megabytes
  max_lv = 0
  max_pv = 0

  physical_volumes {

    pv0 {
      id = "ZBW5qW-dXF2-0bGw-ZCad-2RlV-phwu-1c1RFt"
      device = "/dev/sda"   # Hint only

      status = ["ALLOCATABLE"]
      dev_size = 35964301   # 17.1491 Gigabytes
      pe_start = 384
      pe_count = 4390 # 17.1484 Gigabytes
    }

    ...
  }
  logical_volumes {

    mylv {
      id = "GhUYSF-qVM3-rzQo-a6D2-o0aV-LQet-Ur9OF9"
      status = ["READ", "WRITE", "VISIBLE"]
      segment_count = 2

      segment1 {
        start_extent = 0
        extent_count = 1280   # 5 Gigabytes

        type = "striped"
        stripe_count = 1  # linear

        stripes = [
          "pv0", 0
        ]
      }
      segment2 {
        start_extent = 1280
        extent_count = 1280   # 5 Gigabytes

        type = "striped"
        stripe_count = 1  # linear

        stripes = [
          "pv1", 0
        ]
      }
    }
  }
}

Properties

The metadata is textual and uses properties, where a property is defined as:

<identifier> = <value>

The identifier contains an unique name of the property and <value> is one of the following types:

Note that white space, such as space and new line characters, seem to be ignored.

The # character is used for comments. A comment continues to the end-of-line.

Note that for now it is assumed that the # character is not allowed to be used in any of the values.

Top level properties

Note it is assumed that there can only be 1 volume group object.

Volume group and logical volume names

• Allowed characters: “a-z”, “A-Z”, “0-9”, “_”, “+”, “.”, and “-” (hypen)
• A volume group name should not start with a hypen
• A logical volume name can start with a hypen, but it is reserved for logical volumes that have been removed but are stil being tracked by the format

Note that some implementations use lv_ as the prefix for a logical volume, however this is not enforced by the format.

Volume group object

The volume group object contains the following properties:

Physical volumes object

The physical volumes object contains the following properties:

Physical volume object

Each physical volume object contains the following properties:

Logical volumes object

The logical volumes object contains the following properties:

Logical volume object

Each logical volume object contains the following properties:

Segment object

Each segment object contains the following properties:

Segment types

Note that a comparable list can be retrieved using lvm segtypes.

Stripes list

stripes = [
<physical volume name>, <start extent number>
]

Where:

• <physical volume name> is a string containing the physical volume name e.g. “pv0”.
• <start extent number> the segment start extent number relative from the start of the data area.

start_extent_offset = (
    (start_extent_number * extent_size * sector_size) + physical_volume_data_area_start_offset
)

Status flags

Flags

TODO: complete section

Comments

Textual metadata such as:

# Generated by LVM2 version 2.02.39 (2008-06-27): Sat Jan 17 11:45:29 2009

References

• LVM HOWTO
• RHEL 5 - Logical Volume Manager Administration