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5.3.2. LVM Allocation

When an LVM operation needs to allocate physical extents for one or more logical volumes, the allocation proceeds as follows:
  • The complete set of unallocated physical extents in the volume group is generated for consideration. If you supply any ranges of physical extents at the end of the command line, only unallocated physical extents within those ranges on the specified physical volumes are considered.
  • Each allocation policy is tried in turn, starting with the strictest policy (contiguous) and ending with the allocation policy specified using the --alloc option or set as the default for the particular logical volume or volume group. For each policy, working from the lowest-numbered logical extent of the empty logical volume space that needs to be filled, as much space as possible is allocated, according to the restrictions imposed by the allocation policy. If more space is needed, LVM moves on to the next policy.
The allocation policy restrictions are as follows:
  • An allocation policy of contiguous requires that the physical location of any logical extent that is not the first logical extent of a logical volume is adjacent to the physical location of the logical extent immediately preceding it.
    When a logical volume is striped or mirrored, the contiguous allocation restriction is applied independently to each stripe or mirror image (leg) that needs space.
  • An allocation policy of cling requires that the physical volume used for any logical extent to be added to an existing logical volume is already in use by at least one logical extent earlier in that logical volume. If the configuration parameter allocation/cling_tag_list is defined, then two physical volumes are considered to match if any of the listed tags is present on both physical volumes. This allows groups of physical volumes with similar properties (such as their physical location) to be tagged and treated as equivalent for allocation purposes. For more information on using the cling policy in conjunction with LVM tags to specify which additional physical volumes to use when extending an LVM volume, see Section 5.4.14.3, “Extending a Logical Volume with the cling Allocation Policy”.
    When a Logical Volume is striped or mirrored, the cling allocation restriction is applied independently to each stripe or mirror image (leg) that needs space.
  • An allocation policy of normal will not choose a physical extent that shares the same physical volume as a logical extent already allocated to a parallel logical volume (that is, a different stripe or mirror image/leg) at the same offset within that parallel logical volume.
    When allocating a mirror log at the same time as logical volumes to hold the mirror data, an allocation policy of normal will first try to select different physical volumes for the log and the data. If that is not possible and the allocation/mirror_logs_require_separate_pvs configuration parameter is set to 0, it will then allow the log to share physical volume(s) with part of the data.
    Similarly, when allocating thin pool metadata, an allocation policy of normal will follow the same considerations as for allocation of a mirror log, based on the value of the allocation/thin_pool_metadata_require_separate_pvs configuration parameter.
  • If there are sufficient free extents to satisfy an allocation request but a normal allocation policy would not use them, the anywhere allocation policy will, even if that reduces performance by placing two stripes on the same physical volume.
The allocation policies can be changed using the vgchange command.

Note

If you rely upon any layout behavior beyond that documented in this section according to the defined allocation policies, you should note that this might change in future versions of the code. For example, if you supply on the command line two empty physical volumes that have an identical number of free physical extents available for allocation, LVM currently considers using each of them in the order they are listed; there is no guarantee that future releases will maintain that property. If it is important to obtain a specific layout for a particular Logical Volume, then you should build it up through a sequence of lvcreate and lvconvert steps such that the allocation policies applied to each step leave LVM no discretion over the layout.
To view the way the allocation process currently works in any specific case, you can read the debug logging output, for example by adding the -vvvv option to a command.