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5.4.14.3. Extending a Logical Volume with the cling Allocation Policy

When extending an LVM volume, you can use the --alloc cling option of the lvextend command to specify the cling allocation policy. This policy will choose space on the same physical volumes as the last segment of the existing logical volume. If there is insufficient space on the physical volumes and a list of tags is defined in the lvm.conf file, LVM will check whether any of the tags are attached to the physical volumes and seek to match those physical volume tags between existing extents and new extents.
For example, if you have logical volumes that are mirrored between two sites within a single volume group, you can tag the physical volumes according to where they are situated by tagging the physical volumes with @site1 and @site2 tags and specify the following line in the lvm.conf file: 
cling_tag_list = [ "@site1", "@site2" ]
For information on tagging physical volumes, see Appendix D, LVM Object Tags.
In the following example, the lvm.conf file has been modified to contain the following line: 
cling_tag_list = [ "@A", "@B" ]
Also in this example, a volume group taft has been created that consists of the physical volumes /dev/sdb1, /dev/sdc1, /dev/sdd1, /dev/sde1, /dev/sdf1, /dev/sdg1, and /dev/sdh1. These physical volumes have been tagged with tags A, B, and C. The example does not use the C tag, but this will show that LVM uses the tags to select which physical volumes to use for the mirror legs. 
# pvs -a -o +pv_tags /dev/sd[bcdefgh]1
  PV         VG   Fmt  Attr PSize   PFree   PV Tags
  /dev/sdb1  taft lvm2 a-   135.66g 135.66g A
  /dev/sdc1  taft lvm2 a-   135.66g 135.66g B
  /dev/sdd1  taft lvm2 a-   135.66g 135.66g B
  /dev/sde1  taft lvm2 a-   135.66g 135.66g C
  /dev/sdf1  taft lvm2 a-   135.66g 135.66g C
  /dev/sdg1  taft lvm2 a-   135.66g 135.66g A
  /dev/sdh1  taft lvm2 a-   135.66g 135.66g A
The following command creates a 100GB mirrored volume from the volume group taft. 
# lvcreate -m 1 -n mirror --nosync -L 100G taft
The following command shows which devices are used for the mirror legs and mirror log. 
# lvs -a -o +devices
  LV                VG        Attr   LSize   Log         Copy%  Devices
  mirror            taft      Mwi-a- 100.00g mirror_mlog 100.00
mirror_mimage_0(0),mirror_mimage_1(0)
  [mirror_mimage_0] taft      iwi-ao 100.00g                    /dev/sdb1(0)
  [mirror_mimage_1] taft      iwi-ao 100.00g                    /dev/sdc1(0)
  [mirror_mlog]     taft      lwi-ao   4.00m                    /dev/sdh1(0)
The following command extends the size of the mirrored volume, using the cling allocation policy to indicate that the mirror legs should be extended using physical volumes with the same tag. 
# lvextend --alloc cling -L +100G taft/mirror
  Extending 2 mirror images.
  Extending logical volume mirror to 200.00 GiB
  Logical volume mirror successfully resized
The following display command shows that the mirror legs have been extended using physical volumes with the same tag as the leg. Note that the physical volumes with a tag of C were ignored. 
# lvs -a -o +devices
  LV                VG        Attr   LSize   Log         Copy%  Devices
  mirror            taft      Mwi-a- 200.00g mirror_mlog  50.16
mirror_mimage_0(0),mirror_mimage_1(0)
  [mirror_mimage_0] taft      Iwi-ao 200.00g                    /dev/sdb1(0)
  [mirror_mimage_0] taft      Iwi-ao 200.00g                    /dev/sdg1(0)
  [mirror_mimage_1] taft      Iwi-ao 200.00g                    /dev/sdc1(0)
  [mirror_mimage_1] taft      Iwi-ao 200.00g                    /dev/sdd1(0)
  [mirror_mlog]     taft      lwi-ao   4.00m                    /dev/sdh1(0)