How chunk_size works in LVM thin provisioning and why there is difference between RHEL7.3 and RHEL7.4

Issue

• Prior to RHEL7.4, the lvcreate command could create a thin-provisioned pool with a default chunk_size of 64KiB. After updating to RHEL7.4 and creating an identical thin pool the default chunk_size is different (larger) and value of Meta% is usually significantly smaller. Why does this difference occur?

• When creating LVM2 thin pools and volumes the way things were calculated changed when RHEL 7.4 was released because the LVM2 version was rebased. This changed the calculation of chunk_size between RHEL 7.3 and 7.4.

  This is with lvm2-2.02.166-1.el7_3.5.x86_64:

  # lvcreate -L 3T -T testvg/mythinpool -V1T -n thinvolume
    Using default stripesize 64.00 KiB.
    Logical volume "thinvolume" created.
  # lvs -ao name,size,stripesize,chunksize,metadata_percent
    LV                 LSize  Stripe Chunk  Meta%
    home               72.02g     0      0
    root               50.00g     0      0
    swap               13.67g     0      0
    lvol0              80.00m     0      0
    [lvol1_pmspare]    96.00m     0      0
    mythinpool          3.00t     0  64.00k 12.74
    [mythinpool_tdata]  3.00t     0      0
    [mythinpool_tmeta] 96.00m     0      0
    thinvolume          1.00t     0      0
  

  And this is with lvm2-2.02.171-8.el7.x86_64:

  # lvcreate -L 3T -T testvg/mythinpool -V1T -n thinvolume
    Using default stripesize 64.00 KiB.
    Thin pool volume with chunk size 2.00 MiB can address at most 506.00 TiB of data.
    Logical volume "thinvolume" created.
  # lvs -ao name,size,stripesize,chunksize,metadata_percent
    LV                 LSize  Stripe Chunk Meta%
    home               72.02g     0     0
    root               50.00g     0     0
    swap               13.67g     0     0
    lvol0              80.00m     0     0
    [lvol1_pmspare]    96.00m     0     0
    mythinpool          3.00t     0  2.00m 0.44
    [mythinpool_tdata]  3.00t     0     0
    [mythinpool_tmeta] 96.00m     0     0
    thinvolume          1.00t     0     0
  

  In the first one the chunk_size was 64KiB and it was changed to 2MiB in the later version of LVM.
  Note that some messages are different when creating the thin pool and volume.