Description of problem:
When 2 disk redundancy is required and future expandability is needed, RAID 6 allows for setting up a 3-disk array and expanding it later.

lvm2 does not allow creation of 3-disk RAID6 arrays. While 3-disk RAID1 arrays provide the same redundancy level, migration from 3-disk RAID1 to 4-disk RAID6 requires an intermediate RAID5 step. During migration the data will have just 1 disk redundancy, migration that can take upwards of 20h for 16TB HDDs. For disks that have unrecoverable error rate at 1 per 10^14 bits read level.

Version-Release number of selected component (if applicable):


How reproducible:
always

Steps to Reproduce:
truncate -s 1G disk1
truncate -s 1G disk2
truncate -s 1G disk3
DEVS=($(losetup -f --show /tmp/disk1))
DEVS+=($(losetup -f --show /tmp/disk2))
DEVS+=($(losetup -f --show /tmp/disk3))
for file in "${DEVS[@]}"; do pvcreate "$file"; done
vgcreate redundant ${DEVS[@]}
lvcreate --type raid6 -L 500M --stripes 1 redundant -n data

Actual results:
  Physical volume "/dev/loop1" successfully created.
  Physical volume "/dev/loop2" successfully created.
  Physical volume "/dev/loop3" successfully created.
  Volume group "redundant" successfully created

  Minimum of 3 stripes required for raid6.
  Run `lvcreate --help' for more information.

Expected results:
an LV with RAID 6 using 3 PVs created

Additional info:
mdadm does allow creation of 3-disk RAID6 arrays.

Trying to create a 4-disk RAID6 array fails similarly, using --stripes 3 with just 4 PVs available results in the following error:

  Rounding size 500.00 MiB (125 extents) up to stripe boundary size 504.00 MiB(126 extents).
  Insufficient suitable allocatable extents for logical volume data: 129 more required