lsblk
    fdisk -l
blkid -o list
fdisk /dev/vda

 partprobe
blkid -o list
 mkdir /cache
  mkfs.ext4 /dev/vda3
mount /dev/vda3 /cache
 blkid -o list
nano /etc/fstab

e2label /dev/vda3 "LVM Cache"
 blkid -o list
systemctl daemon-reload
reboot

   lvs

 blkid -o list
device             fs_type   label      mount point            UUID
---------------------------------------------------------------------------------------------------
/dev/mapper/centos7-root
                   xfs                  /                      f3b05d5e-eaa1-4fb1-ba8e-dc7b52aa3509
/dev/vda2          LVM2_member          (in use)               su80uZ-L3qP-4br0-zM7U-SkUl-I7hB-XPuQrg
/dev/vda1          ext4                 /boot                  443dd8ca-b901-4947-a61d-df258f7f2958
/dev/mapper/centos7-swap
                   swap                 <swap>                 9db3c009-809e-4028-9577-0b2203103010
/dev/vda3          ext4      LVM Cache  /cache                 827c2730-9d16-4c0b-add4-f89c9c22e832
/dev/mapper/centos7-home
                   xfs                  /home                  7de649f7-03aa-46a0-8838-45a6bd23ae6a
 

lsblk
NAME             MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vda              252:0    0   50G  0 disk
├─vda1           252:1    0  953M  0 part /boot
├─vda2           252:2    0 35.4G  0 part
│ ├─centos7-root 253:0    0 31.7G  0 lvm  /
│ ├─centos7-swap 253:1    0  2.8G  0 lvm  [SWAP]
│ └─centos7-home 253:2    0  956M  0 lvm  /home
└─vda3           252:3    0 13.7G  0 part

umount /dev/vda3

pvcreate /dev/vda3
WARNING: ext4 signature detected on /dev/vda3 at offset 1080. Wipe it? [y/n]: y
  Wiping ext4 signature on /dev/vda3.
  Physical volume "/dev/vda3" successfully created.

pvscan

PV /dev/vda2   VG centos7         lvm2 [35.40 GiB / 4.00 MiB free]
  PV /dev/vda3                      lvm2 [13.67 GiB]
  Total: 2 [49.06 GiB] / in use: 1 [35.40 GiB] / in no VG: 1 [13.67 GiB]

 vgextend centos7 /dev/vda3
  Volume group "centos7" successfully extended

lvcreate --type cache-pool -L 13.6G -n cpool centos7 /dev/vda3
  Using default stripesize 64.00 KiB.
  Rounding up size to full physical extent 13.60 GiB
  Logical volume "cpool" created.

lvchange -a n /dev/centos7/cpool

lvchange -a y /dev/centos7/cpool

nano /etc/fstab

 -> {to comment-out previously added physical volume dev/vda3}

lvs -a -o name,size,attr,devices centos7
  LV              LSize   Attr       Devices
  cpool            13.60g Cwi---C--- cpool_cdata(0)
  [cpool_cdata]    13.60g Cwi------- /dev/vda3(12)
  [cpool_cmeta]    16.00m ewi------- /dev/vda3(8)
  home            956.00m -wi-ao---- /dev/vda2(8106)
  lvol0            16.00m -wi------- /dev/vda3(0)
  [lvol1_pmspare]  16.00m ewi------- /dev/vda3(4)
  root             31.66g -wi-ao---- /dev/vda2(0)
  swap              2.80g -wi-ao---- /dev/vda2(8345)
#  lvconvert --type cache --cachepool cpool centos7/root
Do you want wipe existing metadata of cache pool volume centos7/cpool? [y/n]: y
  Logical volume centos7/root is now cached.
# lvs -a -o name,size,attr,devices centos7
  LV              LSize   Attr       Devices
  [cpool]          13.60g Cwi---C--- cpool_cdata(0)
  [cpool_cdata]    13.60g Cwi-ao---- /dev/vda3(12)
  [cpool_cmeta]    16.00m ewi-ao---- /dev/vda3(8)
  home            956.00m -wi-ao---- /dev/vda2(8106)
  lvol0            16.00m -wi------- /dev/vda3(0)
  [lvol1_pmspare]  16.00m ewi------- /dev/vda3(4)
  root             31.66g Cwi-aoC--- root_corig(0)
  [root_corig]     31.66g owi-aoC--- /dev/vda2(0)
  swap              2.80g -wi-ao---- /dev/vda2(8345)