Description of problem:

[root@hayes-03 ~]# lvcreate --wipesignatures y  -L 4G -n single writecache_sanity @slow
  Logical volume "single" created.

[root@hayes-03 ~]# lvs -a -o +devices,segtype
  LV     VG                Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices      Type  
  single writecache_sanity -wi-a----- 4.00g                                                     /dev/sde1(0) linear

# I don't want to use all ~1860G
[root@hayes-03 ~]# lvconvert --type writecache --cachedevice /dev/sdn1 --cachedevice /dev/sdm1 writecache_sanity/single
Use all <930.76 GiB from /dev/sdn1 for cache? [y/n]: n
  Use --cachesize SizeMB to use a part of the cachedevice.
  Conversion aborted.

[root@hayes-03 ~]# lvconvert --type writecache --cachedevice /dev/sdn1 --cachesize 2G --cachedevice /dev/sdm1 --cachesize 2G writecache_sanity/single
  Option --cachesize may not be repeated.
  Error during parsing of command line.

# Using the entirety of the devices results in the process being killed due to bug 1855038, and you end up with unused "_cache" devices
[root@hayes-03 ~]# lvconvert --type writecache --cachedevice /dev/sdn1 --cachedevice /dev/sdm1 --cachesize 1800G writecache_sanity/single
  Creating cachevol LV single_cache with size <1.76 TiB.
  Logical volume "single_cache" created.
  Using writecache block size 512 for unknown file system block size, logical block size 512, physical block size 4096.
Killed

[root@hayes-03 ~]# lvs -a -o +devices,segtype
  LV           VG                Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices      Type  
  single       writecache_sanity -wi-a-----  4.00g                                                     /dev/sde1(0) linear
  single_cache writecache_sanity -wi------- <1.76t                                                     /dev/sdn1(0) linear
  single_cache writecache_sanity -wi------- <1.76t                                                     /dev/sdm1(0) linear

[root@hayes-03 ~]# lvremove -f writecache_sanity/single_cache
  Logical volume "single_cache" successfully removed

[root@hayes-03 ~]#  lvconvert --type writecache --cachedevice /dev/sdn1 --cachedevice /dev/sdm1 --cachesize 8G writecache_sanity/single
  Creating cachevol LV single_cache with size 8.00 GiB.
  Logical volume "single_cache" created.
  Using writecache block size 512 for unknown file system block size, logical block size 512, physical block size 4096.
  device-mapper: create ioctl on writecache_sanity-single_cache_cvol LVM-e3lLEZuDwfJugVxfuQk08fHYybd1YF9aH60NmP2roHij0T43mUIxZy0Oaoc09Nzw-cvol failed: Device or resource busy
  Failed to suspend logical volume writecache_sanity/single.

# Reboot to get around the issues caused above...

# Using less space than the first device has will result in dropping the second device on the floor
[root@hayes-03 ~]# lvconvert --type writecache --cachedevice /dev/sdn1 --cachedevice /dev/sdm1 --cachesize 8G writecache_sanity/single
  Creating cachevol LV single_cache with size 8.00 GiB.
  Logical volume "single_cache" created.
  Using writecache block size 512 for unknown file system block size, logical block size 512, physical block size 4096.
  Logical volume writecache_sanity/single now has writecache.

[root@hayes-03 ~]# lvs -a -o +devices
  LV                  VG                Attr       LSize Pool                Origin          Data%  Meta%  Move Log Cpy%Sync Convert Devices         
  single              writecache_sanity Cwi-a-C--- 4.00g [single_cache_cvol] [single_wcorig] 0.00                                    single_wcorig(0)
  [single_cache_cvol] writecache_sanity Cwi-aoC--- 8.00g                                                                             /dev/sdn1(0)    
  [single_wcorig]     writecache_sanity owi-aoC--- 4.00g                                                                             /dev/sde1(0)    


Version-Release number of selected component (if applicable):
kernel-4.18.0-221.el8    BUILT: Thu Jun 25 16:28:39 CDT 2020
lvm2-2.03.09-3.el8    BUILT: Mon Jun 29 13:50:23 CDT 2020
lvm2-libs-2.03.09-3.el8    BUILT: Mon Jun 29 13:50:23 CDT 2020
lvm2-dbusd-2.03.09-3.el8    BUILT: Mon Jun 29 13:53:38 CDT 2020
lvm2-lockd-2.03.09-3.el8    BUILT: Mon Jun 29 13:50:23 CDT 2020
boom-boot-1.2-2.el8    BUILT: Mon Jun 29 08:27:02 CDT 2020
device-mapper-1.02.171-3.el8    BUILT: Mon Jun 29 13:50:23 CDT 2020
device-mapper-libs-1.02.171-3.el8    BUILT: Mon Jun 29 13:50:23 CDT 2020
device-mapper-event-1.02.171-3.el8    BUILT: Mon Jun 29 13:50:23 CDT 2020
device-mapper-event-libs-1.02.171-3.el8    BUILT: Mon Jun 29 13:50:23 CDT 2020