Changing lvmcache mode

[Raman Gupta]

I would like to change the cache mode of an existing lvm cache volume.

I see the following in the man page:

`With the --cachemode option, the cache mode can be set when creating
a cache LV, or changed on an existing cache LV.`

However, it does not seem to work:

# lvs -o cache_mode vg_raid10/lv_var
  CacheMode
  writethrough
# lvconvert --cachemode writeback vg_raid10/lv_var
# lvs -o cache_mode vg_raid10/lv_var
  CacheMode
  writethrough

(Fedora 23, 4.8.14)

The `lvconvert` command produces no output, and there is nothing in
dmesg or the journal.

Regards,
Raman