[linux-lvm] wipe LVM cache

[Xen]

Is there a way to wipe a cache without recreating it?

When I split the cache using --split-cache, then dd if=/dev/zero the 
origin volume, and then combine the cache again, the system will think 
nothing has changed, and the blocks on the cache that were used before, 
will keep being used, and will "mask" blocks in the root volume, I 
believe. (The origin volume).

Dmsetup status will show all blocks in the cache are still used:

linux-root: 0 41943040 cache 8 367/3072 128 121600/121600

It seems weird that I can't just clean the cache. I'd have to either dd 
the entire volume when the cache is combined, or recreate the cache 
itself.