SSD caching an existing btrfs raid1

[Pat Sailor <psalleetsile@gmail.com>]

Hello,

I have a half-filled raid1 on top of six spinning devices. Now I have 
come into a spare SSD I'd like to use for caching, if possible without 
having to rebuild or, failing that, without having to renounce to btrfs 
and flexible reshaping.

I've been reading about the several options out there; I thought that 
EnhanceIO would be the simplest bet but unfortunately I couldn't get it 
to build with my recent kernel (last commits are from years ago).

Failing that, I read that lvmcache could be the way to go. However, I 
can't think of a way of setting it up in which I retain the ability to 
add/remove/replace drives as I can do now with pure btrfs; if I opted to 
drop btrfs to go to ext4 I still would have to offline the filesystem 
for downsizes. Not a frequent occurrence I hope, but now I'm used to 
keep working while I reshape things in btrfs, and it's better if I can 
avoid large downtimes.

Is what I want doable at all? Thanks in advance for any 
suggestions/experiences to proceed.