lvm PV on top of dm-cache

I am running a private cloud (opennebula, KVM) where I am running VMs both directly using LVM Logical Volumes and using disk images on a file system (one is faster, the other is easier to migrate), all on top of raid5.

I managed to procure some SSDs that I can put into my servers, and I would like to use them as cache, most probably using DM-cache (as bcache needs a disk reformat and flashcache is still not mainline).

Now, as I would like to cache multiple LVs, I could create separate metadata and cache partitions for each one, or I could create a new LV, put dm-cache on top of that and then use it as a PV and move the previous LVs there one by one.

So the resulting layering would be:

1. md1 == sda+sdb+sdc+sdd
2. PV0 == md1
3. LV0 <- PV0 (I intend to leave the host OS's volumes uncached)
4. PV1 == LV0 + dmcache
5. (LV1, LV2, LV3, ...) <- PV1

So my question are:

• Does this make sense? :)
• Is there a significant (>5%) performance penalty for an additional LVM layer?
• Is there an inherent incompatibility issue in this setup that I should be aware of?