-----Original Message-----
I'm sorry I don't follow this. It would be appreciated if you could include a simpler example. The way I see it, a VM disk is just a small chunck "LVM LV in my case" of a real disk.
Perhaps if you were to compare and contrast a virtual disk to a raw disk, that would help. If you wanted to use drbd with a raw disk being accessed via a VM guest, that would probably be all right. Might not be "supported" though.
Depending on your virtualization method, raw device passthrough would probably be OK. Otherwise, think about what you're doing - putting a filesystem - on a replicated block device - that's presented through a virtualization layer - that's on a filesystem - that's on a block device. If you're running GFS/GlusterFS/etc on the DRBD disk, and the VM is on VMFS, then you're actually using two clustered filesystems!
Each layer adds a bit of overhead, and each block-on-filesystem layering adds the potential for block misalignments and other issues that will affect your overall performance and throughput. It's just hard to do right.
-Brad