yes you can use LVM snapshots with DRBD, however it’s advisable to make sure that the file system is “frozen” at the time the snapshot is created. That’s not too easily feasible when using DRBD as Xen VBDs. YMMV.

this does not require GFS. Nor OCFS2. Indeed, the point is to avoid the need for a clustering file system.

The helper script makes your VBDs Primary on domU creation, and Secondary on domU destruction. The allow-to-primaries option is there to allow live migration, since Xen assumes to have write access to the backing both the source and the destination host at the time of migration. If you’re not using live migration, you can do fine without allow-two-primaries.

Re question #1: the maximum number of device nodes DRBD can handle is 255 — 32 by default, and up to 255 if you set the minor_count module parameter for the drbd.ko kernel module. I am not sure if there is a lower limit on VBDs per domU in Xen, though.

Re question #2: both internal and external meta data is fine with DRBD. If you can place the meta data on a different spindle than the rest of your data, you might gain a slight performance benefit.