A couple of observations:1. BTRFS currently has no knowledge of multipath or anything like that. In theory it should work fine as long as the multiple device instances all point to the same storage directly (including having identical block addresses), but we still need to add proper handling for it.
2. Be _VERY_ careful using BTRFS on top of _ANY_ kind of shared storage. Most non-clustered filesystems will have issues if multiply mounted, but in almost all cases I've personally seen, it _WILL_ cause irreparable damage to a BTRFS filesystem (we really need to do something like ext4's MMP in BTRFS).
3. See the warnings about doing block level copies and LVM snapshots of BTRFS volumes, the same applies to using it on DRBD currently as well (with the possible exception of remote DRBD nodes (ie, ones without a local copy of the backing store) (in this case, we need to blacklist backing devices for stacked storage (I think the same issue may be present with BTRFS on a MD based RAID1 set).
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