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I'm closing this one as duplicate of Bug #1007074. This BZ added few more checks when metadata could be swapped.
It relates to this BZ as well - since if the thin-pool would be inactive,
not activation for swapped thin-pool is made.
However if someone tries to activate thin-pool with incorrect metadata,
he needs to deal with consequences - which currently may enforce user
to manually remove devices from dm table.
So if the 'newMDA' doesn't have valid metadata, the user will need to swap back valid metadata for the snapper_thinp/POOL device - or has to manually remove entries from dm table and vgcfgrestore metadata without any appearance of the POOL in them (thin data & metadata and thin volumes removed).
Swapping is only meant to be used for manual recovery of damaged pool (which may eventually be stacked over raids).
*** This bug has been marked as a duplicate of bug 1007074 ***