This proposal adds two kinds of pool, each of which can combine MDTs and OSTs:

• Plain pool: limits where new directories and files go. A directory tree bound to the pool keeps new subdirectories on the pool's MDTs and new file objects on the pool's OSTs, at every level below it. Members can be shared with other pools, renames across pools are allowed, and recovery is unchanged. Today's OST pools are plain pools.

• Isolated pool: a failure domain. Its MDTs and OSTs belong to it alone, its whole directory tree stays inside it, and nothing outside it depends on it. When its targets are known to be bad, the administrator takes it offline: the other MDTs finish recovery without waiting for it, and clients get -EIO right away inside it. A pool that is lost for good can be dropped while the rest of the filesystem stays consistent.

MDT0 can't be in either kind of pool, because it can never be skipped or removed. Directories and files can be moved between pools with lfs migrate.