Each node frame is a permanent member of some node pot which is the size of a page. Each node pot is the permanent home of a range of node frames with contiguous CDAs. A node pot has a permanent disk address but may be cached in RAM much like user pages. The CDA of the node pot for a node is a simple arithmetic function of the CDA of the node. Individual nodes, extracted from their pots, are further cached in item space which is a RAM array of node frames where nodes are brought to participate. About the only thing that can happen to a node without coming to item space is to be severed or filled with 16 DK(0)’s.
There are swap pots of nodes too, which hold sets of nodes whose CDAs are discontiguous, when there is not room in RAM or item space for nodes and it is too soon to write them back to their home pots. Swap pots are ephemeral and are swapped between the swap areas and RAM. They are created when available swap pots are full and item space is congested. They disappear after the checkpoint migration process if finished. Checkpoint logic sometimes requires two or more partial system states to be maintained.