Now prove it end to end - write a batch of keys, close the file, open it again from a fresh handle, and confirm every key is still there in order. This is the payoff of moving the tree to disk.
Show that keys written and Closed are all present and ordered after reopening the file from scratch.
This lesson makes the on-disk story concrete: a full lifecycle of open, mutate, close, and reopen, with the assertion that nothing was lost. Because every node was already a page on disk and the meta names the root, reopening is just reading page 0 and resuming - the tree that comes back is byte-for-byte the tree that was closed.
Doing it twice - reopen, mutate more, close, reopen again - proves persistence is not a one-shot trick but a stable property across sessions. This is real durability under a clean shutdown, and for many uses it is enough. But it hides a sharp edge: every write so far overwrites pages in place, so a crash in the middle of a structural change can leave the file with a half-updated tree - a parent pointing at a child that was never finished. Closing the gap between “survives a restart” and “survives a crash” is the whole point of the final chapter.
// t, _ := Open(path); for i := 1..20 { t.Put(i, i*10) }; t.Close()// t2, _ := Open(path); assert t2.Get(i) for all i; assert Scan sorted// t2.Put(21, 210); t2.Delete(1); t2.Close()// t3, _ := Open(path); assert 21 present, 1 gone, still ordered