[toolslive]

it's possible to unify the WAL and the tree. There are some append only B-tree implementations. https://github.com/Incubaid/baardskeerder fe.

[avinassh]

There are also CoW B Trees not entirely similar, but kinda same.

[toolslive]

there's a whole class of persistent persistent (the repetition is intentional here) data structures. Some of them even combine performance with elegance.