If you manage the index using a B-tree, then you can perform partial updates of the B-tree by appending the minimum number of pages needed to represent the changes. At that point, you can append N additional new files to the tail of the archive, and add a new index that re-uses some of the original index.

Just an idea to check the "append" box.

See also B-trees, Shadowing, and Clones https://www.usenix.org/legacy/events/lsf07/tech/rodeh.pdf