YCSB out-of-core tests from Evergreen show that the BTrees for the YCSB collection and index are one level deeper when WiredTiger is configured in disagg mode.

Benchmarks show that traversing an extra cached level of BTree adds insignificant overhead. But it is still desireable to understand why this is happening, since we would expect that the same workload ingesting the same number of records, of the same size, with the same (random) order would produce similarly shaped BTrees.