As many probably already know, PBXT is the first MySQL Storage
Engine to use a log-based architecture. Log-based means that data
that would normally first be written to the transaction log, and
then to the database tables, is just written to the log, and the
log becomes part of the database.
This result is that data is only written once, and is always written sequentially. The advantage when writing is obvious, but there is a down side (as always). The data is written to the disk in write order, which is seldom the order in which the data is retrieved. So this results in a lot of random reads to the disk when accessing the data later.