Parallel replication so far did not work well with TokuDB, as some people who tried it found out. I have now pushed to 10.1 some patches to solve the problems. There are two main fixes: 1. Fix some races where a waiting transaction would miss its wakeup and get a lock timeout on a waiting row lock, even though the lock was released by the holding transaction. This fix is due to great work by Rich Prohaska. This problem is actually not specific to replication, normal transactions on a master will experience it too. But it hurts replication a lot, since replication must commit transactions in order, so one stalled transaction stalls all following transactions as well. 2. Implement the conflict detection and handling necessary for optimistic parallel replication to work. This basically implements thd_rpl_deadlock_check() and hton->kill_query methods. This should solve the problems where optimistic parallel replication with TokuDB breaks with lock wait timeouts. If someone wants to test it, I have made a tree available with just these fixes on top of MariaDB 10.1.19: https://github.com/knielsen/server/tree/tokudb_optimistic_parallel_replicati... The fix should appear in 10.1.20 eventually. The first part of the patch has also been submitted by Rich to upstream. When this is (hopefully) merged upstream, and upstream merged into MariaDB, the MariaDB version of the fix should be replaced with the Percona one. I tried making the MariaDB version of the fix identical to Rich's pull request and keeping it in a separate commit, so this should hopefully be simple to do when the time comes. - Kristian.