Marko Mäkelä added a comment - 2021-10-13 07:02 Thank you. In the stack traces, I see that we have log_checkpoint() executing log_write_flush_to_disk_low() where we have a similar assertion (which did not fail): ut_a(log_sys.n_pending_flushes == 1); /* No other threads here */   bool do_flush = srv_file_flush_method != SRV_O_DSYNC;   if (do_flush) { fil_flush(SRV_LOG_SPACE_FIRST_ID); }     log_mutex_enter(); The thread where the assertion failed in log_write_and_flush() is holding the mutex and thus blocking this thread. It seems to me that the == 1 must be removed from both assertions. Invoking fil_flush() concurrently from multiple threads is safe, because it is protected by fil_system.mutex .