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Mar 19, 2019 · Databases

Root Cause Analysis of Slave IO Thread Hang in MySQL Semi‑Sync Replication with rpl_semi_sync_master_wait_for_slave_count=1

An in‑depth investigation reveals that when MySQL semi‑sync replication is configured with rpl_semi_sync_master_wait_for_slave_count=1, starting a second slave can cause the master’s dump thread to fail, leading to slave_io_thread stalls; the analysis includes reproduction steps, status checks, thread stack traces, and a patch using sched_yield to resolve the lock contention.