<p>MySQL cannot write to the slave relay log, halting replication and raising Error 1595 (ER_SLAVE_RELAY_LOG_WRITE_FAILURE).</p>
<p>MySQL Error 1595: ER_SLAVE_RELAY_LOG_WRITE_FAILURE occurs when the replica cannot write incoming events to its relay log, usually due to disk, permission, or I/O issues. Free disk space, check file permissions, and restart replication to resolve the failure.</p>
Relay log write failure: %s
The error appears when a MySQL replica cannot write new events to its relay log file during replication. The server stops the SQL thread and prints Relay log write failure: %s to the error log.
The relay log stores events fetched from the primary. If the replica cannot append to this file, replication stalls, data diverges, and downstream systems may read stale data.
Most cases trace back to insufficient disk space, OS-level permission changes, or hardware I/O problems affecting the relay log directory. Network interruptions rarely trigger it because events are already downloaded when the write occurs.
Corrupted relay logs or a full file system can also block writes. Upgrading MySQL without updating file ownership often produces the same write failure.
Identify and clear the blocking condition, then restart replication. Always back up the current relay logs before purging them. If corruption is suspected, regenerate the relay logs with RESET SLAVE or CHANGE REPLICATION SOURCE TO RELAY_LOG_FILE=''.
Disk full: free space or move relay logs to a larger volume, then START REPLICA.
Permission error: set correct ownership on the --relay-log path and restart mysqld.
Relay log corruption: flush, reset, and resync from primary.
Monitor disk usage on replicas, place relay logs on resilient storage, and run periodic checksums. Treat replica file permissions as code and version them.
Error 1594 (ER_SLAVE_RELAY_LOG_READ_FAILURE) arises when the relay log cannot be read. Similar fixes apply but focus on read permissions and corruption.
The file system that stores relay logs is full, preventing further writes.
OS user running MySQL lacks write access to the relay log directory after a system change.
Underlying disk or network storage encounters hardware errors causing write operations to fail.
Previous crashes left the relay log in an inconsistent state, blocking appends.
The operating system remounted the partition as read-only after detecting errors.
Read failure on the relay log; indicates corruption or permission problems during read operations.
Replica cannot read binlog from the source, usually due to network or source crash.
Applies events referencing rows missing on the replica; often surfaced after relay log issues.
Restarting alone rarely helps unless it also remounts storage read-write or reloads permissions. Resolve root causes first.
You may purge them after ensuring the replica catches up or by using RESET REPLICA ALL, which downloads fresh events.
No. The error only affects the replica. The primary continues writing its binary log unaffected.
Galaxy alerts on replication lag metrics and lets engineers run corrective SQL fast in its IDE, reducing downtime.
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