How to fix ERROR 1202: Could not create slave thread; check system resources in MySQL

Connect to the slave server and verify its current state:

SHOW SLAVE STATUS\G

Look for the values of Slave_IO_Running and Slave_SQL_Running. If either is "No", replication has stopped. Check the Last_Error field for the exact error message.

Verify that the server has sufficient available memory:

free -h
# or on macOS
vm_stat

Also check MySQL memory allocation:

SHOW GLOBAL STATUS LIKE 'max_used_connections';
SHOW GLOBAL VARIABLES LIKE 'max_connections';

If memory usage is above 85-90% of available RAM, you need to free up resources or add more memory.

On Linux, verify ulimit settings for the mysqld process:

# Check current limits for the MySQL process
cat /proc/$(pidof mysqld)/limits

# Or check as the mysql user
ulimit -a

Key limits to check:
- max user processes: Should be higher than max_connections (default 151)
- open files: Should be at least 65535

If limits are too low, increase them by editing /etc/security/limits.conf:

mysql soft nofile 65535
mysql hard nofile 65535
mysql soft nproc 65535
mysql hard nproc 65535

Then restart MySQL.

Once resources are available, restart the replication process:

-- Stop the slave
STOP SLAVE;

-- Clear the relay logs
RESET SLAVE;

-- If needed, reconfigure replication
CHANGE MASTER TO
  MASTER_HOST='master_host',
  MASTER_USER='replication_user',
  MASTER_PASSWORD='password',
  MASTER_LOG_FILE='mysql-bin.000001',
  MASTER_LOG_POS=154;

-- Start the slave
START SLAVE;

-- Verify status
SHOW SLAVE STATUS\G

Wait a moment and check the status again to ensure both Slave_IO_Running and Slave_SQL_Running are "Yes".

If increasing system limits doesn't resolve the issue, reduce MySQL's thread stack memory usage:

SHOW GLOBAL VARIABLES LIKE 'thread_stack';

The default is usually 262144 bytes (256KB). If memory is still constrained, you can reduce this in your MySQL configuration file:

[mysqld]
thread_stack = 192K

Then restart MySQL. Note: Only reduce this if you're experiencing stack overflow errors; too low a value can cause crashes.

To prevent this error from recurring:

1. Regularly monitor replication status: Set up automated checks for SHOW SLAVE STATUS

2. Monitor system resources: Use tools like htop, Prometheus, or your cloud provider's monitoring

3. Set replication alerts: Configure MySQL to alert when Seconds_Behind_Master exceeds a threshold

4. Plan capacity: Ensure your slave has at least as much memory as the master

5. Use parallel replication (MySQL 5.7+): Enable multithreaded replication for better performance:

STOP SLAVE;
SET GLOBAL slave_parallel_workers = 4;
SET GLOBAL slave_parallel_type = 'LOGICAL_CLOCK';
START SLAVE;

However, on low-resource systems, reduce parallel workers or set to 0.