## Module 1: Modifying Global Persistent System Variables [`tidb_gc_enable`](https://docs.pingcap.com/tidb/stable/system-variables#tidb_gc_enable-new-in-v50) is one of the global persistent TiDB system variables, modify it and observe the result. ### Step 1. Log into the remote Linux + Open three SSH connections to your lab virtual machine, by clicking: + [Terminal 1](ssh://monitor1-ssh#1) + [Terminal 2](ssh://monitor1-ssh#2) + [Terminal 3](ssh://monitor1-ssh#3) + You will now have three active terminal sessions. ### Step 2. Connect to TiDB Server 1 - In [Terminal 1](ssh://monitor1-ssh#1), execute: ```bash ./connect-db1.sh ``` ```output ... tidb:db1> ``` ### Step 3. Create one more connection to TiDB Server 1 - In [Terminal 2](ssh://monitor1-ssh#2), execute: ```bash ./connect-db1.sh ``` ```output ... tidb:db1> ``` ### Step 4. Connect to TiDB Server 2 - In [Terminal 3](ssh://monitor1-ssh#3), execute: ```bash ./connect-db2.sh ``` ```output ... tidb:db2> ``` ### Step 5. In terminal 1, change `tidb_gc_enable` to `OFF` + In [Terminal 1](ssh://monitor1-ssh#1), verify the default value of `tidb_gc_enable`. ```sql SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | ON | +----------------+-------+ 1 row in set (0.00 sec) ``` ```sql SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | ON | +----------------+-------+ 1 row in set (0.00 sec) ``` + Change the value from `ON` to `OFF` globally. ```sql SET GLOBAL tidb_gc_enable=OFF; ``` ```output tidb:db1> SET GLOBAL tidb_gc_enable=OFF; Query OK, 0 rows affected (0.06 sec) ``` ### Step 6. Verify the change result from current session ```sql SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.00 sec) ``` ```sql SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.00 sec) ``` ### Step 7. Verify the change result from another session - Switch to [Terminal 2](ssh://monitor1-ssh#2), execute: ```sql SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.01 sec) ``` ```sql SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.00 sec) ``` ### Step 8. Verify the change result from another TiDB server - Switch to [Terminal 3](ssh://monitor1-ssh#3), execute: ```sql SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db2> SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.01 sec) ``` ```sql SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db2> SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.00 sec) ``` ### Step 9. Try to change the value of `tidb_gc_enable` on session scope + In any terminal, change the value of `tidb_gc_enable` on session scope. ```sql SET SESSION tidb_gc_enable=ON; ``` ```output tidb:db2> SET SESSION tidb_gc_enable=ON; ERROR 1229 (HY000): Variable 'tidb_gc_enable' is a GLOBAL variable and should be set with SET GLOBAL ``` ### Step 10. In all terminals, exit from MySQL client ```bash EXIT ``` ### Step 11. Restart the TiDB cluster - Switch to [Terminal 1](ssh://monitor1-ssh#1), execute the following command. ```bash tiup cluster restart tidb-test --yes ``` ```output ... Restarted cluster `tidb-test` successfully ``` ### Step 12. In all terminals, check the value of `tidb_gc_enable` + Verify that in [Terminal 1](ssh://monitor1-ssh#1), [Terminal 2](ssh://monitor1-ssh#2), and [Terminal 3](ssh://monitor1-ssh#3), the value of `tidb_gc_enable` is `OFF`. We use [Terminal 1](ssh://monitor1-ssh#1) as an example. You need to re-connect to TiDB servers because the previous restart disconnected your sessions. ```bash ./connect-db1.sh ``` ```output ... tidb:db1> ``` ```sql SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW SESSION VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.01 sec) ``` ```sql SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; ``` ```output tidb:db1> SHOW GLOBAL VARIABLES LIKE 'tidb_gc_enable'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | tidb_gc_enable | OFF | +----------------+-------+ 1 row in set (0.00 sec) ``` ### Step 13. In any terminal, restore `tidb_gc_enable` back to `ON` ```sql SET GLOBAL tidb_gc_enable=ON; ``` ```output tidb:db1> SET GLOBAL tidb_gc_enable=ON; Query OK, 0 rows affected (0.07 sec) ``` ### Step 14. Close all terminals ### Summary The global persistent TiDB system variable can only be configured on a global level, and its effects will be immediately applied across all TiDB servers and sessions in the cluster.