Modifying Cluster Parameters in GBase 8a: Instant vs. Permanent

Changing system parameters like gcluster_rebalancing_concurrent_count in GBase 8a can be done in two ways — one gives you instant effect with a catch, the other is permanent but requires a restart. Here's how both work in a gbase database.

Two Approaches at a Glance

Method	How	When It Takes Effect	Persistence	Best For
Config file	Edit .cnf / .conf files	After restarting the module	Permanent	All parameters, especially read-only ones
SET GLOBAL	SET GLOBAL param = value;	New sessions immediately	Lost on restart	Dynamic, tuneable parameters
SET SESSION	SET [SESSION] param = value;	Current session only	Session ends	Debugging or one-off queries

Example: Adjusting Rebalance Concurrency

The parameter gcluster_rebalancing_concurrent_count controls how many tables can be rebalanced in parallel. It's a performance tuning knob that supports dynamic change.

Method 1: SET GLOBAL (Immediate, for Quick Tuning)

SET GLOBAL gcluster_rebalancing_concurrent_count = 5;
-- Verify in a new session
SHOW GLOBAL VARIABLES LIKE 'gcluster_rebalancing_concurrent_count';

New rebalance tasks will use the updated count instantly. Already running tasks are unaffected.

Method 2: Config File (Permanent)

Edit the gcluster config file at $GCLUSTER_BASE/config/gbase_8a_gcluster.cnf, find or add:

gcluster_rebalancing_concurrent_count = 5

Then restart the module:

gcluster_services gcluster restart

After restart, this value becomes the new default. Any SET GLOBAL changes made before the restart will be replaced by the config file value.

Key Points

• Read‑only parameters (e.g., security settings) can only be changed in config files and require a restart.
• Parameter prefixes tell you which file to edit: gcluster_ → gbase_8a_gcluster.cnf, gbase_ → gbase_8a_gbase.cnf.
• Always verify with SHOW GLOBAL VARIABLES and SHOW VARIABLES to confirm the effective value at global and session levels.

Recommended Workflow

Scenario	Method	Why
Online tuning, instant effect	SET GLOBAL	No restart, quick validation
Make permanent, standardize config	Config file + restart	Survives restarts
Temporary debug	SET SESSION	Minimal scope, safe
Read‑only parameter change	Config file + restart	Only option

For performance parameters, the best practice is: test with SET GLOBAL first, then persist the winning value in the config file, and restart during a maintenance window to make it stick. This gives you both agility and long‑term stability in your gbase database.