apache/cassandra · critical · ConfigurationException

Group sync specified, but commitlog_sync_period found. Only

Error message

Group sync specified, but commitlog_sync_period found. Only specify commitlog_sync_group_window when using group sync

What it means

Group commitlog sync is configured with commitlog_sync_group_window_in_ms, while periodic sync uses commitlog_sync_period_in_ms. When commitlog_sync is group, applySimpleConfig rejects any non-zero commitlog_sync_period because the two parameters are mutually exclusive and an active period value indicates stale or conflicting config.

Source

Thrown at src/java/org/apache/cassandra/config/DatabaseDescriptor.java:657

        }

        if (conf.commitlog_sync == CommitLogSync.batch)
        {
            if (conf.commitlog_sync_period.toMilliseconds() != 0)
            {
                throw new ConfigurationException("Batch sync specified, but commitlog_sync_period found.", false);
            }
            logger.debug("Syncing log with batch mode");
        }
        else if (conf.commitlog_sync == CommitLogSync.group)
        {
            if (conf.commitlog_sync_group_window.toMilliseconds() == 0)
            {
                throw new ConfigurationException("Missing value for commitlog_sync_group_window.", false);
            }
            else if (conf.commitlog_sync_period.toMilliseconds() != 0)
            {
                throw new ConfigurationException("Group sync specified, but commitlog_sync_period found. Only specify commitlog_sync_group_window when using group sync", false);
            }
            logger.debug("Syncing log with a group window of {}", conf.commitlog_sync_period.toString());
        }
        else
        {
            if (conf.commitlog_sync_period.toMilliseconds() == 0)
            {
                throw new ConfigurationException("Missing value for commitlog_sync_period.", false);
            }
            logger.debug("Syncing log with a period of {}", conf.commitlog_sync_period.toString());
        }

        if (conf.accord.journal.flushPeriod == null)
        {
            conf.accord.journal.flushPeriod = conf.commitlog_sync_period;
            if (conf.accord.journal.flushMode == PERIODIC && conf.commitlog_sync_period.toMilliseconds() == 0)
            {
                logger.warn("Accord journal is configured in periodic mode, while Cassandra commit log is configured in {} mode", conf.commitlog_sync);

View on GitHub (pinned to 88fd0f6a0e)

Solutions

1. Remove or comment out `commitlog_sync_period_in_ms` from cassandra.yaml when commitlog_sync is group.
2. Keep `commitlog_sync_group_window_in_ms` set to the intended window (e.g. 1000ms).
3. If you actually want periodic sync, change commitlog_sync to periodic instead and delete the group window.

Example fix

// before (cassandra.yaml)
commitlog_sync: group
commitlog_sync_group_window_in_ms: 1000
commitlog_sync_period_in_ms: 10000
// after (cassandra.yaml)
commitlog_sync: group
commitlog_sync_group_window_in_ms: 1000

// Java, before toolInitialization()
var raw = Config.getRawConfig();
if (raw != null && raw.commitlog_sync == CommitLogSync.group
    && raw.commitlog_sync_period != null && raw.commitlog_sync_period.toMilliseconds() != 0)
    throw new IllegalArgumentException("Remove commitlog_sync_period_in_ms when commitlog_sync=group");

When it happens

Trigger: cassandra.yaml contains `commitlog_sync: group` plus a non-zero `commitlog_sync_period_in_ms` (typically left over from switching away from periodic sync), hitting DatabaseDescriptor.applySimpleConfig during toolInitialization/applyAll.

Common situations: Flipping commitlog_sync back to group (or to periodic and then back) without removing the other parameter; merged/partially reverted config files; copy-pasting tuning snippets from both sync modes.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

• Missing required directive CommitLogSync
• Batch sync specified, but commitlog_sync_period found.
• Missing value for commitlog_sync_group_window.
• Missing value for commitlog_sync_period.
• Configured ${configName} "${intf}" could not be found

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/6060e1ae4a7e9cd6. Report an issue: GitHub.