Single Parameter Tweak in GBase 8a Triggered 10 TB Disk Write Storm in Production

A production GBase 8a cluster suffered severe performance degradation after administrators increased the group_concat_max_len parameter from 32 KB to 1 MB to meet a business requirement. A TOP-N query that normally finished in seconds began running for over three hours, while multiple other queries on the same node stalled, with some exceeding 10,000 seconds of execution time. Investigation revealed all slow queries were bottlenecked on node3, where disk utilisation hit 100% and write speeds reached 900 MB/s. The root cause was traced to the database engine typing an intermediate GROUP_CONCAT column as LONGTEXT due to the enlarged parameter, prompting the sort operation to pre-allocate up to 64 MB per row. With 200,000 rows to sort, this ballooned into roughly 12 TB of anticipated data, which spilled entirely to disk as temporary files when memory proved insufficient.