AWS FSx for ONTAP API: Key Failures and Fixes in FlexGroup and S3 Operations

A developer building a 182-operation file portal on AWS FSx for ONTAP documented recurring API errors not covered in official documentation, publishing findings in a technical series. FlexGroup volume creation consistently fails when ONTAP is allowed to auto-select aggregates, because FSx aggregates have tiering enabled and are excluded from automatic placement. The capacity rebalance feature has undocumented runtime bounds and does not start under ONTAP's default settings, while initiating it triggers an irreversible configuration change that cannot be undone by stopping the process. Storage capacity metrics such as space.used behave differently depending on volume type, and clones blocking parent-volume deletion remain invisible through the API. The findings, gathered between mid-August and September 2026 on ONTAP version 9.18.1P3D1 in the ap-northeast-1 region, also highlight five measurement pitfalls — three linked to reads being served from cache — and two documented discrepancies between AWS documentation and actual system behavior.