Uploading documents (especially images) to Marqo eventually started failing with a persistent 429 Too Many Requests: "Marqo-OS received too many requests! Please try reducing the frequency of adddocuments and updatedocuments calls". Searches still worked fine. The 429s persisted indefinitely on that index no matter how long the reporter waited, even though the machine had 16 GB RAM and a 6-core CPU. The OpenSearch logs showed the real cause: the low disk watermark was exceeded, which blocks writes while reads keep working.