While HDFS is indeed used for exporting old data and storing some partition mapping metadata, it's clear from the blog post that MetricsDB is much more reliant on BlobStore as well as MetricsDB-specific services.

> The servers checkpoint in-memory data every two hours to durable storage, Blobstore. We are using Blobstore as durable storage so that our process can be run on our shared compute platform with lower management overhead.

Thanks for this clear summary of the differences.