Strictly speaking, SQS does not necessarily store data on disk and being highly available and fault tolerant does not exclude one from operating a completely volatile data store. Indeed, SQLite itself has an in-memory option which, when appropriately architected, could be used as the primary data store for a highly available and fault tolerant queue.

Having said that, I think it is safe to assume that SQS probably stores information to non-volatile storage.

Nice work on the project!