I think you may be misunderstanding the issue. A flash controller that tries to look like a block device needs to manage a mapping between logical and physical blocks along with various other pieces of metadata. The on-flash representation needs to be structured in a flash-friendly way, which often means that it’s not efficient to find the metadata needed to process an IO request unless that metadata is cached in memory.

On an HMB device, the metadata will be cached in host memory, which will likely adversely affect performance and power efficiency.