  Zoned storage devices, such as zoned namespace (ZNS) solid-state drives
(SSDs) and host-managed shingled magnetic recording (HM-SMR) hard-disk drives
(HDDs), expose interfaces for host-level applications to support fine-grained,
high-performance storage management. Combining ZNS SSDs and HM-SMR HDDs into a
unified hybrid storage system is a natural direction to scale zoned storage at
low cost, yet how to effectively incorporate zoned storage awareness into
hybrid storage is a non-trivial issue. We make a case for key-value (KV) stores
based on log-structured merge trees (LSM-trees) as host-level applications, and
present HHZS, a middleware system that bridges an LSM-tree KV store with hybrid
zoned storage devices based on hints. HHZS leverages hints issued by the
flushing, compaction, and caching operations of the LSM-tree KV store to manage
KV objects in placement, migration, and caching in hybrid ZNS SSD and HM-SMR
HDD zoned storage. Experiments show that our HHZS prototype, when running on
real ZNS SSD and HM-SMR HDD devices, achieves the highest throughput compared
with all baselines under various settings.
