ZapRAID: Toward High-Performance RAID for ZNS SSDs via Zone Append

Zoned Namespace (ZNS) provides the Zone Append primitive to boost the write performance of ZNS SSDs via intrazone parallelism. However, making Zone Append effective for a RAID array of multiple ZNS SSDs is non-trivial, since Zone Append offloads address management to ZNS SSDs and requires hosts to dedicatedly manage RAID stripes across multiple drives. We propose ZapRAID, a high-performance software RAID layer for ZNS SSDs by carefully using Zone Append to achieve high write parallelism and lightweight stripe management. ZapRAID's core idea is a group-based data layout with coarse-grained ordering across multiple groups of stripes, such that it can use small-size metadata for stripe management on a per-group basis. Our prototype evaluation shows that ZapRAID achieves a 2.34x write throughput gain compared with using the Zone Write primitive.