MixStore: Back-End Storage Based on Persistent Memory and SSD

Persistent memory (PMEM) has both the low-latency byte addressing of memory and the persistence characteristics of disk, which will bring revolutionary changes and far-reaching impact on the existing software architecture system. Distributed storage has been widely used in cloud computing and data centers, but the existing back-end storage engines represented by Ceph BlueStore are designed for traditional mechanical disks and solid state disks(SSD), and its original optimization design mechanism is not suitable for exerting the advantages of PMEM. In this paper, a back-end storage MixStore is proposed based on PMEM and SSD. Through the volatile range marking and to-be-deleted list technology, a Concurrent SkipList suitable for persistent memory is implemented, which is used to replace RocksDB to implement the metadata management mechanism. While ensuring transaction consistency, it eliminates the performance jitter and other issues caused by BlueStore’s compaction, and improves the concurrent access performance of metadata. The storage optimization design of data objects based on the metadata management mechanism is adopted, non-aligned small blocks of data objects are stored in PMEM, and the aligned large blocks of data objects are stored in SSD. This makes full use of the byte addressing and durability characteristics of the PMEM, and the large-capacity and low-cost advantage of the SSD. The data update policy is optimized based on the delayed writing and copy-on-write (CoW) technology, eliminating the write amplification caused by the WAL log of BlueStore and improving the performance of small data writing. The test results show that under the same hardware environment, MixStore’s write throughput is increased by 59% and the write latency is reduced by 37% compared with that of BlueStore, which effectively improves the system performance.