eNVy: a non-volatile, main memory storage system

This paper describes the architecture of eNVy, a large non-volatile main memory storage system built primarily with Flash memory. eNVy presents its storage space as a linear, memory mapped array rather than as an emulated disk in order to provide an efficient and easy to use software interface. Flash memories provide persistent storage with solid-state memory access times at a lower cost than other solid-state technologies. However, they have a number of drawbacks. Flash chips are write-once, bulk-erase devices whose contents cannot be updated in-place. They also suffer from slow program times and a limit on the number of program/erase cycles. eNVy uses a copy-on-write scheme, page remapping, a small amount of battery backed SRAM, and high bandwidth parallel data transfers to provide low latency, in-place update semantics. A cleaning algorithm optimized for large Flash arrays is used to reclaim space. The algorithm is designed to evenly wear the array, thereby extending its lifetime. Software simulations of a 2 gigabyte eNVy system show that it can support I/O rates corresponding to approximately 30,000 transactions per second on the TPC-A database benchmark. Despite the added work done to overcome the deficiencies associated with Flash memories, average latencies to the storage system are as low as 180ns for reads and 200ns for writes. The estimated lifetime of this type of storage system is in the 10 year range when exposed to a workload of 10,000 transactions per second.