Non-volatile memory for fast, reliable file systems

Given the decreasing cost of non-volatile RAM (NVRAM), by the late 1990’s it will be feasible for most workstations to include a megabyte or more of NVRAM, enabling the design of higher-performance, more reliable systems. We present the trace-driven simulation and analysis of two uses of NVRAM to improve I/O performance in distributed file systems: non-volatile file caches on client workstations to reduce write traffic to file servers, and write buffers for write-optimized file systems to reduce server disk accesses. Our results show that a megabyte of NVRAM on diskless clients reduces the amount of file data written to the server by 40 to 50%. Increasing the amount of NVRAM shows rapidly diminishing returns, and the particular NVRAM block replacement policy makes little difference to write traffic. Closely integrating the NVRAM with the volatile cache provides the best total traffic reduction. At today’s prices, volatile memory provides a better performance improvement per dollar than NVRAM for client caching, but as volatile cache sizes increase and NVRAM becomes cheaper, NVRAM will become cost effective. On the server side, providing a one-half megabyte write-buffer per file system reduces disk accesses by about 20% on most of the measured logstructured file systems (LFS), and by 90% on one heavilyused file system that includes transaction-processing workloads.