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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 log-structured file systems (LFS), and by 90% on one
heavily-used file system that includes transaction-processing
workloads.
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