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The Direct Access File System (DAFS) is an emerging industrial
standard for network-attached storage. DAFS takes advantage of new
user-level network interface standards. This enables a user-level
file system structure in which client-side functionality for remote
data access resides in a library rather than in the kernel. This
structure addresses longstanding performance problems stemming from
weak integration of buffering layers in the network transport,
kernel-based file systems and applications. The benefits of this
architecture include lightweight, portable and asynchronous access
to network storage and improved application control over data
movement, caching and prefetching.
This paper explores the fundamental performance characteristics of
a user-level file system structure based on DAFS. It presents
experimental results from an open-source DAFS prototype and compares
its performance to a kernel-based NFS implementation optimized for
zero-copy data transfer. The results show that both systems can
deliver file access throughput in excess of 100 MB/s, saturating
network links with similar raw bandwidth. Lower client overhead in
the DAFS configuration can improve application performance by up
to 40% over optimized NFS when application processing and I/O demands
are well-balanced.
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