Simplifying Decentralized File Systems Using a Coherent Shared Logical Disk

	Robert A Shillner	Edward W Felten
	ras@cs.princeton.edu	felten@cs.princeton.edu
	Princeton University Computer Science Department

Decentralized ("serverless") distributed file systems scale to large
numbers of clients by eliminating central performance bottlenecks.
However, previous implementations of decentralized file systems have
been complex, requiring multiple in-memory and on-disk data structures
to ensure safety and reliability in the presence of failures.  To
simplify the construction of decentralized file systems, we introduce
the coherent shared logical disk.

The coherent shared logical disk provides the abstraction of a
cache-coherent, fault-tolerant, entry-consistent, shared array of
persistent data blocks.  To implement a decentralized file system on a
multicomputer or network of workstations, each node runs a local file
system which stores data in a coherent shared logical disk.  Using
locking and cache coherence services provided by the shared logical
disk, the independent file systems cooperate to provide a
decentralized file system.

The key feature of this system is the separation of concerns between
the two layers.  The coherent shared logical disk layer handles the
details of communication, caching, locking, coherence and fault
tolerance, leaving the file system to handle higher-level entities
such as files, directories and inodes.  The result is that the local
file system itself is simple, closely resembling an ordinary
single-node file system.  This property contrasts with earlier
monolithic implementations, in which the file system handles all
aspects of distribution, and which require complex data structures to
keep track of system state and guard against node failures.  It also
contrasts with other shared logical disk systems, which do not provide
coherence or locking and thus require these features to be integrated
into the higher-level file system.

We will describe the design of the coherent shared logical disk and
distributed file system, as well as a prototype implementation for the
Princeton SHRIMP multicomputer.