mod-os@sdcsvax.uucp (01/12/87)
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The following is the abstract for a technical report (CS-092) that has just
been released. If you are interested in obtaining a copy, please address
inquiries to:
Technical Report Librarian
Department of Computer Science and Engineering, C-014
University of California, San Diego
La Jolla, CA 92093
DL
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Block-Level Consistency of Replicated Files
John L. Carroll
Computer Science Group
Department of Mathematical Sciences
San Diego State University
San Diego, CA 92182
Darrell D. E. Long
Jehan-Francois Paris
Computer Systems Research Group
Department of Computer Science and Engineering
University of California, San Diego
La Jolla, Ca 92093
ABSTRACT
To increase the availability and reliability of
files the data are often replicated at several
sites. The usual method is to treat files as logi-
cal entities and to replicate on a per file basis.
This can lead to unnecessary complications for the
implementor in trying to preserve file system
semantics.
We investigate the construction of a reliable
device. Such a device would appear to the file
system as an ordinary block-structured device, but
would be implemented as a set of server processes
on several sites. This allows us to provide repli-
cation while leaving the operating system kernel
and the file system unchanged.
The regular structure of the block-level
replication environment allows the use of con-
sistency control algorithms that are simpler and
less network intensive. We present three algo-
rithms for maintaining file consistency in a
block-level replication environment. The first is
a majority consensus voting algorithm that recov-
ers blocks only when they are required for data
access; the second is a variant of the available
copy scheme modified for replication at the block
level; the third is a naive version of the avail-
able copy scheme that does not maintain any
failure information.
Each scheme is evaluated in terms of availa-
bility and network traffic. While block-level
replication is shown to allow improvements in the
network traffic burden incurred by voting, the
available copy schemes are shown to have better
availability and require significantly less
traffic than the voting schemes. The naive avail-
able copy variant proposed here is shown to be the
algorithm of choice.
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