golding@terra.ucsc.edu (Richard Golding) (01/25/91)
The following technical report is available for anonymous ftp from
midgard.ucsc.edu (128.114.134.15), in ~ftp/pub/tr/ucsc-crl-91-01.ps.Z.
Accessing Replicated Data in a Large-Scale Distributed System
Richard Golding
Darrell D. E. Long
Concurrent Systems Laboratory
Baskin Center for CE & IS
University of California, Santa Cruz
Technical Report UCSC-CRL-91-01
Replicating a data object improves the availability of the data, and
can improve access latency by locating copies of the object near to
their use. When accessing replicated objects across an internetwork,
the time to access different replicas is non-uniform. Further, the
probability that a particular replica is inaccessible is much higher
in an internetwork than in a local-area network (LAN) because of
partitions and the many intermediate hosts and networks that can fail.
We report three replica-accessing algorithms which can be tuned to
minimize either access latency or the number of messages sent. These
algorithms assume only an unreliable datagram mechanism for
communicating with replicas. Our work extends previous investigations
into the performance of replication algorithms by assuming unreliable
communication.
We have investigated the performance of these algorithms by measuring
the communication behavior of the Internet, and by building
discrete-event simulations based on our measurements. We find that
almost all message failures are either transient or due to long-term
host failure, so that retrying messages a few times adds only a small
amount to the overall message traffic while improving both access
latency as long as the probability of message failure is small.
Moreover, the algorithms which retry messages on failure provide
significantly improved availability over those which do not.
Richard A. Golding golding@cello.hpl.hp.com or
UC Santa Cruz CIS Board (grad student) golding@cis.ucsc.edu (best) or
golding@slice.ooc.uva.nl
Post: Baskin Centre for CE & IS, Appl. Sci. Bldg., UC, Santa Cruz CA 95064