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From: darrell@midgard.ucsc.edu (Darrell Long)
Newsgroups: comp.os.research
Subject: TR Available (replication, reliability, regeneration)
Message-ID: <5395@saturn.ucsc.edu>
Date: 7 Nov 88 21:37:04 GMT
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Approved: comp-os-research@jupiter.ucsc.edu

For those of you interested in replication...

The following technical report (UCSC-CRL-88-18) can be ordered from UCSC.

Send your request to:

Jennifer Madden
Baskin Center for Computer Engineering and Information Sciences
University of California
Santa Cruz, CA 95064

   The  Reliability  of  Regeneration-Based   Replica   Control Protocols

             Darrell D.E. Long           John L. Carroll   Kris Stewart
     Computer and Information Sciences     Computer Science Division
         University of California          San Diego State University
           Santa Cruz, CA 95064               San Diego, CA 92182


				 ABSTRACT

	The accessibility of vital information can  be  enhanced  by
	replicating  the  data  on  several  sites,  and employing a
	consistency control protocol to manage the copies.  The most
	common  measures of accessibility include reliability, which
	is the probability that a replicated data object will remain
	continuously  accessible  over  a  given  time  period,  and
	availability, which is the steady-state probability that the
	data object is accessible at any given moment.

	     For many applications, the reliability of a system is a
	more   important   measure   of  its  performance  than  its
	availability.  These applications are characterized  by  the
	property  that  interruptions of service are intolerable and
	usually involve interaction with real-time  processes,  such
	as  process control or data gathering where the data will be
	lost if it is not captured when it is available.

	     The reliability of a replicated data object depends  on
	maintaining  a viable set of current replicas.  When storage
	is limited it may not be feasible to simply replicate a data
	object  at  enough  sites  to  achieve  the desired level of
	reliability.  If new  replicas  of  a  data  object  can  be
	created faster than a system failure can be repaired, better
	reliability can be achieved  by  creating  new  replicas  on
	other   sites   in   response   to  changes  in  the  system
	configuration.   This  technique,  known  as   regeneration,
	approximates the reliability provided by additional replicas
	for a modest increase in storage costs.

	     Several  strategies   for   replica   maintenance   are
	considered,  and  the  benefits of each are analyzed.  While
	the availability afforded by each of the protocols is  quite
	similar, the reliabilites vary greatly.  Formulas describing
	the reliability of the replicated data object are presented,
	and closed-form solutions are given for the tractible cases.
	Numerical solutions, validated by  simulation  results,  are
	used  to  analyze  the  trade-offs  between  reliability and
	storage costs.  With estimates of the  mean  times  to  site
	failure   and  repair  in  a  given  system,  the  numerical
	techniques presented here can  be  applied  to  predict  the
	fewest  number  of  replicas required to provide the desired
	level of reliability.