5 research outputs found
Fault tolerant software technology for distributed computing system
Issued as Monthly reports [nos. 1-23], Interim technical report, Technical guide books [nos. 1-2], and Final report, Project no. G-36-64
Performance modelling of replication protocols
PhD ThesisThis thesis is concerned with the performance modelling of data replication protocols.
Data replication is used to provide fault tolerance and to improve the performance
of a distributed system. Replication not only needs extra storage but also has an
extra cost associated with it when performing an update. It is not always clear which
algorithm will give best performance in a given scenario, how many copies should be
maintained or where these copies should be located to yield the best performance.
The consistency requirements also change with application. One has to choose these
parameters to maximize reliability and speed and minimize cost. A study showing the
effect of change in different parameters on the performance of these protocols would
be helpful in making these decisions. With the use of data replication techniques in
wide-area systems where hundreds or even thousands of sites may be involved, it has
become important to evaluate the performance of the schemes maintaining copies of
data.
This thesis evaluates the performance of replication protocols that provide differ-
ent levels of data consistency ranging from strong to weak consistency. The protocols
that try to integrate strong and weak consistency are also examined. Queueing theory
techniques are used to evaluate the performance of these protocols. The performance
measures of interest are the response times of read and write jobs. These times
are evaluated both when replicas are reliable and when they are subject to random
breakdowns and repairs.Commonwealth Scholarshi
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On Improving the Availability of Replicated Files
To improve the availability and reliability of files the data are often replicated at several sites. A scheme must then be chosen to maintain the consistency of the file contents in the presence of site failures. The most commonly used scheme is voting. Voting is popular because it is simple and robust: voting schemes do not depend on any sophisticated message passing scheme and are unaffected by network partitions.
When network partitions cannot occur, better availabil- ities and reliabilities can be achieved with the available copy scheme. This scheme is somewhat more complex than vot- ing as the recovery algorithm invoked after a failure of all sites has to know which site failed last. We present in this paper a new method aimed at finding this site. It consists of recording those sites which received the most recent up- date; this information can then be used to determine which site holds the most recent version of the file upon site re- covery. Our approach does not require any monitoring of site failures and so has a much lower overhead than other methods.
We also derive, under standard Markovian assumptions, closed-form expressions for the availability of replicated files managed by voting, available copy and a naive scheme that does not keep track of the last copy to fail
On Improving the Availability of Replicated Files
To improve the availability and reliability of files the data are often replicated at several sites. A scheme must then be chosen to maintain the consistency of the file contents in the presence of site failures. The most commonly used scheme is voting. Voting is popular because it is simple and robust: voting schemes do not depend on any sophisticated message passing scheme and are unaffected by network partitions. When network partitions cannot occur, better availabilities and reliabilities can be achieved with the available copy scheme. This scheme is somewhat more complex than voting as the recovery algorithm invoked after a failure of all sites has to know which site failed last. We present in this paper a new method aimed at finding this site. It consists of recording those sites which received the most recent update; this information can then be used to determine which site holds the most recent version of the file upon site recovery. Our approach does not require any monitoring of site failures and so has a much lower overhead than other methods. We also derive, under standard Markovian assumptions, closed-form expressions for the availability of replicated files managed by voting, available copy and a naive scheme that does not keep track of the last copy to fail. 1