File Replication : High availability is a desirable feature of a good distributed file system and file replication is the primary mechanism for improving file availability. Replication is a key strategy for improving reliability, fault tolerance and availability. Therefore duplicating files on multiple machines improves availability and performance. Replicated file : A replicated file is a file that has multiple copies, with each copy located on a separate file server. Each copy of the set of copies that comprises a replicated file is referred to as replica of the replicated file. Replication is often confused with caching, probably because they both deal with multiple copies of data. The two concepts has the following basic differences: A replica is associated with server, whereas a cached copy is associated with a client. The existence of cached copy is primarily dependent on the locality in file access patterns, whereas the existence of a replica normally depends on availability and performance requirements. Satynarayanana [1992] distinguishes a replicated copy from a cached copy by calling the first-class replicas and second-class replicas respectively

1. FILE REPLICATION
TO BE PRESENTED
BY
ZAHARADDEEN KARAMI LAWAL
Department of Computer Science and Engineering,
Faculty of Engineering and
Technology,
Jodhpur National University, Jodhpur
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2. INTRODUCTION
 File Replication : High availability is a desirable feature of a
good distributed file system and file replication is the primary
mechanism for improving file availability. Replication is a key
strategy for improving reliability, fault tolerance and
availability. Therefore duplicating files on multiple machines
improves availability and performance.
 Replicated file : A replicated file is a file that has multiple
copies, with each copy located on a separate file server. Each
copy of the set of copies that comprises a replicated file is
referred to as replica of the replicated file.
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3. Difference between Replication and Caching
 Replication is often confused with caching, probably because
they both deal with multiple copies of data. The two concepts
has the following basic differences:
1. A replica is associated with server, whereas a cached copy is
associated with a client.
2. The existence of cached copy is primarily dependent on the
locality in file access patterns, whereas the existence of a
replica normally depends on availability and performance
requirements.
3. Satynarayanana [1992] distinguishes a replicated copy from a
cached copy by calling the first-class replicas and second-class
replicas respectively
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4. Advantages of file replication
1. Increase availability: one of the most important advantages
of replication is that it masks and tolerates failures in the
network gracefully. In particular, the system remains
operational and available to the users despite failures.
2. Improve response time
3. Increase reliability
4. Improve availability
5. Reduced network traffic
6. Better scalability
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5. Replication Transparency
Naming Of Replicas
 It’s the responsibility of the system to name the various copies of
a resource and to map a user-supplied name of the resource to an
appropriate replica of the resource.
Replication Control
 Replication control determines how many copies of the resource
should be created, where each copy should be placed, and when
should a copy be created /deleted. All this things should be made
entirely automatically by the system in a user transparent manner.
Draw back
The main problem of file replication is consistency. That is
when one copy of replica changes, how does the other copies
reflect that change.
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6. Multi Copy Update Problem
 Maintaining consistency among copies when a replicated
file is updated is the major issue of file system that
supports replication of files. Some commonly used
approaches to handle this issue are described below:
1. Read -Only-Replication
2. Read -Any-Write- All Protocol
3. Available –Copies Protocol
4. Primary-Copy Protocol
5. Quorum-Based Protocol
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7. Multi Copy Update Protocols
1. Read- Only- Replication: This approach allows the replication of
only immutable files, since immutable files are used only in the read-
only mode, because mutable files cannot be replicated.
 This approach is too restrictive in the sense that it allows the
replication of only immutable files.
2. Read-Any-Write-All Protocol: This approach allows the replication
of mutable files. In this method, a read operation on a replicated file
is performed by reading any copy of the file and write operation by
writing to all copies of the file. Some of the lock has to be used to
carryout a write operation. That is, before updating any copy, all
copies are locked, then the they are updated, and finally locks are
released to complete write operation. The protocol is used for
implementing UNIX like Semantics
 The main problem with this approach is that a write operation can’t
be performed if any of the servers having a copy of the replicated file
is down at a time of write operation. 7

8. Contd…
3.Available-Copies Protocol: this Approach allows the write
operation to be carried out even when some of the servers
having a copy of the replicated file are down. In this method the
read operation is performed by reading any available copy, but
a write operation is performed by writing to all available
copies. When the server recovers after a failure, it brings itself
up to date by copying from the other servers before accepting
any user request.
4.Primary-Copy Protocol: another simple method to solve the
multi-copy update problem is the primary –copy protocol. In
this protocol for each replicated file one copy is as the primary
copy and all others are secondary copies. Read operation can
be performed using any copy primary or secondary. Each server
having a copy updates its copy either by receiving notification
of changes from the server having the primary copy or by
requesting the updated copy from it.
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9. Contd…
Draw backs
The read-any-write-all and available –copies protocols cannot handle the
network partition problem in which the copies of a replicated file are
partitioned into two or more active groups. Moreover, the primary-copy –
protocol is too restrictive in the sense that a write operation cannot be
perform if the server having the primary copy is down.
5. Quorum –Based Protocol: This protocol is capable
of handling the network partition problem and can
increase the availability of write operations at the
expense of read operation.
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10. conclusion
 A replicated file is a file that has multiple copies,
with each copy located on a separate file server.
 Each copy of the set of copies that comprises a
replicated file is referred to as a replica.
 Maintaining consistency among copies when a
replicated file is updated is the major issue in file
system that support replication of files.
 Some of the commonly approaches to handle this
issue are read-only replication, read –any write to all
protocol, available copies protocol, primary copy
protocol and quorum-based protocol.
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