This document discusses key aspects of distributed file systems including file caching schemes, file replication, and fault tolerance. It describes different cache locations, modification propagation techniques, and methods for replica creation. File caching schemes aim to reduce network traffic by retaining recently accessed files in memory. File replication provides increased reliability and availability through independent backups. Distributed file systems must also address being stateful or stateless to maintain information about file access and operations.

1. Distributed File Systems
 File Caching Schemes
 File Replication
 Fault Tolerance
Presented by:
Maurvi Dave (T1013CSE2023)
Pooja Upadhyay(T1013CSE2028)

2. File caching schemes
Contents
Introduction
Key decisions of file caching schemes
 Cache location
 Modification propagation

3. Introduction
 To Retain recently accessed files in the main
memory.
 Repeated accesses to the same information can be
handled locally.
 Reduce network traffic

4. Key decisions
 In implementation of file caching scheme, one has to
make several key decisions
 Like granularity of cached data
 Cache size
 Replacement policy

5. Cache location
 It refers to the place at where the cached data is
stored.
 Let’s assume original location of a file is on its
server’s disk.
 Three possible cache location:
1) Server’s main memory
2) Client’s disk
3) Client’s main memory

6. Possible cache locations in file-
caching scheme for DFS

7. Modification Propagation
 Write-through:
when new user modifies the cache entry it is
immediately written to the server.
 Delayed-write:
To decrease continuous network traffic write all updates
to the server periodically or batch them together.
 Single write operation is efficient then multiple

8. File replication
 Provide for increase reliability by having independent
backups of each file.
 Enable file access to continue even if one file server
is down.

9. Advantages of replication
 Improver performance
By distributing workload among multiple servers
 Increase reliability
Copy can be taken from another server if one
crashes
 Availability
By multiple copy of each file on separated servers

10. Key issue in replication
 Replication Transparency
Not transparent to the user.
Entire process is carry behind programmer’s back.

11. Replica creation methods
 Three methods
a) Explicit file replication
b) Lazy file replication
c) File replication using group

12. Explicit file replication
 Entire process controlled by
programmer
 Process makes file on one
server & Make multiple
copies on servers
 Directory server maintain list
of all replicas
 When file is requested, any
one of these copies can be
opened.

13. Lazy file replication
 Only one copy created
on server & server
makes replication for
other servers.
 System can track all
replicas & retrieve one
copy as required.

14. File replication using a group
 System call is sent to all
servers
 Replicas are created
when original is made
 All copies are made at
same time.

15.  Stateful
– Server maintains information about a file opened by a client (e.g.,
file pointer, mode)
– Mechanism: on open, the server provides a “handle” to the client
to use on subsequent operations.
 Stateless
– Server maintains no information about client access to files
– Mechanism: each client operation must provide context
information for that operation
Fault Tolerance

16.  Stateful service
 Information about file operations are kept in the server during
all the file session.
 A communication channel is established between the
client and the server when a the client explicitly solicits the file
opening.
 A number(identifier) is used to define the communication
channel then this identifier will be used to perform file
operations.
 To attend its clients, the server copies data from the storage
devices to memory and let them there till the file closing.

17.  Stateless service
 The stateless service does not establish a communication
channel. there is no necessity for explicit file opening and
closing
 Before executing a file operation the server will automatically
open and close the file.
 Each request sent to the server must define the desired file
likewise, if a read or write
 Operation is requested, it must contain the position in the file
referring to the respective operation.