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1. By: Little Goku

2. Distributed Files Systems (DFS)
• Allows multi-computer systems to share files
– Even when no other IPC or RPC is needed
• Sharing devices
– Special case of sharing files
 • E.g.,
– NFS (Sun’s Network File System)
– Windows NT, 2000, XP
– Andrew File System (AFS) & others …

3. Naming of Distributed Files
• Naming – mapping between logical and physical objects
• A transparent DFS hides the location where in the network
the
file is stored.
• Location transparency – file name does not reveal the
file’s physical storage location.
• Location independence – file name does not need to be
changed when the file’s physical storage location changes.
– Better file abstraction.
– Separates the naming hierarchy from the storage-devices hierarchy

4. DFS – Three Naming Schemes
1. Mount remote directories to local directories, giving the
appearance of a coherent local directory tree
• Mounted remote directories can be accessed transparently.
• Unix/Linux with NFS; Windows with mapped drives
2. Files named by combination of host name and
 local name;
• Guarantees a unique system wide name
• Windows Network Places, Apollo Domain
3. Total integration of component file systems.
• A single global name structure spans all the files in the system.
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5. Mounting Remote Directories (NFS)
.

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

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

8. 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

9. Possible cache locations in file-caching
scheme for DFS
//Do not learn it...Just read it.....

10. 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.

11. 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.

12. Advantages of replication
Improver performance
Increase reliability
High Availability.

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

14. 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.

15. 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.

16. 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.

17. Fault Tolerance
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.

18.  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 open a file.
 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.

19.  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.

20. NFS
• Sun Network File System (NFS) has become de facto
standard for distributed UNIX file access.
• NFS runs over LAN
– even WAN (slowly).
• Any system may be both a client and server
• Basic idea:
– Remote directory is mounted onto local directory
– Remote directory may contain mounted directories within

21. Andrew File System (AFS)
 Completely different kind of file system
 Developed at CMU to support all student computing.
 Consists of workstation clients and dedicated file server machines.
• Single name space
– File has the same names everywhere in the world.
• Lots of local file caching
– On workstation disks
– For long periods of time
– Originally whole files, now 64K file chunks.
• Good for distant operation because of local disk caching
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