3. distributed file system requirements

DISTRIBUTED FILE SYSTEMS
(DFS)

DISTRIBUTED FILE SYSTEMS
• Including
1.Introduction
2.File Service Architecture
3.Case Study: Sun NFS

1. Introduction
• File system: support convenient programming
interface to disk storage, access control and
file looking mechanism.

Distributed Files Systems (DFS)
• Allows multi-computer systems to share files &
Sharing devices
• E.g.,
– NFS (Sun’s Network File System)
– Windows NT, 2000
– Andrew File System (AFS)
Distributed File Systems CS-4513, D-Term 2007 4

Distributed File Systems (continued)
• One of most common uses of distributed file
system is distributed computing
• Goal: provide common view of centralized file
system, but distributed implementation.
– Ability to open & update any file on any machine
on network
– All of synchronization issues and capabilities of
shared local files
Distributed File Systems CS-4513, D-Term 2007 5

Including
1. Characteristics of file system
2. Distributed File system requirements

1. Characteristics of file system
• File system: support convenient programming
interface to disk storage, access control and
file looking mechanism.
 Files contain both data and attributes.
 A typical attribute record structure is
illustrated in Figure 3.

8
Introduction
File length
Creation timestamp
Read timestamp
Write timestamp
Attribute timestamp
Reference count
Owner
File type
Access control list
Figure 3. File attribute record structure
Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005

• Figure 4 summarizes the main operations
on files that are available to applications in
UNIX systems.

10
Introduction
Figure 4. UNIX file system operations
filedes = open(name, mode)
filedes = creat(name, mode)
Opens an existing file with the given name.
Creates a new file with the given name.
Both operations deliver a file descriptor referencing the open
file. The mode is read, write or both.
status = close(filedes) Closes the open file filedes.
count = read(filedes, buffer, n)
count = write(filedes, buffer, n)
Transfers n bytes from the file referenced by filedes to buffer .
Transfers n bytes to the file referenced by filedes from buffer.
Both operations deliver the number of bytes actually transferred
and advance the read-write pointer.
pos = lseek(filedes, offset,
whence)
Moves the read-write pointer to offset (relative or absolute,
depending on whence).
status = unlink(name) Removes the file name from the directory structure. If the file
has no other names, it is deleted.
status = link(name1, name2) Adds a new name (name2) for a file (name1) .
status = stat(name, buffer) Gets the file attributes for file name into buffer.

11
2. Distributed File system requirements
 Related requirements in distributed file systems
are:
1. Transparency
2. Concurrent file updates
3. File Replication
4. Hardware & Operating system Heterogeneity
5. Fault tolerance
6. Consistency
7. Security
8. Efficiency

1. Transparency
• Transparency defined as the concealment
from the user and application programmer of
the separation of components in a distributed
system.
• Client programs should be unaware of the
distribution of files.

1. Transparency
1. Access transparency
2. Location transparency
3. Mobility transparency
4. Performance transparency
5. Scaling transparency

1. Access transparency
• enable local & remote file to be accessed
using identical operations.

2. Location transparency
enables files to be accessed without knowledge
of their physical or network location.

3. Mobility transparency
• Allows the movement of files within a system
without affecting the operation of users or
programs.

4. Performance transparency
• Allows the system to be reconfigured to
improve the performance as loads vary.

5. Scaling transparency
• Allows the service can be expanded.
• increase in size of storage and network size
should be transparent.

2. Concurrent file updates
• Concurrent file updates is protected by
locking.
• Simultaneously accessing or changing the
same file.

3. File Replication
• File service maintains multiple identical copies
of files.
• Load sharing between server makes service
more scalable.

4. Hardware & Operating System
Heterogeneity
• Service can be accessed by clients running on
any OS or any hardware platform.

5. Fault Tolerance
• If the service is replicated, it can continue to
operate even during a server crash.

6. Consistency
• Unix offers one –copy update semantics for
operation on local files.
• Difficult to achieve the same for distributed
file systems.

7. Security
• Must maintain access control and privacy as
for local files.
• Digital signatures and encryption for secret
data.

8. Efficiency
• Goal for distributed file system is usually
performance comparable to local file system.

3. distributed file system requirements

In this document

More Related Content

What's hot

Viewers also liked

Similar to 3. distributed file system requirements

More from AbDul ThaYyal

3. distributed file system requirements