Distributed System

1. Distributed File
System

2. Introduction
 File – collection of data with user view(structure) and a physical
view (blocks)
 DFS – distributed storage and users – provides transparency of
location, access and migration of files – uses replicas for efficiency
and fault tolerance
 Enables programs to store and access remote files as local ones
 Two types
 Andrew File system (AFS)
 Sun Network File system (NFS)
 File systems are abstractions that enables users to read, manipulate
and organize dat. Data is stored in units known as files in a
hierarchical tree – nodes are directories

3.  Enables uniform view – independent of underlying storage devices
(floppy to hard drives and flash memory cards)
 Goals:
 Network transparency – access rremote files
 High availability – easy to access files irrespective of their physical
location

4. Characteristics
 Organization, storage, retrieval, naming sharing and protection of
files
 File – Data + attributes (length, timestamp, file type)
 Directory – provides mapping from names to internal file identifier
 Modules
 Directory module – File names to File IDS
 File module – File IDS to particular files
 Access control module – checks permission for file operation
 Block module – Access and allocates disk blocks
 Device module – Disk IO and buffering

6. Requirements
1. Transparency
 Structure
 Access
 Naming
 Replication
2. User mobility
3. Performance
4. Scalability
5. High Availability
6. High reliability
7. Security

7. File Service Architecture
 Clear separation of main concerns providing access to files is
obtained by structuring the file service as three components – Flat
file service, directory file service, client module

8.  Flat file service
 Implementation of operations on the content of the file
 UFID – Unique File Identifier – refer to files in all requests for flat
file operations
 UFID – long sequence of bits chosen
 Create file – new UFID will be generated and returns UFID

9.  Directory service
 Mapping between text names for the files and their UFIDs
 Get UFID by quoting the text name to directory service
 It supports functions needed to generate directories, add files to
directories

10.  Client module
 Runs on each computer and provides integrated service as a
single API to application programs.
 Holds information about the network locations of flat file,
directory server processes
 Achieves better performance through implementation of cache of
recently used file blocks

11. Flat file service operations
Read (fileId, I, n) -> data
Write (fileId, I, data)
Create() -> fileId
Delete (field)
GetAttributes(FileId) -> attr
SetAttributes(FileId, attr)

12. Terminologies
 Access control
 Hierarchic file system
 File group