14-08-2012, 04:50 PM
Distributed Systems
1Distributed.ppt (Size: 668 KB / Downloads: 158)
Accessing files
FTP, telnet:
Explicit access
User-directed connection to access remote resources
We want more transparency
Allow user to access remote resources just as local ones
Focus on file system for now
NAS: Network Attached Storage
File service types
Upload/Download model
Read file: copy file from server to client
Write file: copy file from client to server
Advantage
Simple
Problems
Wasteful: what if client needs small piece?
Problematic: what if client doesn’t have enough space?
Consistency: what if others need to modify the same file?
File server
File Directory Service
Maps textual names for file to internal locations that can be used by file service
File service
Provides file access interface to clients
Client module (driver)
Client side interface for file and directory service
if done right, helps provide access transparency
e.g. under vnode layer
Session semantics
Relax the rules
Changes to an open file are initially visible only to the process (or machine) that modified it.
Last process to modify the file wins.