Basic

The Network File System (NFS) is a client/server application that lets a computer user view and optionally store and update files on a remote computer as though they were on the user’s own computer. NFS is a distributed file system(DFS).

Client/server architecture

Advantage

• Easy sharing of data across clients
• Centralized administration
• Security

Disadvantage:

• Network overhead
• More components to fail

DFS

Architecture

Stateless - 无状态协议

• Server does not keep track of states of clients
  • Which files are currently open at which clients
  • Current position/offset of file
  • Which clients have read/cached which blocks
• Requests from clients must make sure:
  • the server can deliver all the information needed to complete the requests
  • do not rely on previous requests

file handle

• Volume (file system) identifier
  • Which volume? (e.g. partition C or D if NTFS)
• Inode number
  • Which file in the volume?
• Generation number
  • Needed since inode number may be reused at the server (e.g., after file has been deleted by other clients)

Client uses file handle to communication with server

RPC - Remote procedure call

• Remote server publishes a set of procedures, for example f(args)
  • example: NFSPROC_LOOKUP for lookup file handle ,read, write, create, remove, etc.
• In making RPC calls,(客户端调用)
  • Client notifies remote server of executing f & sends over arguments args for f
  • Server executes f(args) => results
  • Server sends back results

Operations on remote file

All CRUD operations use file handle

Read

First obtain file handle via lookup

• File handle for the root directory may be obtained via the mount protocol

Look up "/foo/more/bar.txt"
– First, use / file handle to obtain foo's handle  
	File handle for the root directory may be obtained via the mount protocol
– Next, use foo's handle to obtain more's handle
– Finally, use more's handle to obtain bar.txt handle

Then use file handle to read data

NFSPROC_READ(file handle, offset, count)
- offset here is explicit
– Return: data + file attributes
– File attributes include modification time, useful for client-size cache validation

Compared to local file system
- n = read(fd, buffer, size)
– n is the number of bytes actually read

Write

First obtain file handle via lookup

Then use file handle to write data

NFSPROC_WRITE(file handle, offset, count, data)
– Return: file attributes
– Note again explicit offset is specified in the call

Compared to local file system
– n = write(fd, buffer, size)
– Offset is again implicit (current position)

Create

NFSPROC_CREATE(directory file handle, name of file in the directory, attributes)
– Return: file handle

Remove

NFSPROC_REMOVE(directory file handle, name of file to be removed)
– Return: Nothing

Failures

Cases

Case1: request lost
Case2: Server down
Case3: Reply lost on way back from Sever

More details

学习资源: https://tools.ietf.org/html/rfc1094