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NFS Protocol Stack / VFS

NFS/DFS:  An Overview

• Unix distributed filesystems are used to

• • centralize administration of disks
  • provide transparent file sharing across a network

• Three main systems:

• • NFS: Network File Systems developed by Sun Microsystems 1984
  • AFS: Andrew Filesystem developed by Carnegie-Mellon University

• Unix NFS packages usually include client and server components

• • A DFS server shares local files on the network
  • A DFS client mounts shared files locally
  • a Unix system can be a client, server or both depending on which commands are executed

• Can be fast in comparasion to many other DFS

• • Very little overhead
  • Simple and stable protocols
  • Based on RPC (The R family and S family)

Overview of NFS

• Developed by Sun Microsystems 1984
• Independent of operating system, network, and transport protocols.
• Available on many platforms including:

• • Linux, Windows, OS/2, MVS, VMS, AIX, HP-UX….

• Restrictions of NFS

• • stateless open architecture
  • Unix filesystem semantics not guaranteed
  • No access to remote special files (devices, etc.)

• Restricted locking

• • file locking is implemented through a separate lock daemon

• Industry standard is currently nfsV3 as default in

• • RedHat, SuSE, OpenBSD, FreeBSD, Slackware, Solaris, HP-UX, Gentoo

• Kernel NFS or UserSpace NFS

Versions of NFS

• Version 2:

• • Supports files up to 4GB long (most common 2GByte)
  • Requires an NFS server to successfully write data to its disks before the write request is considered successful
  • Has a limit of 8KB per read or write request. (1 TCP Window)

• Version 3 is the industry standard:

• • Supports extremely large file sizes of up to 264 – 1 bytes
  • Supports files up to 8 Exabyte
  • Supports the NFS server data updates as being successful when the data is written to the server’s cache
  • Negotiates the data limit per read or write request between the client and server to a mutually decided optimal value.

• Version 4 is coming:

• • File locking and mounting are integrated in the NFS daemon and operate on a single, well known TCP port, making network security easier
  • Support for the bundling of requests from each client provides more efficient processing by the NFS server.
  • File locking is mandatory, whereas before it was optional

NFS Daemons

• Portmap The primary daemon upon which all the RPC rely

• • Manages connections for applications that use the RPC specification
  • Listens to TCP port 111 for initial connection
  • negotiate a range of TCP ports, usually above port 1024, for further comms.
  • You need to run portmap on both the NFS server and client.

• Nfs (rpc.nfsd)

–        Starts the RPC processes needed to serve shared NFS file systems

–        Listens to TCP or UDP port 2049 (port can vary)

–        The nfs daemon needs to be run on the NFS server only.

• Nfslock (rpc.mountd)

• • Used to allow NFS clients to lock files on the server via RPC processes.
  • Neogated port UDP/TCP port
  • The nfslock daemon needs to be run on both the NFS server and client

• netfs

• • Allows RPC processes run on NFS clients to mount NFS filesystems on the server.
  • The nfslock daemon needs to be run on the NFS client only.

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