My Understanding of Network File System (NFS)

NFS
> Stands for Network File System
> Client Server Mechanism
> NFS to be hosted by one single server machine (not suitable for scaling)
> NFS works on TCP/IP layer.

Mounting NFS
> User boots up the machine and NFS is mounted as file system
> OS is required to do same file system calls and NFS mount will handle it

Interaction between client and server
> Client makes a request for a file and translated to NFS
> Client uses TCP/IP to communicate
> Reqeusted file is locked and sent back to client
> Client caches the copy of the file (or part of the file)
> The lock is a time base entity. Say for 5 mins lock will stay alive for client.
> Meanwhile accessing the file client can release the lock, can request lock on the file (to restart the time quota), on go down (NFS will time out and releases the lock)

Cache Consistency or Coherence of files
> As mentioned above the files are stored in the local cache (temporal locality)
> Now if client requires to use the same file, a request goes back to NFS checking if the cache is consistent with file on the server (caching the same copy)
> If the copy is updated then new version of file is fetched else client can use the cached copy of file

How to cache consistency is not 100% in NFS
> Client has already cached a file
> Client requests the file and cache is consistent with NFS copy
> While client is using the cached copy of file another client (client0) requests a lock on that file to NFS
> NFS allocates lock to client0.
> Client0 updates the file while client is reading the file
> So there are chances that client may read a stale version

For more information check NFS @ Wiki.