Consistency Semantics

There should be consistency maintained by the file system for all users sharing the system :

Unix Semantics

• Writes performed by one user are immediately seen by other user(s)
• Can optionally share current location pointer :
  User 1 reads block 510, User 2 then reads block 511 (Single File Image)

Session Semantics

• Writes to open file are not visible to other users
• When file closed, changes are made visible to new sessions, but are not seen by current sessions

Multiple copies of files ?

Immutable Files

• Read only sharing - files cannot be changed

Locks

• Sharing of files locking to give exclusive access
• Prevent others reading while the file is being updated
• Locks can be : On whole files or Individual record of a file