Replication

• Fault Transparency or Tolerance - components in a distributed system can fail independently. A file may be made more available in the face of failures of a file server if it appears on more than one file server. Availability is one of the main measures of the advantage of a replication algorithm.
• Performance - if a file is 'nearer' a user, access will generally be faster. Files may be replicated across local storage devices at creation time, or they may be 'cached' when first accessed. Such caching may be on the basis of a few blocks of the file (NFS), the whole file (AFS 1) or as much of the file as possible (AFS 3).
• Portability - file access should be possible after a workstation is detached from the network if portability is required.

• Storage costs. Whole file copies are relatively expensive
• Consistency of Updates. There is cost in coordinating the update of a replicated file so that further accesses perceive a consistent object. This has an associated execution and communication cost.
• Complexity. Some replication schemes may require complex software to implement. This has an associated implementation and maintenance overhead.

• Optimistic These schemes assume faults are rare and implement recovery schemes to deal with inconsistency
• Pessimistic These schemes assume faults are more common, and attempt to ensure consistency of every access. Schemes that allow access when all copies are not available use voting protocols to decide if enough copies are available to proceed.