The naming facility of a distributed operating system enables users and programs to assign character-string names to objects and subsequently use these names to refer to those objects.

It provides a further level of abstraction when dealing with object replicas. Given an object name, it returns a set of the locations of the object's replicas.·

Naming – mapping between logical and physical objects.

■ Multilevel mapping – abstraction of a file that hides the details of how and where on the disk the file is actually stored.

■ A transparent DFS hides the location where in the network the file is stored.

■ For a file being replicated in several sites, the mapping returns a set of the locations of this file’s replicas; both the existence of multiple copies and their location are hidden.

The naming system plays a very important role in achieving the goal of

location transparency,

facilitating transparent migration and replication of objects,

object sharing.

Naming Structures

■ Location transparency – file name does not reveal the file’s physical storage location.

✦ File name still denotes a specific, although hidden, set of physical disk blocks.

✦ Convenient way to share data.

✦ Can expose correspondence between component units and machines.

■ Location independence – file name does not need to be changed when the file’s physical storage location changes.

✦ Better file abstraction.

✦ Promotes sharing the storage space itself.

✦ Separates the naming hierarchy form the storage-devices hierarchy.

Naming Schemes — Three Main Approaches

■ Files named by combination of their host name and local name; guarantees a unique system wide name.

■ Attach remote directories to local directories, giving the appearance of a coherent directory tree; only previously mounted remote directories can be accessed transparently.

■ Total integration of the component file systems.

✦ A single global name structure spans all the files in the system.

✦ If a server is unavailable, some arbitrary set of directories on different machines also becomes unavailable.

1)Absolute names:

<machine name:="" path="" name="">

• Examples: AppleShare, Win NT

• Advantages:

– Finding a fully specified file name is simple.

– It is easy to add and delete new names.

– No global state.

– Scales easily.

• Disadvantages:

– User must know the complete name and is aware of which files are local and which are remote.

– File is location dependent, and thus cannot move.

– Makes sharing harder.

– Not fault tolerant.

2) Mount Points

• Mount Points (NFS - Sun's Network File System)

– Each host has a set of local names for remote locations.

– Each host has a mount table (/etc/fstab) that specifies <remote path="" name="" @="" machine="" name=""> and a <local path="" name="">.

– At boot time, the local name is bound to the remote name.

– Users then refer to the <local path="" name=""> as if it were local, and the NFS takes care of the mapping

• Advantages:

location transparent, remote name can change across reboots

• Disadvantages:

single unified strategy hard to maintain, same file can have different names

3) Global Name Space

• Single name space: CMU's Andrew and Berkeley's Sprite

– No matter which node you are on, the file names are the same.

– Set of workstation clients, and a set of dedicated file server machines.

– When a client starts up, it gets its file name structure from a server.

– As users access files, the server sends copies to the workstation and the workstation caches the files

Advantages:

– Naming is consistent and easy to keep consistent.

– The global name space insures all the files are the same regardless of where you login.

– Since names are bound late, moving them is easier.

• Disadvantages:

– It is difficult for the OS to keep file contents consistent due to caching.

– Global name space may limit flexibility.

– Performance problems.