1. Naming and Name Resolution:

Name refers to an object such as file or a directory. Name Resoltuion refers to the process of mapping a name to an object that is physical storage. Name space is collection of names. Names can be assigned to files in distributed file system in three ways:

a) Concatenate the host name to the names of files that are stored on that host.
Advantages:

• File name is unique
• Systemwide
• Name resolution is simple as file can  be located easily.

Limitations:

• It conflicts with the goal of network tranparency.
• Moving a file from one host to another requires changes in filename and the application accessing that file that is naming scheme is not location independent.

b) Mount remote directories onto local directories. Mounting a remote directory require that host of directory to be known only once. Once a remote directory is mounted, its files can be referred in location transparent way. This approach resolve file name without consulting any host.
c) Maintain a single global directory where all the files in the system belong to single namespace. The main limitation of this scheme is that it is limited to one computing facility or to a few co-operating computing facilities. This scheme is not used generally.
Name Server: It is responsible for name resolution in distributed system. Generally two approaches are used for maintaining name resolution information.
Way 1: Use a single name server that is all clients send their queries to single server which maps names to objects. Its limitation is: If name server fails, the entire system  is affected and Name server become a bottleneck and degrades the performance of the system.
Way 2: Use several name servers(on different hosts) wherein each server is responsible for ampping objects stored in different domains. This approach is generally used. Whenever a name is to be mapped to an object, the local name server is queried. The local name server may point to remote server for furthur mapping of the name. Example: "a/b/c"- requires a remote server mapping the /b/c part of the filename. This procedure is repeated until the name is completely resolved.
2. Caches on Disk or Main Memory:
Caching refers to storage of data either into the main memory or onto disk space after its first reference by client machine.
Advantages of having cache in main memory:

• Diskless workstations can also take advantage of caching.
• Accessing a cache in main memory is much faster than accessing a cache on local disk.
• The server cache is in the main memory at the server, a single design for a caching mechanism is used for clients and servers.

Limitations:

• Large files cannot be cached completely so caching done block oriented which is more cpomplex.
• It competes with virtual memory system for physical memory space, so a scheme to deal with memory contention cache and virtual memory system is necessary. Thus, more complex cache manager and memory management is required.

Advantages of having cache on a local disk:

• Large files can be cached without affecting performance.
• Virtual memeory management is simple.
• Portable workstation can be incorporated in distributed system.

3. Writing Policy:
This policy decides when a modified cache block at client should be transferred to the server. Following policies are used:
a) Write Through: All writes required by clients applications are also carried out at servers immediately. The main advantage is reliability that is when client crash, little information is lost. This scheme cannot take advantage of caching.
b) Delayed Writing Policy: It delays the writing at the server that is modifications due to write are reflected at server after some delay. This scheme can take advantage of caching. The main limitation is less reliability that is when client crash, large amount of information can be lost.
c) This scheme delays the updating of files at the server until the file is closed at the client. When average period for which files are open is short then this policy is equivalent to write through and when period is large then this policy is euivalent to delayed writing policy.
4. Cache Consistancy:
When multiple clients want to modify or access the same data, then cache consistancy problem arises. Two schemes are used to gurantee that data returned to the client is valid.
a) Server initiated approach: Server inform the cache manager whenever data in client caches becomes  valid. Cache manager at clients then retrieve the new data or invalidate the blocks containing old data in cache in cache. Server has maintain reliable records on what data blocks are cached by which cache managers. Co operation between servers and cache manager is required.
b) Client-initiated approach: It is the responsibilty of cache manager at the clients to validate data with server before returning it to the clients. This approach does not take benefit of caching as the cache manager consult the server for validation of cached block each time.
5. Availability:
It is one of the important issue is design of Distributed file system. Server failure or communication network can attract the availability of files.
Replication: The primary mechanism used for enhancing availability of files is replication. In this mechanism, many copies or replicas of files are maintained at fifferent servers.
Limitations:

• Extra storage space is required to store replicas.
• Extra overhead is required in maintained all replicas up to date.

Following situations cause inconsistancy among replicas:

• Replica is not updated due to failure of server storing the replica
• All the file servers storing the replicas of file are not reachable from all clients due to network partition and replicas of file in different partition are updated differently.

Unit of Replication: The main design issue in replication is the unit of replication. Following units can be used for replication of data:
a) The most basic unit is file: This unit allow the replication of only those files that need to have higher availability. This unit results in expansive replica management. Protection rights associated with directory have to individually stored with each replica. Replica of common directory may not have common file servers so require extra name resolution to locate each replica in case of modification of file or directory.
b) Group of files called volumes can be used: Volume may represent files of single user or files that are in a server. This scheme is used in coda file system. In this scheme, replica management is simple that is protection rights can be associated with volume instead with each individual file replica. Volume replication results in wasteful as a user needs higher availability for only a few files in the volume.
c) Combination of volume and single file replication can be used: All the files of a user form a file group called primary pack. A replica of primary pack, called a pack, is allowed to contain a subset of files in primary pack. Corresponding to primary pack, one or more of packs can be obtained according to requirement.
6. Scalability:
The design of DFS should be such that new systems can be easily introduced without affecting it. Generally, client-server organisation is used to define DFS structure. Caching is used in this organisation to improve performance. Server initiated cache invalidation is used to maintain cache consistancy. In this approach, server maintain a record based information regarding all the clients sharing file stored on it. This information represents server state. As the system grows both the size of server state and load due to inalidations increases on server. Following schemes can be used to reduce server state and server load:
a) Exploit knowledge about usage of files that is it is found that most commonly used and shared files are accessed in read only mode. So, there is no need to check the validity of these files of maintain the list of clients at servers for validation purpose.
b) Generally, data required by a client is found in another client's cache so a client can obtain required data from another client rather than server.
Structure of server process play am important role. If server is designed with single proces, then many clients have to wait for a long time whenevr a disk input/ output is initiated. This can be avoided if separate process is assigned to each client.
7. Semantics:
The semantic of a file system represent the affects of acceses on file. The basic semantic is that a read operation will return the data (stored ) due to latest write operation. The semantic can be guranteed in two ways: All read and writes from various clients will have to go through the server. Sharing will have to be disallowed either by server or by the use of locks by application. In first way, the server become bottleneck and in second way, the file is not available for certain clients.

• Written by Aastha.
• In category Engineering.
• 14 years ago.

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