I MPLEMENTING FILES

Contiguous Allocation:  The simplest allocation scheme is to store each file as a contiguous run of disk blocks (a 50-KB file would be allocated as 50 consecutive blocks).

Advantages:  Simple to implement (keeping track of where a file’s stored).  Read performance is excellent (because the entire file can be read from the disk in a single operation). Drawback:  The disk becomes fragmented.

Linked List Allocation:  The second method for storing files is to keep each one as a linked list of disk blocks Drawback:  Random access is extremely slow & some amount of memory is wasted in storing pointers..

 The disadvantage of linked list allocation can be eliminated by taking the pointer word from each disk block and putting it in a table or index in memory.  Disadvantage: Entire table must be in memory all the time to work with it. Linked List Using Index:

 Method to keep track of which blocks belong to which file is to associate with each file a little table called i-node.  It list the attributes and disk address of the file’s block. I-nodes:

Implementing Directories:  Files on a UNIX system are organized into groups called directories ; which contains files and other directories.  Directories are arranged into a hierarchy, which is tree of a file system.

File Sharing:  In a multiuser system there is always a need to share a file among a number of users.  File sharing determines the manner in which authorized users of a file may share a file and the manner in which the results of their file manipulation is visible to one another.  Two kinds of file sharing mode: Sequential sharing Concurrent sharing  In Sequential sharing, files which are shared can be accessed by only one program at any point in time.  In C oncurrent sharing, it’s essential to avoid mutual interface between various users of shared file.  It’s also required to change access rights according to requirement.

 Files are normally stored on disk, so management of disk space is a major concern to file system designers.  When a file is deleted, its disk space is added to the free space list. This can be implemented in may ways: Bit vector Linked List Grouping Counting 1.Bit Vector:  Free space list is implemented as a bit vector.  Each block is represented by 1 bit. If block is free the bit is 1; if block is allocated the bit is 0. Disk Space Management:

2. Linked List:  Here, all the free blocks are linked keeping a pointer to the first free block in a special location on a disk.  It just needs space for the pointer to the beginning of the chain and the length of the first portion. 3. Grouping:  It stores the address of n free blocks in the first free block.  The last block contains the addresses of another n free blocks.  This last block is often called as index block. 4. Counting:  Every entry in the free list contains the address of the first free block and the number of consecutive free blocks.  Helps in fast allocation of free blocks.

File System Reliability:  File system reliability concerns the ability of a file system to function correctly despite occasional faults in the system.  There are 2 aspects of system reliability: Ensuring correctness of the file creation, deletion & updates. Preventing loss of data in files. File System Reliability Techniques: 1.Recovery: It used when failure is noticed. Restores the data to its original state. 2. Fault Tolerance: Used to prevent against the loss of integrity of the file system when the fault occurs.

File System Performance:  There are many techniques used to ensure good performance of a file system.  Most of them are concerned with faster access of data.  Hash tables and B+ trees are used to make directory searches more efficient.  The techniques of caching & buffering are used to speed up the accesses to file data.  Caching is most basic technique for speeding up access to the information.  It involves keeping the most accessed data item in the memory to speed-up repeated accesses to the data item.

 Buffering loads the information in memory in advance of future references. (for example, directories are cached in the memory when accessed for the first time. Thus, a directory used to decide a pathname is kept in the memory cache to speed-up future references to files located in it.)