1 File Management
2 File Directories Associated with any file management system and collection of files is a file directories The directory contains information about the files, including attributes, location and ownership.
File System Implementation File systems are on disk. Most disks can be divided up into one or more partitions, with independent file system on each partition. Sector 0 of the disk is called MBR ( Master Boot Record) and is used to boot the computer. The end of the MBR contains the partition table. This table gives starting and ending addresses of each partition. One of the partitions in the table is marked as active. When the computer is booted, the BIOS reads in and executes the MBR. 3
The first thing the MBR program does is locate the driver partition, read in its first block, called the boot block, and execute it. The program in the boot block loads the operating system contained in that partition. For uniformity, every partition start with a boot block, even if it does not contain a bootable operating system. 4
5 Implementing Files Contiguous Allocation The simplest allocation scheme is to store each file as a contiguous run of disk blocks. Contiguous disk space allocation has two significant advantages. First, it is simple to implement because keeping track of where file s blocks are reduced to remembering two numbers: the disk address of the first block and the number of blocks in the file. Given the number of the first block, the number of any other block can be found by a simple addition.
6 Characteristic of contagious file allocation 1. It supports variable size portions. 2. Pre-allocation is required 3. It requires only single entry for a file. 4. Allocation frequency is only once. Advantages 1. It supports variable size portion 2. Easy to retrieve single block. 3. Accessing a file is easy. 4. It provides good performance. Disadvantages 1. It suffers from external fragmentation. 2. Pre-allocation is required.
Linked Allocation Linked allocation solve the problem of contiguous allocation. This allocation is on the basis of an individual block. Each block contains a pointer to the next block in the chain. The disk blocks may be scattered anywhere on the disk. The directory ( file allocation table) contains a pointer to the first and last blocks of the file. To create a new file, simply create a new entry in the directory. With linked allocation, each directory entry has a pointer to the first disk block of the file. This pointer is initialized to nil signify an empty file. Size file is also set to 0. there is no external fragmentation to worry about because only one block at a time is needed. 7
The size of file does not need to be declared when the file is created. A file can continue to grow as long as free blocks are available. It is never necessary to compact disk space. Characteristics 1. It supports fixed size portions. 2. Pre-allocation is possible. 3. File allocation table size is one entry for a file. 4. Allocation frequency is low to high. 8
Advantages 1. There is no external fragmentation. 2. It is never necessary to compact disk space. 3. Pre-allocation is not required. Disadvantages 1. Files are accessed only sequentially 2. Space required for pointers. 3. Reliability is not good. 4. Can not support direct access 9
Indexed Allocation The file allocation table contains a separate one level index for each file. The index has one entry for each portion allocated to the file. Typically file indexes are not physically stored as part of the file allocation table. Rather the file index for a file is kept in separate block, and the entry for the file in the file allocation table points to that block. Allocation may be on the basis of either fixed size blocks or variable size portions. When the file is created, all pointer in the index block are set to nil. 10
11 Advantages 1. It supports sequential and direct access 2. No external fragmentation 3. Faster than other two methods. 4. It support fixed and variable size blocks Disadvantages 1. Indexed allocation does suffer wasted space 2. Pointer overhead is generally faster
I-Node The administrative information about a file is kept in a structure known as an i-node. I-nodes in a file system, in general, are structured as an array known as an i-node table. An i-node number, which is an index to the i-node table, uniquely identifies a file in a file system. 12
I-nodes Keeping track of which blocks belong to which file is to associate with each file a data structure called i- node(index-node), which lists the attribute and disk addresses of the file s block. A simple example is shown in figure (next slide). Given the i-node, it is then possible to find all the blocks of the file. The big advantage of this scheme over linked files using an in-memory table is that the i-node need only be in memory when the corresponding file is open. If each i-node occupies n bytes and a maximum of k files may be open at once, the total memory occupied by the array holding the i-nodes for the open files is only kn 13 this much space need be reserved in advance.
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This array is usually far smaller than the space occupied by the file table. The reason is simple. The table for holding the linked list of all disk blocks is proportional in size to the disk itself. If the disk has n blocks, the table needs n entries. As disks grow larger, this table grows linearly with them. In contrast, the i-node scheme requires an array in memory whose size is proportional to the maximum number of files that may be open at once. One problem with i-nodes is that if each one has room for a fixed number of disk addresses, what 15 hen a file grows beyond the limit?
One solution is to reserve the last disk address not for a data block, but the instead for the address of a block containing more disk block addresses. Even more advanced would be two or more such blocks containing disk addresses or even disk blocks pointing to other disk block full of addresses. 16
Free Space Management To keep track of free disk space, the system maintains a free space in list. The free space list records all free disks blocks those not allocated to some file or directory. We need a disk allocation table in addition to a file allocation table. Four techniques are in common use 1. Bit Vector 2. Linked List 3. Grouping 4. Counting 17
Bit Vector This method use a vector containing 1 bit for each block on the disk. Each entry of 0 corresponds to a free block, and each 1 corresponds to a block in use. For example, consider a disk where blocks 2,3,4,5,8,9,10,11,12,13,17,18,25,26,27are free and rest of the blocks are allocated. The free space would be 1100001100000011100111111000. Main advantage of this method is that it is relatively easy to find one or a contiguous group of free blocks. Second advantage is that it is as small as possible and can be kept in main memory 18
Linked List In linked list, all free space disk blocks are linked, keeping a pointer to the first free block in special location on a disk and caching in memory This method has negligible space overhead because there is no need for a disk allocation table, merely for a pointer to the beginning of the chain and the length of the first portion. This method is suited to all the file allocation methods. 19
Grouping It stores the addresses of n free blocks in the first free block. The first n-1 of these block are actually free. The last block contains of another n free blocks. Addresses of a large number of free blocks can be found quickly. 20
Counting It keeps the address of the first free block and the number n of free contiguous block that follow the first block. Each entry in the free space list then consists of a disk address and a count. 21
DIRECTORY STRUCTURE IN DOS Directory is just like a file folder, which contain all the logically related files. DOS files are organized in a hierarchical or an inverted tree-like structure. DOS enables the user to organize the files in a disk into directories and sub-directories Adirectory within another directory is called a subdirectory Of course, there may be sub-directories of sub-directories, subsub. 22
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Originally invented as a method for storing data on floppy disks, first version has single directory Later used by MS-DOS with supports for a hierarchical directory structure, and fixed disks as well as floppy disks directory entries are fixed 32 bytes different versions support FAT-12, FAT-16 or FAT-32 24
Block Sizes and FAT Sizes Disk Size FAT-12 FAT-16 FAT-32 FAT entry 12 bits 16 bits 28 bits Max # of blocks 4086 ~212 65526 ~216 268435456 ~228 Block size 0.5KB to 4KB 2KB to 32KB 4KB to 32KB FAT size 8KB 128KB 1GB 25
UNIX Directory Directories are structured in a hierarchical tree. Each directory can contain files and other directories. A directory that is inside another directory is referred to as a subdirectory. Fig. shows the UNIXdirectories and i-node I1 Name 1 I2 Name 2 I3 Name 3 26
A directory is a file whose data is a sequence of entries, each consisting of an i-node number and the name of a file contained in the directory. Each directory entry contains a name for the associated file or subdirectory plus an integer called the i-number. When the file or directory is accessed, its i-number is used an index into the i-node table. Unix system V restricts components names to a maximum of 14 characters, with a 2 byte entry for the i-node number, the size of a directory entry is 16 byte. 27