Allocation Methods | File Allocation Methods | Contiguous | linked | indexed | chain | os | files
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- เผยแพร่เมื่อ 17 ต.ค. 2024
- #AllocationMethods #FileAllocationMethods #operatingsystemlectures
Allocation Methods
There are various methods which can be used to allocate disk space to the files. Selection of an appropriate allocation method will significantly affect the performance and efficiency of the system. Allocation method provides a way in which the disk will be utilized and the files will be accessed.
There are following methods which can be used for allocation.
Contiguous Allocation.
Extents
Linked Allocation
Clustering
FAT
Indexed Allocation
Linked Indexed Allocation
Multilevel Indexed Allocation
Inode
We will discuss three of the most used methods in detail.
Contiguous Allocation
If the blocks are allocated to the file in such a way that all the logical blocks of the file get the contiguous physical block in the hard disk then such allocation scheme is known as contiguous allocation.
In the image shown below, there are three files in the directory. The starting block and the length of each file are mentioned in the table. We can check in the table that the contiguous blocks are assigned to each file as per its need.
Advantages
It is simple to implement.
We will get Excellent read performance.
Supports Random Access into files.
Disadvantages
The disk will become fragmented.
It may be difficult to have a file grow.
Linked List Allocation
Linked List allocation solves all problems of contiguous allocation. In linked list allocation, each file is considered as the linked list of disk blocks. However, the disks blocks allocated to a particular file need not to be contiguous on the disk. Each disk block allocated to a file contains a pointer which points to the next disk block allocated to the same file.
Advantages
There is no external fragmentation with linked allocation.
Any free block can be utilized in order to satisfy the file block requests.
File can continue to grow as long as the free blocks are available.
Directory entry will only contain the starting block address.
Disadvantages
Random Access is not provided.
Pointers require some space in the disk blocks.
Any of the pointers in the linked list must not be broken otherwise the file will get corrupted.
Need to traverse each block.
Indexed Allocation
Limitation of FAT
Limitation in the existing technology causes the evolution of a new technology. Till now, we have seen various allocation methods; each of them was carrying several advantages and disadvantages.
File allocation table tries to solve as many problems as possible but leads to a drawback. The more the number of blocks, the more will be the size of FAT.
Therefore, we need to allocate more space to a file allocation table. Since, file allocation table needs to be cached therefore it is impossible to have as many space in cache. Here we need a new technology which can solve such problems.
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Sir thank you, what is the default file allocation method of NTFS?
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sir your lectures are intresting and clear, thanks for that sir. I got a doubt sir... in logical allocation the block 1 has the address of block 2 as u said but why it is connected to block 4...and why 5 is connected to 9? can you please clarify my doubt sir..
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In linux os, what methods are used for free space management?
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Sir, in the linked allocation the pointer is not pointing to the next block. Instead it is directly jumping to next row. How is it possible. Because u told that the pointer will pint to the address of next block.
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Sir is this also called the file allocation policy?
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sir why did you say that indexed allocation requires less memory than linked allocation although the no of pointers are same in both the methods
At one node many node addresses will be available thats why. Coming to linked allocation each node should contains address of next node
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is file start and length are given in question ?
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Easily understanding the concept
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