Explain the single and multiple partition techniques?

There are a number of allocation techniques available and all have different properties and allocate memory based on different principles. One prominent type of allocation is the partitioned allocation. 

- In partitioned allocation the primary or the main memory of the system is divided into a number of contiguous memory blocks which are commonly known as the memory partitions. 

- Each of these partitions consists of all the information that might be required for carrying out a specific task. 

- The task of allocating these memory partitions to various jobs and processes and de-allocating them after use is the duty of the memory management unit.  

- But partitioned allocation cannot be carried out by the help of software alone. 

- It requires some hardware support. 

- This support prevents interference of the various jobs in to each other and with the operating system as well. 

- For example, a lock and key technique was used by the IBM system/ 360. 

- Some other systems made use of the registers called the base and bound registers containing the partition limits and these were also used for flagging if any invalid access was made. 

- Limits register was used by the UNIVAC 1108 having separate base and bound data and instructions. 

- A technique called the memory interleaving was used by this system for placing so called I banks and d banks in different memory modules. 

Partitions are of two types namely:

Ø  Static partitions: These are defined at the boot time or IPL (initial program load) or sometimes by the computer operator. An example of system using static partitions is IBM system/360 operating system multi-programming with MFT (fixed number of tasks).

Ø  Dynamic partitions: These are created automatically for the specified job. An example is of the IBM system/ 360 operating system multi-programming with MVT (variable number of tasks).

- The hardware typed memory such as the base and bound registers (GE – 635, PDP – 10 etc.), Burroughs corporation B5500 etc. is used for relocating the memory partitions. 

- The partitions that can be relocated can be compacted to form larger contiguous memory chunks in the main memory. 

- Some systems allow for swapping out the partitions to the secondary storage and in turn to some additional memory.

The partitioned allocation offers two types of allocation techniques namely:

1. Single partition techniques
2. Multiple partition techniques

- Single partition techniques are the ones that are used for the single time sharing partition for swapping in and out the memory partitions. 

- These techniques are used by the IBM’s TSO (time sharing option). 

- The multiple partition techniques are used in the multiple time sharing partition. 

- In DOS systems when the disk is partitioned, each of the memory partitions act as if it is an individual disk drive. 

- Partitioning is useful for the systems where there are more than one operating system. 

- Partitioning techniques are meant for increasing the efficiency of the disk. 

- Hard and soft partitioning is used on the apple Macintosh computers. 

- The creation, relocation and deletion of the memory partitions can be harmful for the data. 

- That’s why it is good to have back up of the data stored on your system. 

- Several issues have to be considered if you want to install more than one operating system on your computer. 

- Day by day disks are becoming less expensive and bigger. 

- You can go for separate disks for storing data and installing Oss. 

Explain the various Disk Allocation methods? – Part 2

In this article we discuss about the non-contiguous disk allocation methods i.e., the linked allocation and the indexed allocation. 

What is a Linked Allocation?

- In linked allocation a single file might be stored all over the disk and these scattered parts are linked to each other just like a linked list. 

- Few bytes in the memory block are used for storing the address of the following linked block.

- This type of allocation has two major advantages mentioned below:

1. Simplicity and
2. Non – requirement of disk compaction

- Since the nature of the allocation method is non-contiguous, it does not lead to any external fragmentation of the disk space.

- And since all the memory blocks have been linked to each other, a memory block available anywhere in the memory can be used for satisfying the requests made by the processes. 

- Declaring the file size for the linked allocation is not required during its creation. 

- There are no issues even if the file continues to grow as long as free blocks are available and since the blocks can always be linked up. 

- As a consequence of all this, the need for disk compaction is eliminated. 

Disadvantages of Linked Allocation

But there are disadvantages of using linked allocation. They are:

Direct access to the disk blocks becomes slow: 

For finding a particular block of the file, the search has to begin at the starting point of the file and the successive pointers have to be followed till the destination block is reached.

Space required by the pointers: 

- Suppose out of 512 words of a memory block, 2 are required for storing the pointers, then we have 39 percent of the total disk being used by the pointers rather than for data. 

- This adds to the space requirement of the file blocks.

Reliability: 

- Because of all the blocks being linked via the pointers, even if one pointer gets damaged or wrong, the successive blocks can become inaccessible. 

- This problem is quite common and thus most of the operating systems avoid this problem by keeping redundant copies of these pointers in a special file. 

- The basic idea here is to keep the list of the pointers in the physical memory of the system. 

- This also allows for the faster access to the disk blocks.

What is Indexed allocation?

- The linked allocation method does not provide support for the direct access and this problem is solved by the indexed allocation method. 

- In this method, all the pointers are placed over an index. 

- Thus, these pointers together form the index block. 

- The address of this address block is then stored in the directory. 

- The pointer at the nth number in the index block points to the nth block of the associated file. 

- The purpose of the index blocks is somewhat similar to that of the page map tables; however both of these are implemented in a different way.
- First level index is used for searching the index of second level and the second one is used for searching the third one and the process may continue till the fourth level. 

- But in most of the cases the indexes of the first two levels are sufficient. 

- With this method, second level index blocks (128 x 128) can be easily addressed and files of size of up to 8mb can be supported. 

- Assuming the same thing, files of size up to 1 gb can be addressed.

Advantages and Disadvantage of Indexed Allocation

- The major advantage of this allocation technique is that it does not give rise to external fragmentation and offers a high level of efficiency in random accessing. 

- Also, using this technique mapping can be done around the disk blocks that are known to be bad. 

-Bit mapping can be used for indexing the free space. 

The biggest disadvantage of this allocation technique is the large number disk accesses required for the retrieval of the address of the destination block in the memory. 

Explain the various Disk Allocation methods? – Part 1

Management of space of the secondary data storage devices is one of the most necessary functions of the file system. This includes tracking which blocks of memory or disk are being allocated to the files and which blocks are free to be allocated. 

There are two main problems faced by the system during allocation of the space to different files. Firstly, the access to the files has to be fast and secondly, the disk space has to be effectively utilized. Both of these combine to form the big problem of disk management. These two problems are more common with the physical memory of the system. 

However, the secondary storage of the system also introduces 2 additional problems which are long disk access time and blocks of larger size to deal with. In spite of all these problems, there are considerations that are common for both the storage such as the non – contiguous and contiguous space allocation. 

Types of Disk Allocation Methods

The following are the 3 widely used allocation techniques:

Ø  Contiguous

Ø  Linked

Ø  Indexed

- The linked and the indexed allocation techniques fall under the category of the non-contiguous space allocation. 

- All the methods have their own pros and cons.

- It is in the term of blocks that all the input and output operations are carried out on the disk. 

- Software is responsible for converting from the logical record to physical storage blocks.

Contiguous Allocation: 

- This allocation technique assigns only the contiguous memory blocks to the files. 

- The size of the memory required is mentioned by the user in advance for the holding the file. 

- The file is then created only if that much amount of memory is available otherwise not. 

- It is the advantage of the contiguous memory allocation technique that all the successive file records are saved adjacent to each other physically. 

- This causes an increase on the disk access time of the files. 

- This can be concluded from the fact that if the files are scattered all about the disk, then it takes a lot more time to access them. 

- Accessing files when they have been organized in a proper order is quite easy.

- To access the files sequentially, the system uses the address of the last memory block and if required moves to the next one. 

- Both direct and sequential accesses are supported by the contiguous memory allocation technique. 

- The problem with this allocation method is that every time a new contiguous space cannot be found for the new files if a majority of the memory is already in use and if the file size is larger than the available memory.  

Non–Contiguous Memory Allocation: 

- It happens a lot of time that the files grow or shrink in size with usage and time. 

- Therefore, it becomes difficult to determine the exact space required for the files since the users do not have any advance knowledge about the growing size of their files. 

- This is why the systems using the contiguous memory allocation techniques are being replaced by the ones with the non-contiguous storage allocation which is more practical and dynamic in nature. 

- The linked allocation technique mentioned above in the article, is a disk – based implementation of the linked list. 

- Each file is considered to be a list of the disk blocks linked to each other. 

- It does not matter whether the blocks are scattered about the disk. 

- In each block, few bytes are reserved for storing the address of the next block in chain.

- The pointer to the first and the last block of the file is stored in the directory. 

What are Rational Unified Process building blocks?

Whenever we talk about iterative software development process frame works, the first name that comes to our minds is of the rational unified process. It is not just a single hard coded prescriptive process rather it is quite an adaptable and flexible process frame work. 

This frame work comes with several facilities, the best one being that your organization can tailor it according to their needs. 

What is Rational Unified Process?

- Unified process when specifically implemented is called as the rational unified process. 

- Rational unified process is counted among the best products of IBM and the credit for its development goes to the rational software division.

- The RUP comes with a hyper linked base consisting of the descriptions of several types of activities and a few sample artifacts. 

- A part of the IBM’s rational method composer (RMC) is occupied by the rational unified process.

- It allows the users to customize the process as per their needs. 

What are building blocks of Rational Unified Process?

In this article we are to talk about the building blocks of the rational unified process.

- These basic best practices of the rational unified process were developed as a result of the combination of the experience of many companies. 

- The building blocks are:

1. Iterative development with risk as its primary iteration driver.
2. Management of the requirements.
3. Employment of an architecture based up on components.
4. Visual modeling of the software.
5. Continuous verification of the quality.
6. Controlling the changes

- These best practices are used in the following two ways:

1. For driving the development process of the rational’s products.
2. To be used by the rational’s field teams so as to assist the customers in improving the predictability as well as the quality of the software development efforts.

- The task involves the assembling of the explicit process framework for the field of modern software engineering. 

- The delivery mechanism developed by the objector was based up on the HTML and employed in accomplishing this task.

- This task resulted in the creation of the rational unified process. 

- A set of content elements or the building blocks form the foundation for the rational unified process and give a description of the product that is to be produced and the required necessary skills. 

- They also give a detailed step by step explanation for achieving the specific development goals. 

Now we shall list all the building blocks of the rational unified process and discuss them in detail:

1. Roles: This building block can be defined as set consisting of related skills, responsibilities as well as competencies.
2. Work products: This building block gives the representation of thing that would result when a task would be completed inclusive of all the models and documentation produced during the course of the completion of that task.
3. Tasks: This building block gives a description of the units of works that are assigned to an element from the role which will produce a result that would be meaningful.

There are 9 disciplines in to which the tasks are categorized within each iteration. There are 6 engineering disciplines and 3 supporting disciplines which together make up total 9 disciplines. The 6 engineering disciplines are:

1. Business modeling
2. Requirements
3. Analysis and design
4. Implementation
5. Test and
6. Deployment

The following are the three supporting disciplines:

1. Configuration and change management
2. Environment and
3. Project management

The organization and the management of the above mentioned building blocks need to be solid and flexible in order to make the rational unified process a success which otherwise cannot be achieved.