Advantages and disadvantages of direct access file

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People also asked. What are the advantages and disadvantages of direct file organization? View results. What are advantages and disadvantages of direct memory access? It is not quite so obvious how sequential on the alternate keys is achieved.

This is covered in the unit on Indexed files. In the animation below you can see a representation of an Indexed file and its overlying primary key index.

Note that the index records point to the actual data records which are held in ascending primary key sequence. This animation shows how a direct read on the primary key is done. The record to be read has a key value of "Ni". In the animation we see how the index is used to find the required record we. The "hit rate" refers to the number of records in the file that are affected when updating a file. For instance, if only records are to affected by an insert, delete or amend operation in a file of 10, records then the hit rate is low.

But if 9, records are affected then the hit rate is high. Sequential files are very slow to update when the hit rate is low because the entire file must be read and then written to a new file, just to update a few records.

Sequential files are also complicated to change. Changes to Sequential files are batched together into a transaction file to minimize the low hit rate problem but this makes updating the Sequential file much more complicated than updating a direct access file. The complications come from having to match the records in the transaction file with those in the master file i.

When the hit rate is high this is the fastest method of updating a file because the record position does not have to be calculated and no indexes have to be traversed. This is the most storage efficient of all the file organizations. No indexes are required. Space from deleted records is recovered. Only room actually required to hold the records is allocated to the file.

If the file is only partially populated with records then this is a very wasteful file organization. The file will be allocated enough room to hold records from 1 to the highest Relative Record Number used, even if only a few records have actually been written to the file. For instance, if the first record written to the file has a Relative Record Number of 10, then room for that many records is allocated to the file. The fact that there is only one key and that it must be numeric and must take a value between 1 and the highest record number is limiting.

When a record is deleted in a Relative file, it is simply marked as deleted but the actual space that used to be occupied by the record is still allocated to the file see record position in Figure So if a Relative file is K in size when full, it will still be K when you have deleted half the records. The single key is limiting because it is often the case that we need to access the file on more than one key.

The fact that the key must be in the range 1 to the highest key value and that the file system allocates space for all the records between 1 and the highest Relative Record Number used, imposes severe constraints on the key. For instance even though the StudentId is numeric we couldn't use it as a key because the file system would allocate space for records from 1 to the highest StudentId written to the file. Suppose the highest StudentId written to the file was The file system would allocate space for 9,, records.

Sometimes we can get around the limitations of the key by using a transformation function to map the actual key on to the range of Relative Record Numbers. There are a number of possible transformation, or "hashing", functions, including truncation only using some of the digits in the key as the Relative Record Number , folding breaking the key into two or more parts and summing the parts , digit manipulation manipulating some of the digits in the key to produce a Relative Record Number and modulus division using the remainder of a division operation as the Relative Record Number.

Some transformation functions require special code to deal with duplication that occurs when the transformation of two different keys produces the same Relative Record Number. This is the fastest direct access organization. To reach a particular record, only a few simple calculations have to be done. Unlike Indexed files, which must store the indexes as well as the data, Relative files have only a small storage overhead.

As well as allowing direct access, Relative files allow sequential access to the records in the file. Because Indexed file achieve direct access by traversing a number of levels of index this is the slowest direct access organization. Because of this, Indexed files are substantially slower than Relative files.

They are especially slow when writing or deleting records because the primary key index and the alternate key indexes may need to be rebuilt. Indexed files require more storage than other file organizations because file must store;.

Indexed files can have multiple alphanumeric keys and only the primary key has to be unique. When we compare the number of disadvantages of Indexed files with the advantages we might be forgiven for thinking "Why would we ever use Indexed files? But the versatility of its keys overrides all its disadvantages with the result that Indexed files are the most widely used direct access file organization.

There is no one best file organization. Choosing an appropriate file organization is a case of "horses for courses". Direct access to networks was first time introduced in windows server , then in Windows 7 and Windows 8.

Old tape drives use sequential access while hard drives use direct access to read and write to files. In random access it may take longer time to read a large amount of data, the reason behind is that as data is stored in different random locations so hard drive head has to move to different locations many times. In mobile networks we use direct access for faster connection between different devices and data is retrieved instantly.

Differentiate between assembler, compiler and interpreter by Junaid Rehman. Sequential access vs direct access vs random access in operating Post Pagination Next Post Next. There is no restriction on the order of reading and writing for a direct access file. A block number provided by the user to the operating system is normally a relative block number , the first relative block of the file is 0 and then 1 and so on.

Index sequential method — It is the other method of accessing a file that is built on the top of the sequential access method. These methods construct an index for the file.

The index, like an index in the back of a book, contains the pointer to the various blocks. To find a record in the file, we first search the index, and then by the help of pointer we access the file directly.

Key points: It is built on top of Sequential access. It control the pointer by using index. Next File Allocation Methods. Recommended Articles.