DBMS STORAGE MEDIA
FILE ORGANIZATION : STORAGE MEDIA
introduction:
the physical or internal level, organization of a database system is concerned with the
efficient storage of information in the secondary storage devices. At this
level the user's are no longer concerned
with the view of the database (how data is stored). User can retrieve the data
from the physical level to the logical level by providing necessary mapping
from the physical level to the logical level . so, the basic problem in
physical database representation is to select a suitable file system to store the desired information and retrieve it later. The file consist of records and a record may consist of several fields.
The efficiency of a file system depends on how efficiently the operations : Retrieve, Insert, Update and Delete, can be performed on the file. However, the efficiency of these operations on a particular file may depend on the type of queries the file system needs to process. For example, a sequential file system is more efficient. Where the storage and retrieval of records are performed sequentially. The same file system turns out to be inefficient in an environment where records are to be retrieved or updated randomly.
The efficiency of a file system depends on how efficiently the operations : Retrieve, Insert, Update and Delete, can be performed on the file. However, the efficiency of these operations on a particular file may depend on the type of queries the file system needs to process. For example, a sequential file system is more efficient. Where the storage and retrieval of records are performed sequentially. The same file system turns out to be inefficient in an environment where records are to be retrieved or updated randomly.
Data in a DBMS is stored on storage devices such as disks
and tapes. The disk space manager is responsible for keeping track of available
disk space. the file manager, which provides the abstraction of a file of records to higher levels of DBMS code, issues requests to the disk space
manager to obtain space on disk. the file management layer requests and frees
disk space in units of a page; the size of a page is a DBMS parameter and
typical values are 4 KB or 8 KB. the file management layer is responsible for
keeping track of the pages in a file and for arranging records within pages.
When a record is needed for processing, it must be fetched from disk to main memory. The page on which the records resides is determined by the file manager. Sometimes, the file manager uses supporting data structures to quickly identify the page that contains a desired record. After identifying the required page, the file manager issues a request for the page to a layer of DBMS code called the buffer manager. The buffer manager fetches a requested page from disk into a region of main memory called the buffer pool and tells the file manager, the location of the requested page.
Thus, the glimpse of the physical level in DBMS focuses us on the study of details of memory hierarchy, redundant arrays of independent disks (RAID), disk space management etc. records and a record may consist of several fields.
When a record is needed for processing, it must be fetched from disk to main memory. The page on which the records resides is determined by the file manager. Sometimes, the file manager uses supporting data structures to quickly identify the page that contains a desired record. After identifying the required page, the file manager issues a request for the page to a layer of DBMS code called the buffer manager. The buffer manager fetches a requested page from disk into a region of main memory called the buffer pool and tells the file manager, the location of the requested page.
No comments:
Post a Comment