DBMS Storage Structure and Access Method

DBMS Storage Structure

Databases are stored in several file formats, which contains records. To store the database files or records there are several types of data storage exist in the computer systems. These storage media are classified by the speed with which data can be accessed, by the cost per unit of data to buy the medium, and by the medium's reliability. At the physical level, the actual data is stored in electromagnetic format on some device. These storage can be categorized into three types :

Primary Storage : The memory storage that is directly accessible to the CPU comes under this category. CPU's internal memory(registers), fast memory (cache), and main memory (RAM) are directly accessible to the CPU, as they are all placed on the motherboard or CPU chip-set. This storage is typically very small, ultra-fast, and volatile. Primary storage requires continuous power supply in order to maintain its state. In case of power failure, all its data is lost.

Secondary Storage : Secondary storage is used to protect interactive data written from a primary storage array to a non-volatile tier of disk, flash or tape. Secondary storage is synonymous with the terms secondary memory, auxiliary storage and external storage. Secondary storage is a trade-off between high performance and economical long-term archiving. Because it is accessed less frequently, data can be migrated to secondary storage devices with lower performance and costs. Secondary storage devices are used to store data for future use or as backup. Secondary storage devices are used to store data for future use or as backup. Secondary storage includes memory devices that are not a part of the CPU chip-set or motherboard, for example, magnetic disks, optional disks (DVD, CD, etc.), hard disks, flash drives, and magnetic tapes.

Tertiary Storage : The main objective of the tertiary storage level is to provide huge storage capacity at low cost. Several types of storage devices are available to be used at the tertiary storage level in Hierarchical Storage Systems (HSS). They include: • Magnetic tapes • Optical disks • Optical tapes These storage devices are composed of fixed storage drives and removable media units. The storage drives are fixed to the computer system. The removable media unit can be removed from the drives so that the storage capacity can be expanded with more media units. When data on a media are accessed, the media unit is accessed from their normal location. One of the storage drives on the computer system is chosen. If there is a media unit in the storage drive, the old media unit is unloaded and ejected. The new media unit is then loaded to the drive. Each type of storage drive may handle the storage drives and media units differently.

DBMS Access Methods : 

An access method is a software component, operating system service or network interface that handles the storage/retrieval and sending/receipt of data. Access methods provide an application programming interface (API) for programmers to perform these services, which rely on low-level, specialized instructions. Access methods provide a convenient service to programmers for managing data storage and transmission. While providing the programmer with flexibility, the abstraction hides the many details regarding low-level disk access and communication protocol operations. Access methods include internal structures to organize data as data sets, system-provided programs or macros to define data sets, and utility programs for data set processing. Error detection and correction capabilities are also provided. For example When a transaction is executed, then different memory blocks are assigned to the transaction to hold the data. The data residing in the database are stored in the physical memory like hard disk. The data location in the physical memory is called as physical blocks. When a transaction retrieves the data from these physical blocks, the same is copied to the main memory and are stored in buffer blocks. These are temporary storage blocks and are helpful in accessing the data again and again. This reduces the access to physical blocks and hence time / cost are also reduced.

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