Overview of physical storage media

Overview of Physical Storage Media

• Several types of data storage exist in most 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.

Types Of Storage Media

• cache• main memory• flash memory• magnetic disk• optical disk• magnetic tapes

Cache

• The cache is the fastest and most costly form of storage.

• Cachememory is relatively small• its use is managed by the computer system

hardware.

• We shall not be concerned about managing cache storage in the database system.

• It is, however, worth noting that database implementors do pay attention to cache effects when designing query processing data structures and algorithms.

Main memory

• The storage medium used for data that are available to be operated on is main memory.

• Although main memory may contain several gigabytes of data on a personal computer, or even hundreds of gigabytes of data in large server systems, it is generally too small (or too expensive) for storing the entire database.

• The contents of main memory are usually lost if a power failure or system crash occurs.

Flash memory

• Flash memory differs from main memory in that stored data are retained even if power is turned off (or fails).

• Two Types of Flash Memory:– Nand– Nor

• NAND flash has a much higher storage capacity for a given cost, and is widely used for data storage in devices such as cameras, music players, and cell phones, and increasingly, in laptop computers as well.

• Flash memory has a lower cost per byte than main memory, in addition to being nonvolatile; that is, it retains stored data even if power is switched off.

• Flash memory is alsowidely used for storing data in “USB keys,” which can be plugged into the Universal Serial Bus (USB) slots of computing devices.

• Flash memory is also increasingly used as a replacement for magnetic disks for storing moderate amounts of data.

• Such disk-drive replacements are called solid-state drives

• As of 2009, a 64 GB solid-state hard drive costs less than $200, and capacities range up to 160 GB.

• Further, flash memory is increasingly being used in server systems to improve performance by caching frequently used data, since it providesfaster access than disk, with larger storage capacity than main memory (for a given cost).

Magnetic-disk storage

• The primary medium for the long-term online storage of data is the magnetic disk.

• Usually, the entire database is stored on magnetic disk.

• The system must move the data from disk to main memory so that they can be accessed.

• After the system has performed the designated

• operations, the data that have been modified must be written to disk.

• As of 2009, the size of magnetic disks ranges from 80 gigabytes to 1.5 terabytes, and a 1 terabyte disk costs about $100.

• Disk capacities have been growing at about 50 percent per year, and we can expect disks of much larger capacity every year.

• Disk storage survives power failures and system crashes.

• Disk-storage devices themselves may sometimes fail and thus destroy data,

• but such failures usually occur much less frequently than do system crashes.

Optical storage.

• The most popular forms of optical storage are the

• Compact disk (CD), which can hold about 700 megabytes of data and has a playtime of about 80 minutes

• Digital video disk (DVD), which can hold 4.7 or 8.5 gigabytes of data per side of the disk (or up to 17 gigabytes on a two-sided disk).

• The expression digital versatile disk is also used in place of digital video disk, since DVDs can hold any digital data, not just video data.

• Data are stored optically on a disk, and are read by a laser.

• A higher capacity format called Blu-ray DVD can store 27 gigabytes per layer, or 54 gigabytes in a double-layer disk.

• The optical disks used in read-only compact disks (CD-ROM) or read-only digital video disks (DVD-ROM) cannot be written, but are supplied with data prerecorded.

• There are also “record-once” versions of compact disk (called CD-R) and digital video disk (called DVD-R and DVD+R), which can be written only once. such disks are also called write-once, read-many (WORM) disks.

• There are also “multiple-write” versions of compact disk (called CD-RW) and digital video disk (DVD-RW, DVD+RW, and DVD-RAM), which can be written multiple times.

• Optical disk jukebox systems contain a few drives and numerous disks that can be loaded into one of the drives automatically (by a robot arm) on demand.

Tape storage

• Tape storage is used primarily for backup and archival data.

• Although magnetic tape is cheaper than disks, access to data is much slower, because the tape must be accessed sequentially from the beginning.

• For this reason, tape storage is referred to as sequential-access storage.

• In contrast, disk storage is referred to as direct-access storage because it is possible to read data from any location on disk.

• Tape libraries (jukeboxes) are used to hold exceptionally large collections of data such as data from satellites,which could include as much as hundreds of terabytes (1 terabyte = 1012 bytes), or even multiple petabytes (1 petabyte = 1015 bytes) of data in a few cases.

• The fastest storage media—for example, cache and main memory—are referredto as primary storage.

• The media in the next level in the hierarchy—for example, magnetic disks—are referred to as secondary storage, or online storage.

• The media in the lowest level in the hierarchy—for example, magnetic tape and optical-disk jukeboxes—are referred to as tertiary storage, or offline storage.

• In addition to the speed and cost of the various storage systems, there is also the issue of storage volatility. Volatile storage loses its contents when the power to the device is removed

• In the hierarchy shown in Figure 10.1, the storage systems from main memory up are volatile, whereas the storage systems belowmain memory are nonvolatile. Data must be written to nonvolatile storage for safekeeping.

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