Physical Storage Media

Physical Storage Media

It differentiates the storage into two categories and they are:

• Volatile storage: It loses contents when power is switched off.
• Non-volatile storage: The contents persist even when power is switched off. The non-volatile storage includes secondary and tertiary storage.

Storage Type:

Primary Storage:Primary storage is considered as the fastest media but is volatile. Some of the examples of primary storage are cache memory and main memory.

Secondary Storage: Secondary storage is taken to the next level in the hierarchy. It is non-volatile and has moderately fast access time. It is also called as on-line storage. Examples of secondary storage are flash memory, magnetic disks.

TertiaryStorage: Tertiary storage is the lowest level in the hierarchy. It is non-volatile and has slow access time. It is also called as off-line storage. Examples of tertiary storage are magnetic tape, optical storage.

Cache: Cache is the fastest and most costly form of storage. It is volatile and managed by the computer system hardware.

Main Memory: It has fast access of about 10s to 100s of nanoseconds which is 10^-9 seconds. Main Memory is generally too small or too expensive to store the entire database. It has capacities of up to a few Gigabytes widely used currently. Now the capacities have gone up and costs per byte have decreased steadily and rapidly. Main memory is volatile as contents of main memory are usually lost if a power failure or system crash takes place.

Flash Memory: In flash memory data survives even in power failures. Data can be written at a location only once but location can be erased and written again. Flash memory can support only a limited number of write or erase cycles say 10K to 1M. Erasing of memory has to be done to an entire bank of memory. Reads in the flash memory are roughly as fast as in main memory but writes are slow and erase is slower. The cost per unit of storage is roughly similar to main memory. Flash memory is widely used in embedded devices such as digital cameras. It is a type of EEPROM where EEPROM stands for Electrically Erasable Programmable Read-Only Memory).

Magnetic-disk:In magnetic disk data is stored on spinning disk and read or writes magnetically. The magnetic disk is the primary medium for the long-term storage of disks that typically stores entire database. Data must be moved from disk to main memory for access, and written back to storage. It has much slower access than main memory. It has a direct access which means it is possible to read data on disk in any order, unlike magnetic tape. Capacities of magnetic disk range up to 1000 GB roughly. It has the much larger capacity and cost or byte than main memory or flashes memory. The capacity is growing rapidly and constantly with technology improvements. Data stored on magnetic disk survives power failures and system crashes. However, disk failure can destroy data but is rare in the case of the magnetic disk.

Read-write head: The read-write head is positioned very close to the platter surface. It reads or writes the magnetically encoded information. The surface of the platter is divided into circular tracks. There are over 50K-100K tracks per platter on typical hard disks.

Each track is divided into sectors where a sector is the smallest units of data that can be read or written. Sector size is typically 512 bytes. Typical sectors per track are 500 (on inner tracks) to 1000 (on outer tracks). In order to read or write a sector, we should swing disk arm to position head on the right track. The platter spins continually where data is read or written as sector passes under the head. Head-disk assemblies are the multiple disk platters on a single spindle usually 1 to 5. One head per platter is mounted on a common arm. Cylinder i consists of i^th track of all the platters. TheDisk Controller interfaces between the computer system and the disk drive hardware. It accepts high-level commands to read or write a sector. The controller initiates actions such as moving the disk arm to the right track and actually reading or writing the data. It computes and attaches checksums to each sector to verify that data is read back correctly. If data is corrupted with very high probability then the stored checksum won't match precomputed checksums. The controller ensures successful writing by reading back sector after writing it. It also performs remapping of bad sectors.

Performance Measures of Disks

• Access time is the time it takes from when a read or write request is issued to when data transfer begins.
• Seek time is the time it takes to re-position the arm over the correct track. Average seek time is half the worst case seek time. It would be one-third if all tracks had the same number of sectors.
• Rotational latency is the time it takes for the sector to be accessed to appear under the head. Average latency is half of the worst case latency.
• Data transfer rate is the rate at which data can be retrieved from or stored to the disk.

Optical Storage: It is non-volatile and data is read optically from a spinning disk using a laser. CD-ROM of size 640 MB and DVD of size ranging from 4.7 to 17 GB are the most popular forms of optical storage. Write-one, read-many (WORM) optical disks are used for archival storage (CD-R, DVD-R, DVD+R). Multiple write versions are also available (CD-RW, DVD-RW, DVD+RW, and DVD-RAM). Reads and writes are slower with optical storage than with magnetic disk. Luke-box systems with a large number of removable disks, a few drivers and a mechanism for automatic loading or unloading of disks are available for storing large volumes of data.

Tape Storage:Tape storage is non-volatile and used primarily for backup in order to recover from disk failure and for archival data. It has a sequential access and thus much slower than disk. However, it has very high capacity and about 40 to 300 GB tapes are available. The tapes can be removed from drive because storage costs are much cheaper than disk but drivers are expensive. Tape jukeboxes are available for storing the massive amount of data.

References:

1. H.F.Korth and A. Silberschatz,"Database system concepts",McGraw Hill,2010
2. A.K.Majumdar and p, Bhatt acharya,"Database Management Systems",Tata McGraw Hill,India,2004
3. F.Korth, Henry. Database System Concepts. 6th edition.