Physical Storage Media

TRENDS IN DATABASE TECHNOLOGY

Classification of Physical Storage Media

Based on Speed with which data can be accessed Based on Cost per unit of data Based on eliability Data loss on power failure or system crash Physical failure of the storage device H Based on life of storage Volatile storage: loses contents when power is switched off 4 Non-volatile storage:

Contents persist even when power is switched off. Includes secondary and tertiary storage, as well as batterbacked up main-memory.

Physical Storage Media

Cache

The fastest and most costly form of storage Volatile Managed by the computer system hardware.

Main memory

4 Fast access (10s to 100s of nanoseconds; 1 nanosecond = 10–9 seconds) 4 Generally too small (or too expensive) to store the entire database

Capacities of up to a few Gigabytes widely used currently Capacities have gone up and per-byte costs have decreased steadily and rapidly

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

Flash memory

Data survives power failureData can be written at a location only once, but location can be erased and written to again Can support only a limited number of write/erase cycles.

Erasing of memory has to be done to an entire bank of memory.

Reads are roughly as fast as main memory But writes are slow (few microseconds), erase is slower Cost per unit of storage roughly similar to main memory Widely used in embedded devices such as digital cameras Also known as EEPROM

Magnetic-disk

Data is stored on spinning disk,and read/written magneticallyPrimary medium for the long-term storage of data;typically stores entire database.Data must be moved from disk to main memory foraccess, and written back for storage

Much slower accessthan main memory (more on this later)

Direct-access – possible to read data on disk in any order,unlike magnetic tapeHard disks vs. floppy disks Capacities range up to roughly 100 GB currently

Much larger capacity and cost/byte than main memory/ flash memory Growing constantly and rapidly with technology improvements

Survives power failures and system crashes

Disk failure candestroy data, but is very rare.

Optical storage

Non-volatile, data is read optically from a spinning disk using a laser CD-ROM (640 MB) and DVD (4.7 to 17 GB) most popular forms Write-one, read-many (WORM) optical disks used for archival storage (CD-R and DVD-R) Multiple write versions also available (CD-RW, DVDRW, and DVD-RAM) 4 Reads and writes are slower than with magnetic disk Juke-box systems, with large numbers of removable disks, a few drives, and a mechanism for automatic loading/unloading of disks available for storing large volumes of data.

Optical Disks

Compact disk-read only memory (CD-ROM) Disks can be loaded into or removed from a drive 4 High storage capacity (640 MB per disk) High seek times or about 100 msec (optical read head is heavier and slower) Higher latency (3000 RPM) and lower data-transfer rates (3-6 MB/s) compared to magnetic disks

Digital Video Disk (DVD)

DVD-5 holds 4.7 GB, and DVD-9 holds 8.5 GB DVD-10 and DVD-18 are double sided formats with capacities of 9.4 GB & 17 GB Other characteristics similar to CD-ROM 4 Record once versions (CD-R and DVD-R) Data can only be written once, and cannot be erased. High capacity and long lifetime; used for archival storage 4Multi-write versions (CD-RW, DVD-RW and DVDRAM) also available

Tape storage

Non-volatile, used primarily for backup (to recover from disk failure), and for archival data Sequential-access – much slower than disk Very high capacity (40 to 300 GB tapes available) Tape can be removed from drive storage costs much cheaper than disk, but drives are expensive 4 Tape jukeboxes available for storing massive amounts of data Hundreds of terabytes (1 terabyte = 109 bytes) to even apetabyte (1 petabyte = 1012 bytes)

Storage Hierarchy

Primary storage: Fastest media but volatile (cache, main memory).

Secondary storage: next level in hierarchy, non-volatile, moderately fast access time. Also called on-line storage

4E.g. flash memory, magnetic disks

Tertiary storage: lowest level in hierarchy, non-volatile, slow access time Also called off-line storage

E.g. magnetic tape, optical storage

Magnetic Hard Disk Mechanism

n Read-write head Positioned very close to the platter surface (almost touching it) Reads or writes magnetically encoded information. Surface of platter divided into circular tracks Over 17,000 tracks per platter on typical hard disks Each track is divided into sectors. A sector is the smallest unit of data that can be read or written. H Sector size typically 512 bytes Typical sectors per track: 200 (on inner tracks) to 400 (on outer tracks) To read/write a sector Disk arm swings to position head on right track Platter spins continually; data is read/written as sector passes under head Head-disk assemblies Multiple disk platters on a single spindle (typically 2 to 4) One head per platter, mounted on a common arm. Cylinder i consists of ith track of all the platters

Performance Measures of Disks

Access Time – the time it takes from when a read or write request is issued to when data transfer begins. Consists of:

Seek Time – time it takes to reposition the arm over the correct track.

Average Seek Time is 1/2 the worst case seek time.

– Would be 1/3 if all tracks had the same number of sectors, and we ignore the time to start and stop arm movement 4 to 10 milliseconds on typical disks

Rotational latency – time it takes for the sector to be accessed to appear under the head.

Average latency is 1/2 of the worst-case latency. 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) n Data-Transfer Rate – the rate at which data can be retrieved from or stored to the disk. H4 to 8 MB per second is typical

Multiple disks may share a controller, so rate that controller can handle is also important Mean Time To Failure (MTTF) – the average time the disk is expected to run continuously without any failure. Typically 3 to 5 years Probability of failure of new disks is quite low, corresponding to a ―theoretical MTTF‖ of 30,000 to 1,200,000 hours for a new disk.