Electronic Data Is a Sequence of Bits. This Data Can Either Reside In

Secondary Storage

Electronic data is a sequence of bits. This data can either reside in

•primary storage - main memory (RAM), relatively small, fast access, expensive (cost per MB), volatile (go away when power goes off)

•secondary storage - disks, tape, large amounts of data, slower access, cheap (cost per MB), persistent (remain even when power is off)

We will focus on secondary storage since the collections of data in databases are usually both too large to fit in primary storage and must be persistent.

Hard Disks

Features

•spinning platter of special material

•mechanical arm with read/write head must be close to the platter to read/write data

•data is stored magnetically (if you'd like to keep your data it is usually best to avoid using powerful magnets near your hard disk)

•sometimes the mechanical arm digs into platter, resulting in a very bad crash and subsequent loss of data on part of your hard disk

•storage capacity is commonly between 2GB - 11GB

•disks are random access meaning data can be read/written anywhere on the disk

•to read a piece of data, the mechanical arm must be repositioned over the place in the platter where that data is stored, this is called the disk seek. 8 to 15 milliseconds is a common seek time.

•once the arm has been positioned the data transfer rate varies, but commonly is between 1MB and 10MB a second

•a 5GB hard disk will cost anywhere from $300 to $1500, there are many options and vendors

•SCSI (Small Computer System Interface), special hardware to improve throughput, 100s MB per second transfer rates

•solid state hard disks, with no mechanical parts, are starting to become commercially available, they are generally faster and more expensive

Diskette or Floppy Disk

•spinning platter of special material

•information stored by magnetically

•read/write head positioned by mechanical arm

•storage capacity is at a few MBs

•random access

•seek time from 10 to 40 milliseconds

•easily portable

Removable Hard Disk

•like hard disk; designed to permit disk and/or disk drive to be removed and slotted into another machine within seconds.

•more expensive than hard disk

•less reliable

Optical Disks

•CD-ROM - read only (books, software releases)

•WORM - write once, read many (archival storage)

•laser encoding, not magnetic

•30-50 ms seek times

•640MB - 17GB storage capacity

•cheaper than hard disks per MB of storage capacity, but slower

•portable

•Jukeboxes of optical disks are becoming popular for storing really, really large collections of data. The Mercury-20 jukebox (no I'm not selling these, just using it as a typical example) provides access to up to 150 CD-ROMs, or in other words 94GBs of storage capacity. The Mercury jukebox takes a maximum of four seconds to exchange

and load a disc into a drive, 2.5 seconds to spin up and access the data and 10 seconds to transfer a 6.0 MB file to the computer or server