Recap of Feb 20 Database Design Goals Normalization Normal Forms Goals for designing a database a schema with simple easy to phrase queries avoids redundancies repetition of information avoids anomalies good performance Normalization decompose complex relations Lossy decompositions Functional Dependencies Normal Forms 1NF 2NF 3NF BCNF BCNF or 3NF lossless decomposition in both BCNF can t always ensure dependency preservation 3NF sometimes requires null values or redundant information Getting Physical Storage and File Structure Chapter 11 Up until now we have examined database design from a high level conceptual view passing over actual implementation and underlying hardware Appropriate focus for database users But hardware does have an influence on implementation and implementation does have an influence on what conceptual designs will be more efficient and useful Now we get physical examine physical storage media to give a background for later focus on implementation of the data models and languages already described Chapter 11 At this point we are focussing on the following sections 11 1 Overview of Physical Storage Media 11 2 Magnetic Disks 11 3 RAID very briefly 11 4 Tertiary Storage 11 5 Storage Access 11 6 File Organization 11 7 Organization of Records in Files 11 8 Data Dictionary Storage Midterm As per the syllabus the Midterm will cover the pre midterm lecture notes plus sections 1 2 3 except 3 4 3 5 4 6 7 7 7 11 except 11 3 and 11 9 of the text We have examined all that material except chapter 11 which starts today Scheduling the midterm depends upon the speed with which we get through the material It looks as if we ll have enough time to schedule one day for review before the midterm My current guess is that we might be able to schedule the midterm as early as March 13 certainly no later than March 20 I m aiming for Tuesday March 18 Midterm Study and Homework 2 Material you are responsible for All material presented in class before the midterm Textbook sections 1 2 3 except 3 4 3 5 4 6 7 7 7 11 except 11 3 and 11 9 The homework questions from assignment 1 exercises 1 1 1 2 1 3 and 2 1 2 6 are all useful study aids as are the questions from homework assignment 2 3 2 3 3 3 5 3 6 3 9 3 16 4 1 4 2 4 4 4 8 7 2 7 4 7 5 7 11 7 12 7 15 7 16 7 21 7 23 Homework 2 is due Tuesday March 11 I ll try to have them back to you graded Thursday March 13 so you can use them as a study aid for the exam Tuesday March 18 Classification of Physical Storage Media Media are classified according to three characteristics speed of access cost per unit of data reliability data loss on power failure or system crash physical failure of the storage device We can also differentiate storage as either volatile storage non volative storage Physical Storage Media Overview 11 1 Typical media available are Cache Main memory Flash memory Mag disk Optical storage CD or DVD Tape storage Physical Storage Media Cache and Main Memory Cache fastest and most costly form of storage volatile managed by computer system hardware Main memory fast access 10s to 100s of nanoseconds generally too small or expensive to hold the entire database current capacities commonly used are up to a few Gigabites capacities have gone up and per byte costs have decreased steadily roughly a factor of 2 every 2 3 years volatile Physical Storage Media Flash Memory Also known as EEPROM Electrically Erasable Programmable ReadOnly Memory non volatile reading data is comparable to main memory speeds writing is more complex can t overwrite a single location a whole bank of memory must be erased to permit writing within that bank erasing is only supported a limited number of times 10 000 to one million erase cycles writes are slow a few microseconds and erases are slower cost comparable to main memory widely used in computer systems embedded in other devices such as digital cameras and hand held computers Physical Storage Media Magnetic Disk data is stored on a spinning disk and read written magnetically primary medium for long term storage of data typically stores entire database data must be moved from disk to main memory for access and written back for storage much slower access than main memory about which more later direct access possible to read data on disk in any order unlike magnetic tape capacities up to 100 gig much larger capacity and cheaper cost byte than main memory or flash memory capacity doubles every two or three years survives power failures and system crashes disk failure can destroy data but this is more rare than system crashes Physical Storage Media 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 once Read many WORM optical disks used for archival storage CD R and DCD R Multiple write versions also available CD RW DVD RW and DVDRAM Reads and writes are slower than with magnetic disk Juke box systems available for storing large volumes of data large numbers of removable disks several drives mechanism for automatic loading unloading of disks Physical Storage Media 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 40 300 GB tapes available tape can be removed from drive storage costs much cheaper than disk but drives are expensive data is read optically from a spinning disk using a laser Juke box systems available for storing large volumes of data e g remote sensing data possibly hundreds of terabytes 1012 bytes or even a petabyte 1015 bytes Storage Hierarchy Primary storage fastest media but volatile cache main memory secondary storage next level in hierarchy moderately fast access time non volatile also called on line storage flash memory magnetic disks tertiary storage lowest level in hierarchy slower access time nonvolatile also called off line storage optical storage magnetic tape Magnetic Disks 11 2 Read write head positioned very close to the platter surface almost touching it Reads or writes magnetically coded information Surface of platter divided into circular tracks over 16 000 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 sector size is typically 512 bytes typically 200 on inner tracks to 400 outer tracks sectors per track Magnetic Disks cont To read write a sector disk arm swings to position head on the right
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