Database Management Systems,Shri Prasad Sawant. 1 Storing Data: Disks and Files Unit 1 Mr.Prasad Sawant

Database Management Systems,Shri Prasad Sawant. 2 Introduction  DBMS has to store data somewhere  Choices:  Main memory Expensive – compared to secondary and tertiary storage Fast – in memory operations are fast Volatile – not possible to save data from one run to its next Used for storing current data  Secondary storage (hard disk) Less expensive – compared to main memory Slower – compared to main memory, faster compared to tapes Persistent – data from one run can be saved to the disk to be used in the next run Used for storing the database  Tertiary storage (tapes) Cheapest Slowest – sequential data access Used for data archives

Database Management Systems,Shri Prasad Sawant. 3 Why Not Store Everything in Main Memory?  Costs too much.  Main memory is volatile. We want data to be saved between runs. (Obviously!)  Typical storage hierarchy:  Main memory (RAM) for currently used data.  Disk for the main database (secondary storage).  Tapes for archiving older versions of the data (tertiary storage).

Database Management Systems,Shri Prasad Sawant. 4 Disks  Secondary storage device of choice.  Main advantage over tapes: random access vs. sequential.  Data is stored and retrieved in units called disk blocks or pages.

Database Management Systems,Shri Prasad Sawant. 5 Components of a Disk Platters  The platters spin (say, 90rps). Spindle  The arm assembly is moved in or out to position a head on a desired track. Tracks under heads make a cylinder (imaginary!). Disk head Arm movement Arm assembly  Only one head reads/writes at any one time. Tracks Sector  Block size is a multiple of sector size (which is fixed).

Database Management Systems,Shri Prasad Sawant. 6 Animated

Database Management Systems,Shri Prasad Sawant. 7 Accessing a Disk Page  Time to access (read/write) a disk block:  seek time ( moving arms to position disk head on track )  rotational delay ( waiting for block to rotate under head )  transfer time ( actually moving data to/from disk surface )  Seek time and rotational delay dominate.  Seek time varies from about 1 to 20msec  Rotational delay varies from 0 to 10msec  Transfer rate is about 1msec per 4KB page

Database Management Systems,Shri Prasad Sawant. 8 File Organization Three types of file organization  Unordered or Heap files  Ordered or sequential files  Hash files

Database Management Systems,Shri Prasad Sawant. 9 Unordered Or Heap File  Records are stored in the same order in which they are created  Insert operation  Fast – because the incoming record is written at the end of the last page of the file  Search (or update) operation  Slow – because linear search is performed on pages  Delete Operation  Slow – because the record to be deleted is first searched for  Deleting the record creates a hole in the page  Periodic file compacting work required to reclaim the wasted space

Database Management Systems,Shri Prasad Sawant. 10 Ordered or Sequential File  Records are sorted on the values of one or more fields  Ordering field – the field on which the records are sorted  Ordering key – the key of the file when it is used for record sorting  Search (or update) Operation  Fast – because binary search is performed on sorted records  Update the ordering field?  Delete Operation  Fast – because searching the record is fast  Periodic file compacting work is, of course, required  Insert Operation  Poor – because if we insert the new record in the correct position we need to shift all the subsequent records in the file  Alternatively an ‘overflow file’ is created which contains all the new records as a heap  Periodically overflow file is merged with the main file  If overflow file is created search and delete operations for records in the overflow file have to be linear!

Database Management Systems,Shri Prasad Sawant. 11 Files of Records  Page or block is OK when doing I/O, but higher levels of DBMS operate on records, and files of records.  FILE : A collection of pages, each containing a collection of records. Must support:  insert/delete/modify record  read a particular record (specified using record id )  scan all records (possibly with some conditions on the records to be retrieved)

Database Management Systems,Shri Prasad Sawant. 12 System Catalogs  For each index:  structure (e.g., B+ tree) and search key fields  For each relation:  name, file name, file structure (e.g., Heap file)  attribute name and type, for each attribute  index name, for each index  integrity constraints  For each view:  view name and definition  Plus statistics, authorization, buffer pool size, etc. * Catalogs are themselves stored as relations !