Advance Database System Lecture # 2 Department of Computer Science The University of Lahore April 16, 2017 ADBS: Storage

Disk Storage, Basic File Structures April 16, 2017 ADBS: Storage

Review of Previous Lecture Overview of Database Design Process Example Database Application (COMPANY) ER Model Concepts Entities and Attributes Entity Types, Value Sets, and Key Attributes Relationships and Relationship Types ER Diagrams - Notation ER Diagram for COMPANY Schema April 16, 2017 ADBS: Storage

Scope of Today Lecture The Storage Hierarchy Disk Storage Devices Records Blocking Files of Records Unordered Files Ordered Files April 16, 2017 ADBS: Storage

Storage Medium The collection of data that makes up a computerized database must be stored physically on some computer storage medium. The DBMS software can then retrieve, update, and process this data as needed. Computer storage media form a storage hierarchy that includes two main categories. Primary storage Secondary storage April 16, 2017 ADBS: Storage

- The Storage Hierarchy April 16, 2017 ADBS: Storage

Disk Storage Devices Preferred secondary storage device for high storage capacity and low cost. Data stored as magnetized areas on magnetic disk surfaces. A disk pack contains several magnetic disks connected to a rotating spindle. Disks are divided into concentric circular tracks on each disk surface. Track capacities vary typically from 4 to 50 Kbytes. April 16, 2017 ADBS: Storage

Disk Storage Devices Because a track usually contains a large amount of information, it is divided into smaller blocks or sectors. The division of a track into sectors is hard-coded on the disk surface and cannot be changed. One type of sector organization calls a portion of a track that subtends a fixed angle at the center as a sector. A track is divided into blocks. The block size B is fixed for each system. Typical block sizes range from B=512 bytes to B=4096 bytes. Whole blocks are transferred between disk and main memory for processing. April 16, 2017 ADBS: Storage

… - Disk Storage Devices … April 16, 2017 ADBS: Storage

Disk Storage Devices The division of a track into equal sized disk blocks is set by the operating system during disk formatting (initialization of disk). Disk blocks are separate by fixed size inter block gap, which included special coded information at time of initialization. A disk controller, typically embedded in the disk drive, control the disk drive and interfaces it to the computer system. The time required to find the desire track is called seek time. The time required to find the desire block is called rotational delay April 16, 2017 ADBS: Storage

… - Disk Storage Devices … April 16, 2017 ADBS: Storage

Hard Drive Internal April 16, 2017 ADBS: Storage

Seek/Rotation Time April 16, 2017 ADBS: Storage

Example What is the total capacity of a track? Block/sector size B = 512 bytes Interblock gap size G = 128 bytes Number of blocks per track = 20 Number of tracks per surface = 400. A disk pack consists of 15 double-sided disks. What is the total capacity of a track? Total Track Capacity = (Block Size + Gap Size) * Total No. Of Block on One Track Total track capacity = (512 + 128) * 20 = 12800 bytes April 16, 2017 ADBS: Storage

Example How many cylinders are there? Block/sector size B = 512 bytes Interblock gap size G = 128 bytes Number of blocks per track = 20 Number of tracks per surface = 400. A disk pack consists of 15 double-sided disks. How many cylinders are there? Total No Of Cylinders = Total No Of Tracks Per Surface Total No Of Cylinders = 400 April 16, 2017 ADBS: Storage

Example What are the total capacity of a cylinder ? Block/sector size B = 512 bytes Interblock gap size G = 128 bytes Number of blocks per track = 20 Number of tracks per surface = 400. A disk pack consists of 15 double-sided disks. What are the total capacity of a cylinder ? Total Capacity of Cylinder = Total Capacity of One Track * Total No of Surfaces of Hard Disk Total Capacity of Cylinder = 12800 * 30 = 384000 Bytes April 16, 2017 ADBS: Storage

Example What are the total capacity of a disk pack ? Block/sector size B = 512 bytes Interblock gap size G = 128 bytes Number of blocks per track = 20 Number of tracks per surface = 400. A disk pack consists of 15 double-sided disks. What are the total capacity of a disk pack ? Total Capacity of Disk Pack = 2 (Total No of Tracks per Surface) * Total Track Capacity Total Capacity of Disk Pack = 2 (400) * 12800 = 10240000Bytes April 16, 2017 ADBS: Storage

Example The average seek time is = 30 msec The average rotational delay is = 30 msec. Block transfer time per block = btt is 0.17 msec. How much time does it take (on the average) in msec to locate and transfer a single block, given its block address? Average Time to Transfer Single Block = Seek Time + Rotational Delay + Block Transfer Time Average Time to Transfer Single Block = 30 + 30 + 0.17 = 60.17 msec April 16, 2017 ADBS: Storage

Formulas rpm = 1000 Rotational delay rd in msec = (60 * 1000) / (2 * rpm) Transfer rate tr = track capacity in bytes / (60 * 1000/rpm) One Block transfer time Btt = B / tr April 16, 2017 ADBS: Storage

Buffering of Blocks We have two schemes in execution Single buffering Interleave Parallel Single buffering Double buffering April 16, 2017 ADBS: Storage

Records Fixed and variable length records Records contain fields which have values of a particular type (e.g., amount, date, time, age) Fields themselves may be fixed length or variable length Variable length fields can be mixed into one record: separator characters or length fields are needed so that the record can be “parsed”. April 16, 2017 ADBS: Storage

Blocking Blocking: refers to storing a number of records in one block on the disk. Blocking factor (bfr) refers to the number of records per block. [bfr = flooring (B/R)] There may be empty space in a block if an integral number of records do not fit in one block. [B - bfr * R] Spanned Records: refer to records that exceed the size of one or more blocks and hence span a number of blocks. April 16, 2017 ADBS: Storage

Files of Records A file is a sequence of records, where each record is a collection of data values (or data items). A file descriptor (or file header ) includes information that describes the file, such as the field names and their data types, and the addresses of the file blocks on disk. Records are stored on disk blocks. The blocking factor bfr for a file is the (average) number of file records stored in a disk block. A file can have fixed-length records or variable-length records. April 16, 2017 ADBS: Storage

Files of Records File records can be unspanned (no record can span two blocks) or spanned (a record can be stored in more than one block). The physical disk blocks that are allocated to hold the records of a file can be contiguous, linked, or indexed. In a file of fixed-length records, all records have the same format. Files of variable-length records require additional information to be stored in each record, such as separator characters and field types. April 16, 2017 ADBS: Storage

Unordered Files Also called a heap or a pile file. New records are inserted at the end of the file. To search for a record, a linear search through the file records is necessary. This requires reading and searching half the file blocks on the average, and is hence quite expensive. Record insertion is quite efficient. Reading the records in order of a particular field requires sorting the file records. April 16, 2017 ADBS: Storage

Ordered Files Also called a sequential file. File records are kept sorted by the values of an ordering field. Insertion is expensive: records must be inserted in the correct order. A binary search can be used to search for a record on its ordering field value. This requires reading and searching log2 of the file blocks on the average, an improvement over linear search. Reading the records in order of the ordering field is quite efficient. April 16, 2017 ADBS: Storage

Ordered Files April 16, 2017 ADBS: Storage

Average Access Times The following table shows the average access time to access a specific record for a given type of file April 16, 2017 ADBS: Storage

Example Calculate the record size R in bytes? r = 20,000 STUDENT records of fixed length. Each record has the following fields NAME (30 bytes), SSN (9 bytes), ADDRESS (40 bytes), PHDNE (9 bytes), BIRTHDATE (8 bytes), SEX (l byte), MAJORDEPTCODE (4 bytes), MINORDEPTCODE (4 bytes), CLASSCODE (4 bytes, integer), and DEGREEPROGRAM (3 bytes). An additional byte is used as a deletion marker. Block size B = 512 bytes Calculate the record size R in bytes? R = 30+9+40+9+8+1+4+4+4+3+1 = 113 bytes Calculate the blocking factor bfr and the number of file blocks b, assuming an unspanned organization Bfr = flooring(B/R) Bfr = flooring(512/113) = 4/block. b = ceiling (r/bfr = ceiling(20000/4)) = 5000 block April 16, 2017 ADBS: Storage

Term Paper / Project Directions Implementing Hashing Other Hashing Techniques in Dynamic Hashing File Organization in OODBMS File Organization in Spatial DBMS File Organization in Multi-Media DBMS April 16, 2017 ADBS: Storage

Review of Lecture The Storage Hierarchy Disk Storage Devices Records Blocking Files of Records Unordered Files Ordered Files April 16, 2017 ADBS: Storage