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Data Storage and Access Methods
Min SongIS698
Database Design Process
ConceptualModel
LogicalModel
External Model
Conceptual requirements
Conceptual requirements
Conceptual requirements
Conceptual requirements
Application 1
Application 1
Application 2 Application 3 Application 4
Application 2
Application 3
Application 4
External Model
External Model
External Model
Internal Model
PhysicalDesign
Physical Database Design Many physical database design decisions are
implicit in the technology adopted Also, organizations may have standards or
an “information architecture” that specifies operating systems, DBMS, and data access languages -- thus constraining the range of possible physical implementations.
We will be concerned with some of the possible physical implementation issues
Physical Database Design
The primary goal of physical database design is data processing efficiency
We will concentrate on choices often available to optimize performance of database services
Physical Database Design requires information gathered during earlier stages of the design process
Physical Design Information Information needed for physical file and
database design includes: Normalized relations plus size estimates for them Definitions of each attribute Descriptions of where and when data are used
entered, retrieved, deleted, updated, and how often
Expectations and requirements for response time, and data security, backup, recovery, retention and integrity
Descriptions of the technologies used to implement the database
Physical Design Decisions
There are several critical decisions that will affect the integrity and performance of the system Storage Format Physical record composition Data arrangement Indexes Query optimization and performance
tuning
Storage Format
Choosing the storage format of each field (attribute). The DBMS provides some set of data types that can be used for the physical storage of fields in the database
Data Type (format) is chosen to minimize storage space and maximize data integrity
Objectives of data type selection Minimize storage space Represent all possible values Improve data integrity Support all data manipulations The correct data type should, in minimal
space, represent every possible value (but eliminate illegal values) for the associated attribute and can support the required data manipulations (e.g. numerical or string operations)
Access Data Types Numeric (1, 2, 4, 8 bytes, fixed or float) Text (255 max) Memo (64000 max) Date/Time (8 bytes) Currency (8 bytes, 15 digits + 4 digits decimal) Autonumber (4 bytes) Yes/No (1 bit) OLE (limited only by disk space) Hyperlinks (up to 64000 chars)
Access Numeric types Byte
Stores numbers from 0 to 255 (no fractions). 1 byte Integer
Stores numbers from –32,768 to 32,767 (no fractions) 2 bytes
Long Integer (Default) Stores numbers from –2,147,483,648 to 2,147,483,647 (no
fractions). 4 bytes Single
Stores numbers from -3.402823E38 to –1.401298E–45 for negative values and from 1.401298E–45 to 3.402823E38 for positive values. 4 bytes
Double Stores numbers from –1.79769313486231E308 to –
4.94065645841247E–324 for negative values and from 1.79769313486231E308 to 4.94065645841247E–324 for positive values. 15 8 bytes
Replication ID Globally unique identifier (GUID) N/A 16 bytes
Designing Physical Records
A physical record is a group of fields stored in adjacent memory locations and retrieved together as a unit
Fixed Length and variable fields
Data Storage
Storing Data: Disks Buffer manager Representing relational data in a disk
The Memory Hierarchy
Main Memory = Disk Cache•Volatile• 256M-1G•Access time: 10-100 nanoseconds
•Persistent •10-100 GB storage• speed:
•Rate=5-10 MB/S•Access time=
10-15 msecs.
• 1.5 MB/S transfer rate• 280 GB typical capacity• Only sequential access• Not for operational data
Processor Cache:• access time 10 nano’s• 512K
Disk Tape
Main Memory Fastest, most expensive (excluding
cache) Today: 512MB are common even on
PCs Many databases could fit in memory
New industry trend: Main Memory Database
E.g TimesTen Main issue is volatility
Secondary Storage
Disks Slower, cheaper than main memory Persistent !!! The unit of disk I/O = block
Typically 1 block = 4k A disk block is also called a disk page or
simply a page Used with a main memory buffer
Block Blocking factor (bfr) for a file is the
average number of records stored in a disk block.
Suppose the block size of a database system is 2000 bytes. Customer table has an average record length of 190 bytes. Assume the overhead of a block for the data is 100 bytes. What is the blocking factor?
The Mechanics of Disk
Mechanical characteristics: Rotation speed (5400RPM) Number of platters (1-30) Number of tracks (<=10000) Number of sectors (256/track) Number of bytes / sector (29=512) Block size (212=4096)
Platters
Spindle
Disk head
Arm movement
Arm assembly
Tracks
Sector
Cylinder
Important Disk Access Characteristics
Block access time = Disk latency + transfer time Disk latency = seek time + rotational latency Seek time = time for the head to reach the right track
10ms – 40ms Rotational latency = rotation time to get to the right
sector Time for one rotation = 10ms Average rotation latency = 10ms/2
Transfer time = typically 5-10MB/s Disks read/write one block at a time (typically 4kB)
Representing Data Elements
Relational database elements:CREATE TABLE Product (
pid INT PRIMARY KEY,name CHAR(20),description VARCHAR(200),maker CHAR(10) REFERENCES Company(name))
A tuple is represented as a record
Record Formats: Fixed Length
Information about field types same for all records in a file; stored in system catalogs.
Finding i’th field requires scan of record. Note the importance of schema information!
Base address (B)
L1 L2 L3 L4
F1 F2 F3 F4
Address = B+L1+L2
Record Header
L1 L2 L3 L4
F1 F2 F3 F4
To schema
length
timestamp
Need the header because:•The schema may change
for a while new+old may coexist•Records from different relations may coexist
header
Variable Length Records
L1 L2 L3 L4
F1 F2 F3 F4
Other header information
length
Place the fixed fields first: F1, F2Then the variable length fields: F3, F4Null values take 2 bytes onlySometimes they take 0 bytes (when at the end)
header
Records With Referencing Fields
L1 L2 L3
F1 F2 F3
Other header information
length
header
E.g. to represent one-many or many-many relationships
Storing Records in Blocks
Blocks have fixed size (typically 4k)
R1R2R3
BLOCK
R4
Spanning Records Across Blocks
When records are very large Or even medium size: saves space in
blocks
blockheader
blockheader
R1 R2 R2 R3
BLOB
Binary large objects Supported by modern database
systems E.g. images, sounds, etc. Storage: attempt to cluster blocks
together
Modifications: Insertion File is unsorted
add it to the end File is sorted:
Is there space in the right block ? Yes: we are lucky, store it there
Is there space in a neighboring block ? Look 1-2 blocks to the left/right, shift records
If anything else fails, create overflow block
Overflow Blocks
After a while the file starts being dominated by overflow blocks: time to reorganize
Blockn-1 Blockn Blockn+1
Overflow
Modifications: Deletions
Free space in block, shift records Maybe be able to eliminate an
overflow block
Modifications: Updates
If new record is shorter than previous, easy
If it is longer, need to shift records, create overflow blocks
Physical Addresses Each block and each record have a physical
address that consists of: The host The disk The cylinder number The track number The block within the track For records: an offset in the block
sometimes this is in the block’s header
Logical Addresses
Logical address: a string of bytes (10-16)
More flexible: can blocks/records around
But need translation table:
Logical addressPhysical address
L1 P1
L2 P2
L3 P3
Main Memory Address
When the block is read in main memory, it receives a main memory address
Buffer manager has another translation table
Memory address
Logical address
M1 L1
M2 L2
M3 L3
Designing Physical/Internal Model
Overview terminology Access methods
Physical Design
Internal Model/Physical Model
OperatingSystem
Access Methods
DataBase
User request
DBMSInternal ModelAccess Methods
External Model
Interface 1
Interface 3
Interface 2
Physical Design Interface 1: User request to the DBMS.
The user presents a query, the DBMS determines which physical DBs are needed to resolve the query
Interface 2: The DBMS uses an internal model access method to access the data stored in a logical database.
Interface 3: The internal model access methods and OS access methods access the physical records of the database.
Physical File Design A Physical file is a portion of secondary
storage (disk space) allocated for the purpose of storing physical records
Pointers - a field of data that can be used to locate a related field or record of data
Access Methods - An operating system algorithm for storing and locating data in secondary storage
Pages - The amount of data read or written in one disk input or output operation
Internal Model Access Methods
Many types of access methods: Physical Sequential Indexed Sequential Indexed Random Inverted Direct Hashed
Differences in Access Efficiency Storage Efficiency
Physical Sequential
Key values of the physical records are in logical sequence
Main use is for “dump” and “restore” Access method may be used for
storage as well as retrieval Storage Efficiency is near 100% Access Efficiency is poor (unless fixed
size physical records)
Indexed Sequential Key values of the physical records are in logical
sequence Access method may be used for storage and
retrieval Index of key values is maintained with entries
for the highest key values per block(s) Access Efficiency depends on the levels of
index, storage allocated for index, number of database records, and amount of overflow
Storage Efficiency depends on size of index and volatility of database
Index SequentialData File
Block 1
Block 2
Block 3
AddressBlockNumber
1
2
3
…
ActualValue
Dumpling
Harty
Texaci
...
AdamsBecker
Dumpling
GettaHarty
MobileSunociTexaci
Indexed Sequential: Two Levels
Address
7
8
9
…
Key Value
385
678
805
001003
.
.150
705710
.
.785
251..
385
455480
.
.536
605610
.
.678
791..
805
Address
1
2
Key Value
150
385
Address
3
4
Key Value
536
678
Address
5
6
Key Value
785
805
Indexed Random Key values of the physical records are not
necessarily in logical sequence Index may be stored and accessed with
Indexed Sequential Access Method Index has an entry for every data base record.
These are in ascending order. The index keys are in logical sequence. Database records are not necessarily in ascending sequence.
Access method may be used for storage and retrieval
Indexed Random
AddressBlockNumber
2
1
3
2
1
ActualValue
Adams
Becker
Dumpling
Getta
Harty
BeckerHarty
AdamsGetta
Dumpling
BtreeF | | P | | Z |
R | | S | | Z |H | | L | | P |B | | D | | F |
Devils
AcesBoilersCars
MinorsPanthers
Seminoles
Flyers
HawkeyesHoosiers
Inverted Key values of the physical records are
not necessarily in logical sequence Access Method is better used for
retrieval An index for every field to be inverted
may be built Access efficiency depends on number
of database records, levels of index, and storage allocated for index
Inverted
AddressBlockNumber
1
2
3
…
ActualValue
CH 145
CS 201
CS 623
PH 345
CH 145101, 103,104
CS 201102
CS 623
105, 106
Adams
Becker
Dumpling
Getta
Harty
Mobile
Studentname
CourseNumber
CH145
cs201
ch145
ch145
cs623
cs623
Direct Key values of the physical records are
not necessarily in logical sequence There is a one-to-one correspondence
between a record key and the physical address of the record
May be used for storage and retrieval Access efficiency always 1 Storage efficiency depends on density of
keys No duplicate keys permitted
Hashing Key values of the physical records are not
necessarily in logical sequence Many key values may share the same physical
address (block) May be used for storage and retrieval Access efficiency depends on distribution of
keys, algorithm for key transformation and space allocated
Storage efficiency depends on distibution of keys and algorithm used for key transformation
Comparative Access Methods
IndexedNo wasted space for databut extra space for index
Moderately Fast
Moderately FastVery fast with multiple indexesOK if dynamic OK if dynamic
Easy but requiresMaintenance ofindexes
FactorStorage spaceSequential retrieval on primary keyRandom Retr.Multiple Key Retr.Deleting records
Adding records
Updating records
SequentialNo wasted space
Very fast
ImpracticalPossible but needsa full scancan create wasted spacerequires rewriting fileusually requires rewriting file
Hashedmore space needed foraddition and deletion ofrecords after initial load
Impractical
Very fast
Not possiblevery easy
very easy
very easy
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