Chapter 10Storage and File Structure

Yonsei University2nd Semester, 2013

Sanghyun Park

Outline Overview of Physical Storage Media Magnetic Disks and Flash Storage File Organization Organization of Records in Files Data Dictionary Storage

Classification of Physical Storage Media

Speed with which data can be accessed

Cost per unit of data

Reliability Data loss on power failure or system crash Physical failure of the storage device

Can differentiate storage into: Volatile storage: loses contents when power is switched off Non-volatile storage:

Contents persist even when power is switched off Includes secondary and tertiary storage, as well as battery-backed

up main-memory

Storage Hierarchy (1/2)

Storage Hierarchy (2/2) Primary storage

Fastest media but volatile Cache, main memory

Secondary storage Next level in hierarchy, non-volatile, moderately fast access time Also called on-line storage Flash memory, magnetic disks

Tertiary storage Lowest level in hierarchy, non-volatile, slow access time Also called off-line storage Optical disk, magnetic tape

Magnetic Hard Disk Mechanism

Optimization of Disk-Block Access (1/2)

Block A contiguous sequence of sectors from a single track Data is transferred between disk and main memory in blocks Sizes range from 512 bytes to several kilobytes

Disk-arm-scheduling algorithms order pending accesses to tracks so that disk arm movement is minimized (e.g., elevator algorithm)

R1 R5 R2 R4R3R6

Inner track Outer track

Optimization of Disk-Block Access (2/2)

To reduce block-access time, we can organize blocks on disk in a way that corresponds to how data will be accessed

For example, we can store related information on the same or nearby cylinders

However, blocks may get scattered all over the disk by insertion or deletion (fragmentation)

Some systems have utilities for de-fragmentation

Flash Storage (1/2)

There are two types of flash memory, called NAND and NOR flash

NAND flash has a much higher storage capacity for a given cost, and is widely used for data storage in devices such as cameras, music players, cell phones, and increasingly laptop computers

Has a lower cost per byte than main memory, in addition to being non-volatile

Requires page-at-a-time read (page: 512 bytes to 4KB)

Transfer rate is around 20 MB/sec

Solid state disks: use multiple flash storage devices to provide higher transfer rate of 100 to 200 MB/sec

Flash Storage (2/2)

Once written, a flash page cannot be directly overwritten; instead,it has be erased and rewritten subsequently

Erase operation is performed on a number of pages, called an erase block, at once, and takes about 1 to 2 milliseconds

There is a limit to how many times a flash page can be erased, typically around 100,000 to 1,000,000 times (wear leveling)

Flash memory systems reduce the impact of these problems using a software layer called FTL (Flash Translation Layer)

File Organization A database is stored as a collection of files, each file is organized

logically as a sequence of records

Each file is also logically partitioned into fixed-length storage units called blocks, which are the units of both storage allocation and data transfer

One approach: Assume record size is fixed Each file has records of one particular type only Different files are used for different relations This approach is easiest to implement; we will consider variable length

records later

Fixed-Length Records Simple approach

Store record i starting from byte n i, where n is the size of each record

Record access is simple but records may cross blocks

Deletion of record i: alternatives: Move records i + 1, . . ., m

to i, . . . , m – 1 Move record m to i Do not move records, but

link all free records on afree list

Free Lists Store the address of the first deleted record in the file header Use this first record to store the address of the second deleted

record, and so on Can think of these stored addresses as pointers More space efficient representation: reuse space for normal

attributes of free records to store pointers

Variable-Length Records Variable-length records arise in database systems in several ways:

Storage of multiple record types in a file Record types that allow variable lengths for one or more fields Record types that allow repeating fields (used in some older data

models)

Byte-string representation Store each record as a string of consecutive bytes Attach an end-of-record () control character to the end of each record Not easy to reuse space occupied formerly by a deleted record There is no space, in general, for records to grow longer Thus, the basic byte-string representation is not usually used for

implementing variable-length records

Variable-Length Records:Slotted Page Structure

Slotted page header contains: Number of record entries End of free space in the block Location and size of each record

Records can be moved around within a page to keep them contiguous with no empty space between them; entry in the header must be updated

Pointers should not point directly to record — instead they should point to the entry for the record in header

Organization of Records in Files So far, we have studied how records are represented in a file

structure; Given a set of records, the next question is how to organize them in a file Heap – a record can be placed anywhere in the file where there is

space Sequential – store records in sequential order, based on the value of

the “search key” of each record Hashing – a hash function is computed on some attribute of each

record; the result specifies in which block of the file the record should be placed

Records of each relation may be stored in a separate file.In a multitable clustering file organization records of several different relations can be stored in the same file Motivation: store related records on the same block to minimize I/O

Sequential File Organization (1/2) Suitable for applications that require sequential processing of the

entire file The records in the file are ordered by a search-key A search key is any attribute or set of attributes; it need not be the

primary key, or even a superkey To permit fast retrieval of records in search-key order, we chain

together records by pointers

Sequential File Organization (2/2) Deletion – use pointer chains Insertion – locate the position where the record is to be inserted

If there is free space in the block, insert there If no free space, insert the record in an overflow block In either case, pointer chain must be updated

Need to reorganize the filefrom time to time to restoresequential order

Multitable Clustering File Organization (1/2)

Simple file structure stores each relation in a separate file

Can instead store several relations in one file using a multitable clustering file organization

E.g., multitable clustering organization of department and instructor: Good for queries joining department and instructor relations Bad for queries involving only department Results in variable size records

Multitable Clustering File Organization (2/2)

department

instructor

multitable clusteringof department and instructor

Data Dictionary Storage (1/2)Data dictionary (also called system catalog) stores metadata; that isdata about data, such as Information about relations

Names of relations Names and types of attributes of each relation Names and definitions of views Integrity constraints

User and accounting information, including passwords Statistical and descriptive data

Number of tuples in each relation Physical file organization information

How relation is stored (sequential/hash/…) Physical location of relation

Information about indices (Chapter 11)

Data Dictionary Storage (2/2)