Storage in Database Systems
CMPSCI 445
Fall 2010
Storage Topics
Architecture and Overview
Disks
Buffer management
Files of records
DBMS Architecture
Disk Space Manager
File & Access Methods
Buffer Manager Query Parser Query Rewriter Query Optimizer
Query Executor
Lock Manager Log Manager
Data on External Storage
Disks: Can retrieve random page at fixed cost
But reading several consecutive pages is much cheaper than reading them in random order
Tapes: Can only read pages in sequence
Cheaper than disks; used for archival storage
Page: Unit of information read from or written to disk
Size of page: DBMS parameter, 4KB, 8KB
Disk space manager:
Abstraction: a collection of pages.
Allocate/de-allocate a page.
Read/write a page.
Buffer Management
Architecture:
Data is read into memory for processing
Data is written to disk for persistent storage
Buffer manager stages pages between external storage and main memory buffer pool.
Access method layer makes calls to the buffer
manager.
Access Methods
Access methods: routines to manage various disk-based data structures.
Files of records
Various kinds of indexes
File of records:
Important abstraction of external storage in a DBMS!
Record id (rid) is sufficient to physically locate a record
Indexes:
Auxiliary data structures
File organizations & access methods
Many alternatives exist, each ideal for some situations, and not so good in others:
Heap (unordered) files: Suitable when typical access is a file scan retrieving all records.
Sorted Files: Best if records must be retrieved in some order, or only a `range’ of records is needed.
Indexes: Data structures to organize records via trees or hashing.
• Like sorted files, they speed up searches for a subset of records, based on values in certain (“search key”) fields
• Updates are much faster than in sorted files.
Disks
Disks and DBMS Design
DBMS stores information on disks.
This has major implications for DBMS design!
READ: transfer data from disk to main memory (RAM) for data processing.
WRITE: transfer data from RAM to disk for persistent storage.
Both are high-cost operations, relative to in-memory
operations, so must be planned carefully!
Why Not Store Everything in Main Memory?
Main memory is volatile. We want data to be saved between runs. (Obviously!)
Costs too much. $100 will buy you either 4GB of RAM or 1TB of disk today.
32-bit addressing limitation.
2
32bytes can be directly addressed in memory.
Number of objects cannot exceed this number.
Basics of Disks
Unit of storage and retrieval: disk block or page.
A disk block/page is a contiguous sequence of bytes.
Size of a DBMS parameter, 4KB or 8KB.
Disks support direct access to a page.
Unlike RAM, time to retrieve a page varies!
It depends upon the location on disk.
Therefore, relative placement of pages on disk has
major impact on DBMS performance!
Components of a Disk
Platters
Platters spin (say, 7200rpm).
Spindle
Arm assembly is moved in or out to position a head on a desired track.
Disk head
Arm movement
Only one head reads/
writes at any one time.
Tracks under heads make a cylinder (imaginary!).
Sector Track
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 2 to 30 msec
Rotational delay varies from 0 to 11 msec
Transfer rate between 25 to 100 MB/s
Key to lower I/O cost: reduce seek/rotation
Arranging Pages on Disk
`Next’ block concept:
blocks on same track, followed by
blocks on same cylinder, followed by
blocks on adjacent cylinder
Blocks in a file should be arranged
sequentially on disk (by `next’), to minimize seek and rotational delay.
For a sequential scan, pre-fetching several
Buffer Management
Buffer Management in a DBMS
DB
MAIN MEMORY DISK
disk page free frame
Page Requests from Higher Levels
BUFFER POOL
choice of frame dictated by replacement policy
More on Buffer Management
Requestor of page must unpin it, and indicate whether page has been modified:
dirty bit is used for this.
Page in pool may be requested many times,
a pin count is used. A page is a candidate for replacement iff pin count = 0.
CC & recovery may entail additional I/O when a frame is chosen for replacement.
(Write-Ahead Log protocol; more later.)
When a Page is Requested ...
If requested page is not in pool:
Choose a frame for replacement
If frame is dirty, write it to disk
Read requested page into chosen frame
Pin the page and return its address.
If requests can be predicted (e.g., sequential scans)
Buffer Replacement Policy
Frame is chosen for replacement by a replacement policy:
Least-recently-used (LRU), Clock, MRU etc.
Policy can have big impact on # of I/O’s;
depends on the access pattern.
Sequential flooding: Nasty situation caused by LRU + repeated sequential scans.
# buffer frames < # pages in file means each page
request causes an I/O. MRU much better in this
DBMS vs. OS File System
OS does disk space & buffer mgmt: why not let OS manage these tasks?
Differences in OS support: portability issues
Some limitations, e.g., files can’t span disks.
Buffer management in DBMS requires ability to:
pin a page in buffer pool, force a page to disk
(important for implementing CC & recovery),
Files
Files
Access method layer offers an abstraction of data on disk: a file of records residing on
multiple pages
A number of fields are organized in a record
A collection of records are organized in a page
A collection of pages are organized in a file
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)
Unordered (Heap) Files
Simplest file structure contains records in no particular order.
As file grows and shrinks, disk pages are allocated and de-allocated.
To support record level operations, we must:
keep track of the pages in a file
keep track of free space on pages
Heap File Implemented as a List
(heap file name, header page id) stored in a known place.
Two doubly linked lists, for full pages & pages with space.
Each page contains 2 `pointers’ plus data.
Header Page
Data
Page Data
Page Data
Page
Data
Page Data
Page Data
Page Pages with Free Space Full Pages
Heap File Using a Page Directory
A directory entry per page; it can include the number of free bytes on the page.
Data Page 1 Data Page 2
Data Page N Header
Page
DIRECTORY
Page Format
How to store a collection of records on a page?
Consider a page as a collection of slots, one for each record.
A record is identified by rid = <page id, slot #>
Record ids (rids) are used in indexes
(Alternatives 2 and 3).
Page Format: Fixed Length Records
Slot 1 Slot 2
Slot N
PACKED N
Free Space
number of records
. . .
M 1 . . . 0
M ... 3 2 1
UNPACKED, BITMAP Slot 1
Slot 2
Slot N Slot M
1 1
number of slots
. . .
Page Format: Variable Length Records
Can move records on page without changing rid; so, attractive
Page i Rid = (i,N)
Rid = (i,2)
Rid = (i,1)
20 16 24 N Pointer
to start of free space
SLOT DIRECTORY
N . . . 2 1 # slots
Rid of record is <page id, slot #>
Record Format: Fixed Length
Information of a record type e.g., the number of fields and field types is stored in the system catalog.
Fixed length record: (1) the number of fields is fixed,
Base address (B)
L1 L2 L3 L4
F1 F2 F3 F4
Address = B+L1+L2
Record Format: Variable Length
Variable length record: (1) number of fields is fixed, (2) some fields are variable length
Two alternatives:
$ $ $ $
Scan
Fields Delimited by Special Symbols
F1 F2 F3 F4
F1 F2 F3 F4 S1 S2 S3 S4 E4
