Database Management Systems: Understanding Data Storage and Access, Slides of Database Management Systems (DBMS)

Download Database Management Systems: Understanding Data Storage and Access and more Slides Database Management Systems (DBMS) in PDF only on Docsity! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Storing Data: Disks and Files Chapter 7 “Yea, from the table of my memory I’ll wipe away all trivial fond records.” -- Shakespeare, Hamlet Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2 Disks and Files DBMS stores information on (“hard”) disks. This has major implications for DBMS design!
READ: transfer data from disk to main memory (RAM).
WRITE: transfer data from RAM to disk.
Both are high-cost operations, relative to in-memory operations, so must be planned carefully! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Why Not Store Everything in Main Memory? Costs too much. $1000 will buy you either 128MB of RAM or 7.5GB of disk today. 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 3ed, R. Ramakrishnan and J. Gehrke 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. Unlike RAM, time to retrieve a disk page varies depending upon location on disk.
Therefore, relative placement of pages on disk has major impact on DBMS performance! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 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 3ed, R. Ramakrishnan and J. Gehrke 6 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 Key to lower I/O cost: reduce seek/rotation delays! Hardware vs. software solutions? Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 13 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) pages can be pre-fetched several pages at a time! Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 14 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.) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 15 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 situation (but not in all situations, of course). Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 16 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),
adjust replacement policy, and pre-fetch pages based on access patterns in typical DB operations. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 17 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. Base address (B) L1 L2 L3 L4 F1 F2 F3 F4 Address = B+L1+L2 Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 18 Record Formats: Variable Length Two alternative formats (# fields is fixed): * Second offers direct access to i’th field, efficient storage of nulls (special don’t know value); small directory overhead. 4 $ $ $ $ Field Count Fields Delimited by Special Symbols F1 F2 F3 F4 F1 F2 F3 F4 Array of Field Offsets Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 19 Page Formats: Fixed Length Records * Record id = <page id, slot #>. In first alternative, moving records for free space management changes rid; may not be acceptable. Slot 1 Slot 2 Slot N . . . . . . N M10. . . M ... 3 2 1 PACKED UNPACKED, BITMAP Slot 1 Slot 2 Slot N Free Space Slot M 11 number of records number of slots Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 20 Page Formats: Variable Length Records * Can move records on page without changing rid; so, attractive for fixed-length records too. Page i Rid = (i,N) Rid = (i,2) Rid = (i,1) Pointer to start of free space SLOT DIRECTORY N . . . 2 1 20 16 24 N # slots Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 21 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)