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The Relational Model (cont’d) Introduction to Disks and Storage

The Relational Model (cont’d) Introduction to Disks and Storage. CS 186, Spring 2007, Lecture 3 Cow book Section 1.5, Chapter 3 (cont’d) Cow book Chapter 9 Mary Roth. Administrivia. Homework 1 available today from class web site Submit team members online

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The Relational Model (cont’d) Introduction to Disks and Storage

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  1. The Relational Model (cont’d) Introduction to Disks and Storage CS 186, Spring 2007, Lecture 3 Cow book Section 1.5, Chapter 3 (cont’d) Cow book Chapter 9 Mary Roth

  2. Administrivia • Homework 1 available today from class web site Submit team members online Read thru description; we’ll talk more about it in today’s lecture

  3. A Quick Survey…

  4. Outline • What we learned last time • Components of a DBMS • Relational Data Model • New stuff • Buffer Replacement and Files

  5. Review: Components of a DBMS Query Optimization and Execution Relational Operators Files and Access Methods Buffer Management Disk Space Management Today we go here… DB • A DBMS is like an ogre; it has layers

  6. Review • Relational Model • Schema vs Instance • Primary, Foreign Keys • Disks vs RAM • Disk Properties • DBMS Disk Manager • DBMS Buffer Manager

  7. Buffer Management Query Optimization and Execution Relational Operators Files and Access Methods Buffer Management Disk Space Management DB • Data must be in RAM for DBMS to operate on it! • Buffer Mgr hides the fact that not all data is in RAM You are here…

  8. Buffer Management in a DBMS DB Page Requests from Higher Levels • Buffer pool information table contains: <frame#, pageid, pin_count, dirty> BUFFER POOL disk page free frame MAIN MEMORY DISK choice of frame dictated by replacement policy 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 … 999

  9. Requesting a page I need page 3 Higher level DBMS component Buf Mgr BUFFER POOL 22 disk page 3 I need page 3 3 free frames MAIN MEMORY Disk Mgr DISK 1 2 3 … 22 … 90 • If requests can be predicted (e.g., sequential scans) pages can be pre-fetchedseveral pages at a time!

  10. Releasing a page I read page 3 and I’m done with it Higher level DBMS component Buf Mgr BUFFER POOL 22 disk page 3 free frames MAIN MEMORY Disk Mgr DISK 1 2 3 … 22 … 90

  11. Releasing a page I wrote on page 3 and I’m done with it Higher level DBMS component Buf Mgr BUFFER POOL 22 disk page 3’ free frames 3’ MAIN MEMORY Disk Mgr DISK 3’ 1 2 3 … 22 … 90

  12. More on Buffer Management • Requestor of page must eventually unpin it, and indicate whether page has been modified: • dirtybit is used for this. • Page in pool may be requested many times, • a pin count is used. • To pin a page, pin_count++ • A page is a candidate for replacement iff pin count == 0 (“unpinned”) • CC & recovery may entail additional I/O when a frame is chosen for replacement. • Write-Ahead Log protocol; more later!

  13. What if the buffer pool is full? ... • If requested page is not in pool: • Choose a frame for replacement.Only “un-pinned” pages are candidates! • If frame is “dirty”, write it to disk • Read requested page into chosen frame • Pin the page and return its address.

  14. Buffer Replacement Policy • Frame is chosen for replacement by a replacement policy: • Least-recently-used (LRU), MRU, Clock, etc. • Policy can have big impact on # of I/O’s; depends on the access pattern.

  15. LRU Replacement Policy • Least Recently Used (LRU) • for each page in buffer pool, keep track of time when last unpinned • replace the frame which has the oldest (earliest) time • very common policy: intuitive and simple • Works well for repeated accesses to popular pages • Problems? • Problem: Sequential flooding • LRU + repeated sequential scans. • # buffer frames < # pages in file means each page request causes an I/O. • Idea: MRU better in this scenario?

  16. LRU causes sequential flooding in a sequential scan I need page 1 I need page 2 I need page 3 I need page 4 Higher level DBMS component I need page 1 I need page 2…ARG!!! Buf Mgr BUFFER POOL 1 4 2 1 3 Disk Mgr MAIN MEMORY DISK 1 2 3 4

  17. “Clock” Replacement Policy A(1) B(p) D(1) C(1) • An approximation of LRU • Arrange frames into a cycle, store one reference bit per frame • Can think of this as the 2nd chance bit • When pin count reduces to 0, turn on ref. bit Questions: How like LRU? Problems?

  18. “Clock” Replacement Policy Frame 1 Higher level DBMS component do for each page in cycle { if (pincount == 0 && ref bit is on) turn off ref bit; else if (pincount == 0 && ref bit is off) choose this page for replacement; } until a page is chosen; I need page 5 I need page 6 Frame 4 Frame 2 Frame 3 ref Buf Mgr 1 5 2 6 3 4 1 2 3 4 5 6

  19. DBMS vs. OS File System OS does disk space & buffer mgmt: why not let OS manage these tasks? • 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 & order writes (important for implementing CC & recovery) • adjust replacement policy, and pre-fetch pages based on access patterns in typical DB operations.

  20. HW1 – Buffer Mgr • Questions?

  21. What is in Database Pages? • Database contains files (every page either part of a file, or empty) • Files hold relational data (tuples) or indexes (metadata)

  22. Files of Records • Blocks interface for 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 • fetch a particular record (specified using record id) • scan all records (possibly with some conditions on the records to be retrieved)

  23. Record Formats: Fixed Length • Information about field types same for all records in a file; stored in systemcatalogs. • Finding i’th field done via arithmetic. F3 F4 F1 F2 L3 L4 L1 L2 Address = B+L1+L2 Base address (B)

  24. 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 1 Slot 2 Slot 2 Free Space . . . . . . Slot N Slot N Slot M . . . N 1 1 1 M 0 M ... 3 2 1 number of slots number of records PACKED UNPACKED, BITMAP

  25. Record Formats: Variable Length • Two alternative formats (# fields is fixed): F1 F2 F3 F4 $ $ $ $ Fields Delimited by Special Symbols F1 F2 F3 F4 Array of Field Offsets • Second offers direct access to i’th field, efficient storage • of nulls(special don’t know value); small directory overhead.

  26. Page Formats: Variable Length Records • Can move records on page without changing rid; so, attractive for fixed-length records too. Rid = (i,N) Page i Rid = (i,2) Rid = (i,1) N Pointer to start of free space 16 24 20 N . . . 2 1 # slots SLOT DIRECTORY

  27. 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 spaceon pages • keep track of the records on a page • There are many alternatives for keeping track of this. • We’ll consider 2

  28. Heap File Implemented as a List • The header page id and Heap file name must be stored someplace. • Database “catalog” • Each page contains 2 `pointers’ plus data. Data Page Data Page Data Page Full Pages Header Page Data Page Data Page Data Page Pages with Free Space

  29. Heap File Using a Page Directory Data Page 1 Header Page Data Page 2 Data Page N DIRECTORY • The entry for a page can include the number of free bytes on the page. • The directory is a collection of pages; linked list implementation is just one alternative. • Much smaller than linked list of all HF pages!

  30. Indexes (a sneak preview) • A Heap file allows us to retrieve records: • by specifying the rid, or • by scanning all records sequentially • Sometimes, we want to retrieve records by specifying the values in one or more fields, e.g., • Find all students in the “CS” department • Find all students with a gpa > 3 • Indexes are file structures that enable us to answer such value-based queries efficiently.

  31. System Catalogs • For each relation: • name, file location, file structure (e.g., Heap file) • attribute name and type, for each attribute • index name, for each index • integrity constraints • For each index: • structure (e.g., B+ tree) and search key fields • For each view: • view name and definition • Plus statistics, authorization, buffer pool size, etc. • Catalogs are themselves stored as relations!

  32. Attr_Cat(attr_name, rel_name, type, position)

  33. Summary • Disks provide cheap, non-volatile storage. • Random access, but cost depends on location of page on disk; important to arrange data sequentially to minimize seek and rotation delays. • Buffer manager brings pages into RAM. • Page stays in RAM until released by requestor. • Written to disk when frame chosen for replacement (which is sometime after requestor releases the page). • Choice of frame to replace based on replacement policy. • Tries to pre-fetch several pages at a time.

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