lt22-bufferMgr

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Database Systems & Applications

Lecture 22

Buffer Manager


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Buffer Management

Query Optimizationand Execution

Relational Operators

Files and Access Methods

Buffer Management

Disk Space Management

DB

You are here

Data must be in RAM for DBMS to operate on it!

Buffer Mgr hides the fact that not all data is in RAM


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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

Buffer pool information table contains:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 999


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Requesting a page

22

MAIN MEMORY

DISK

disk page

free frames

BUFFER POOL

1 2 3 22 90

Higher level DBMS

component

I need

page 3

Disk Mgr

Buf Mgr

I need

page 33

3

If requests can be predicted (e.g., sequential scans) pages can

bepre-fetchedseveral pages at a time!


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Releasing a page

22

MAIN MEMORY

DISK

disk page

free frames

BUFFER POOL

1 2 3 22 90

Higher level DBMS

component

I read page 3

and Im donewith it

Disk Mgr

Buf Mgr

3


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Releasing a page

22

MAIN MEMORY

DISK

disk page

free frames

BUFFER POOL

1 2 3 22 90

Higher level DBMS

component

I wrote on page

3 and Im donewith it

Disk Mgr

Buf Mgr

3

3

3


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More on Buffer Management

Requestor of page must eventually unpin it, andindicate whether page has been modified:

dirtybit is used for this.

Page in pool may be requested many times, apin countis used.

To pin a page, pin_count++

A page is a candidate for replacement iffpin count== 0(unpinned)

CC & recovery may entail additional I/O when aframe is chosen for replacement.

Write-Ahead Log protocol; more later!


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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.

If no page in the BP has pin count 0 ??????


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Buffer Replacement Policy

Frame is chosen for replacement by areplacement policy:

Least-recently-used (LRU), MRU, Clock, etc.

Policy can have big impact on # of I/Os;

depends on the access pattern.


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LRU Replacement Policy Least Recently Used (LRU)

This can be implemented using a queue of pointers to frames withpin count 0.

A frame is added at the end of the queue when it became thecandidate for replacement (when its pin count goes to 0)

The page chosen for the replacement is the one in the frame at the

head of the queue. 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 anI/O.

Idea: MRU better in this scenario?


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2114

LRU causes sequential flooding in a sequential

scan

MAIN MEMORY

BUFFER POOL

1 2 3 4

Higher level DBMS

component

I need

page 1

Disk Mgr

Buf Mgr

I need

page 2

3

I need

page 3

I need

page 4

DISK

I need

page 1

I need page

2!!!


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Clock Replacement Policy

An approximation of LRU

Arrange frames into a cycle, store one reference bitperframe

Can think of this as the 2nd chance bit

When pin count reduces to 0, turn on ref. bit

A(1)

B(p)

C(1)

D(1)


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32 6

Clock Replacement Policy

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;

Frame 1

1 2 3 4

1

I needpage 5

4

Frame 2

Frame 3

Frame 4

5

ref

Higher level DBMScomponent

Buf Mgr5

6

I needpage 6

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