Lecture 10: Buffer Manager and File Organization · Lecture 10 > Section 3 •Simplest file...

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Lecture 10: Buffer Manager and File Organization Lecture 10

Transcript of Lecture 10: Buffer Manager and File Organization · Lecture 10 > Section 3 •Simplest file...

Page 1: Lecture 10: Buffer Manager and File Organization · Lecture 10 > Section 3 •Simplest file structure contains records in no particular order. •As file grows and shrinks, disk pages

Lecture10:BufferManagerandFile

Organization

Lecture10

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Today’sLecture

1. Recap:BufferManager

2. ReplacementPolicies

3. FilesandRecords

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Lecture10

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1. BufferManager

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Lecture10>Section1

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Whatyouwilllearnaboutinthissection

1. BufferPool

2. BufferManager

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Lecture10>Section1

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TheBuffer(Pool)

Lecture10>Section1

Disk

MainMemory

Buffer(Pool)• Abuffer isaregionofphysicalmemoryusedtostoretemporarydata

• Inthislecture:aregioninmainmemoryusedtostoreintermediatedatabetweendiskandprocesses

• Keyidea:Reading/writingtodiskisslow-needtocachedata!

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• DatamustbeinRAMforDBMStooperateonit!• Can’tkeepalltheDBMSpagesinmainmemory

• BufferManager:Efficientlyusesmainmemory• Memorydividedintobufferframes:slotsforholdingdiskpages

Upperlevels:• releasepageswhendone

• indicateifapageismodified

DB

MAINMEMORY

DISK

diskpage

freeframe

PageRequestsfromHigherLevelsBUFFERPOOL

choiceofframedictatedbythereplacementpolicy

BufferManagementinaDBMS

Lecture10>Section1

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BufferManager

Lecture10>Section1

• Bookkeepingperframe:• Pincount:#usersofthepageintheframe• Pinning :Indicatethatthepageisinuse• Unpinning :Releasethepage,andalsoindicateifthepageisdirtied

• Dirtybit :Indicatesifchangesmustbepropagatedtodisk

2requestorswanttomodifythesamepage?Handledbytheconcurrencycontrolmanager(usinglocks)

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BufferManager

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•WhenaPageisrequested:• Inbufferpool->returnahandletotheframe.Done!• Incrementthepincount

• Notinthebufferpool:• Chooseaframeforreplacement(Onlyreplacepageswithpincount==0)

• Ifframeisdirty,writeittodisk• Readrequestedpageintochosenframe• Pinthepageandreturnitsaddress

Canyoutellthe#currentusersofapageintheBP?PinCount!

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2. ReplacementPolicies

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Lecture10>Section2

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Whatyouwilllearnaboutinthissection

1. ReplacementPolicy

2. LRUandClock

3. SequentialFlooding

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Lecture10>Section2

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• Howdowechooseaframeforreplacement?• LRU(LeastRecentlyUsed)• Clock• MRU(MostRecentlyUsed)• FIFO,random,…

• Thereplacementpolicyhasbigimpacton#ofI/O’s(dependsontheaccesspattern)

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Bufferreplacementpolicy

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LRU

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• usesaqueue ofpointerstoframesthathavepincount=0

• apagerequestusesframesonlyfromthehead ofthequeue

• whenathepincountofaframegoesto0,itisaddedtotheendofthequeue

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LRUExample

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Bufferpoolwith4frames

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LRUExample

Lecture10>Section3

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LRUExample

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LRUExample

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LRUExample

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Whichpageisevictednext?

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LRUExample

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Clock

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• VariantofLRUwithlowermemoryoverhead• TheN framesareorganizedintoacycle• Eachframehasareferencedbitthatissetto1whenpincountbecomes0

• Acurrentvariablepointstoaframe• Whenaframeisconsidered:

• Ifpincount>0,incrementcurrent• Ifreferenced=1,setto0andincrement• Ifreferenced=0andpincount=0,choosethepage

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ClockExample

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Referencedbit

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ClockExample

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ClockExample

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ClockExample

Lecture10>Section2

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ClockExample

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ClockExample

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ClockExample

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ClockExample

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ClockExample

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ClockExample

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ClockExample

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ClockExample

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ClockExample

Lecture10>Section2

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ClockExample

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SequentialFlooding

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• NastysituationcausedbyLRUpolicy+repeatedsequentialscans• #bufferframes<#pagesinfile• eachpagerequestcausesanI/O!!• MRUmuchbetterinthissituation

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

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SequentialFloodingExample

Lecture10>Section2

NestedLoop

LRUhappenstoevictexactlythepagewhichwe

willneednext!!!

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SequentialFlooding

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• NastysituationcausedbyLRUpolicy+repeatedsequentialscans• #bufferframes<#pagesinfile• eachpagerequestcausesanI/O!!• MRUmuchbetterinthissituation

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3. FilesandRecords

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Lecture10>Section3

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Whatyouwilllearnaboutinthissection

1. FileOrganization

2. PageOrganization

3. BONUS:ColumnStores

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Lecture10>Section3

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ManagingDiskSpace

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Lecture10>Section3

PageorblockisOKforI/O,buthigherlevelsoperateonrecords,andfilesofrecords.

• Thediskspaceisorganizedintofiles

• Filesaremadeupofpages

• Pagescontainrecords

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FileOperations

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Lecture10>Section3

• Thediskspaceisorganizedintofiles

• Filesaremadeupofpages

• Pagescontainrecords

Fileoperations:

• insert/delete/modifyrecord• readaparticularrecord(specifiedusingtherecordid)• scanallrecords(possiblywithsomeconditionsontherecordstoberetrieved)

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FileOrganization:Unordered(Heap)Files

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Lecture10>Section3

• Simplestfilestructurecontainsrecordsinnoparticularorder.

• Asfilegrowsandshrinks,diskpagesareallocatedandde-allocated.• Tosupportrecordleveloperations,wemust:

• keeptrackofthepages inafile: pageid(pid)• keeptrackoffreespaceonpages• keeptrackoftherecords onapage:recordid(rid)• Manyalternativesforkeepingtrackofthisinformation

• Operations:create/destroyfile,insert/deleterecord,fetcharecordwithaspecifiedrid,scanallrecords

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HeapFileasaList

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Lecture10>Section3

HeaderPage

DataPage

DataPage

DataPage

Fullpages

DataPage

DataPage

DataPage

Pageswithfreespace

• (heapfilename,headerpageid)recordedinaknownlocation• Eachpagecontainstwopointers plusdata:Pointer=PageID(pid)• Pagesinthefreespacelisthave“some”freespace

Q:Whathappenswithvariablelengthrecords?

A:Allpagesaregoingtohavefreespace,butmaybewewillhavetogothroughalotofthem

beforewefindonewithenoughspace.

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HeapFileasaPageDirectory

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Lecture10>Section3

• Eachentryforapagekeepstrackof:• isthepagefreeorfull?• howmanyfreebytesare?

• Wecannowlocatepagesfornewtuplesfaster!

DataPageN

DataPage2

DataPage1

…

Headerpage

DIRECTORY

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ManagingDiskSpace

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Lecture10>Section3

• Filesmadeupofpages

• andpagescontainrecords

• Butfileoperationsareonrecords:

Fileoperations:

• insert/delete/modifyrecord• readaparticularrecord(specifiedusingtherecordid)• scanallrecords(possiblywithsomeconditionsontherecordstoberetrieved)

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PageOrganization:PageFormats

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Lecture10>Section3

• Apageiscollectionofrecords

• Slottedpageformat• Apageisacollectionofslots• Eachslotcontainsarecord

• rid=<pageid,slotnumber>

• Therearemanyslottedpageorganizations

• Weneedtohavesupportfor:• search,insert,deleterecordsonapage

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Slot1Slot2

SlotN

...

N

PACKED

FreeSpace

numberofrecords

PageFormats:FixedLengthRecords

Lecture10>Section3

Packedorganization: N recordsarealwaysstoredinthefirstN slots.

Movingrecordschangesrid!Maynotbeacceptable.

Recordid=<pageid,slot#>

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PageFormats:FixedLengthRecords

Lecture10>Section3

UnpackedOrganization: useabitmap tolocaterecordsinthepage.

Recordid=<pageid,slot#>

FreeSpace ...

M10...M...321

UNPACKED,BITMAP

Slot1Slot2

SlotN

SlotM

11

numberofslots

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PageFormats:VariableLengthRecords

Lecture10>Section3

Book-keeping0, 70

-1,0

560,90

-1, 0

120,40 670,

50012345

Slot directoryFree SpacePointer

• Directorygrowsbackwards!• Moverecordsonsamepage;

ridunchanged!Goodforfixed-lengthrecordstoo.

Rid=?Rid= (11, 1)Page num = 11

Delete a record? Slot Entry: Offset, record length Number of slots

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PageFormats:VariableLengthRecords

Lecture10>Section3

• Deletion:• offsetissetto-1

• Insertion:• useanyavailableslot• ifnospaceisavailable,reorganize

• rid remainsunchangedwhenwemovetherecord(sinceitisdefinedbytheslotnumber)

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PageFormats:VariableLengthRecords

Lecture10>Section3

Book-keeping0, 70

-1,0

560,90

-1, 0

120,40 670,

50012345

Slot directoryFree SpacePointer

• Directorygrowsbackwards!• Moverecordsonsamepage;

ridunchanged!Goodforfixed-lengthrecordstoo.

Rid=?Rid= (11, 1)Page num = 11

Delete a recordOffset is set to -1

Slot Entry: Offset, record length Number of slots

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• Allrecordsonthepagearethesamelength• Informationaboutfieldtypessameforallrecordsinafile;storedinsystem catalogs.

Baseaddress(B)

L1 L2 L3 L4

F1 F2 F3 F4

Address=B+L1+L2

RecordFormats:FixedLength

Lecture10>Section3

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Twoalternativeformats(#fieldsisfixed):

ArrayofOffsets

4 $ $ $ $

FieldCount

F1F2F3F4

Fielddelimiter(specialsymbol)

n Secondalternativeoffersdirectaccesstoi’th fieldn Efficientstorageofnullsn Smalldirectoryoverhead.

n Issueswithgrowingrecords!n changesinattributevalue,add/dropattributes

n Recordslargerthanpages

RecordFormats:VariableLength

Lecture10>Section3

Useanarrayofintegeroffsetsinthebeginning

Page 60: Lecture 10: Buffer Manager and File Organization · Lecture 10 > Section 3 •Simplest file structure contains records in no particular order. •As file grows and shrinks, disk pages

ColumnStores:Motivation

• Consideratable:• Foo(aINTEGER,bINTEGER,cVARCHAR(255),…)

• Andthequery`:• SELECTa FROMFooWHEREa>10

• WhathappenswiththepreviousrecordformatintermsofthebytesthathavetobereadfromtheIOsubsystem?

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Lecture10>Section3

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• Storedata“vertically”• Contrastthatwitha“row-store”thatstoresalltheattributesofatuple/recordcontiguously• Thepreviousrecordformatsare“rowstores”

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111 212 Itwasacoldmorning222 222 Warmandsunnyhere333 232 Articwinterconditions444 242 Tropicalweather

111 212 Itwasacoldmorning222 222 Warmandsunnyhere333 232 Articwinterconditions444 242 Tropicalweather

File1 File2 File3

ColumnStores:Motivation

Lecture10>Section3

Eachfileisasetofpages.Columnscanbestoredincompressedform

Page 62: Lecture 10: Buffer Manager and File Organization · Lecture 10 > Section 3 •Simplest file structure contains records in no particular order. •As file grows and shrinks, disk pages

• Arethereanydisadvantagesassociatedwithcolumnstores?

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1. Updatesareslower

2. Retrievingbackmorethanoneattributecanbeslower,e.g.QuerieslikeSELECT*areslower

ColumnStores:Motivation

Lecture10>Section3


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Lecture 9: File Systems. Lecture 9 / Page 2AE4B33OSS 2011 Contents Files & File System Interface Directories & their Organization File System Implementation.

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