Oracle Rdb Buffering A Comparative Study

Magnus WeimanPaul Mead

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Agenda

• Explore various ways Rdb buffers data• Examine pros and cons of buffering features

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Test Case

• Multi-user test• Rdb V7.1-401• Modified MF_PERSONNEL database• Three users reading, inserting, deleting in

EMPLOYEES table• Short Transactions• Results varied a bit from one run to the next

• Standalone ES40 with HSG80• VMS V7.3-2• No other users• Not clustered

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Effects of Number of Buffers

• More buffers increases the chance thatrecently accessed pages will still be inmemory

• More buffers require more overhead tomanage

• More pages in memory increase chances oflock conflicts and deadlocks

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Local Buffers With XFC vsGlobal Buffers Without XFC

• By sharing a common buffer pool manyprocesses can take advantage of the I/Odone by a single process

• Keeping unreferenced data in memory savesI/O if referenced again

• Rdb global buffers and XFC have similargoals

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Effects of Number of BuffersI/O for Local vs Global Buffers

0

50000

100000

150000

200000

250000

300000

20 80 320

1280

5120

Buffers

I/Os LB I/O

GB I/O

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Effects of Number of BuffersCPU Time

0

5

10

15

20

25

30

35

20 80 320

1280

5120

Buffers

CP

U S

eco

nd

s

LB XFC CPUSeconds

GB CPU Seconds

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Effects of Number of BuffersElapsed Time

0

20

40

60

80

100

120

140

160

20 80 320

1280

5120

Buffers

Ela

pse

d S

eco

nd

s

LB XFC ElapsedSeconds

GB ElapsedSeconds

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Effects of Different FeaturesLocal / Global Buffers, XFC

320 Buffers

0

5

10

15

20

LBXFC

LB noXFC

GBXFC

GB noXFC

CPU Seconds

0

20

40

60

80

100

120

LBXFC

LB noXFC

GBXFC

GB noXFC

Elapsed Seconds

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The Quest for More Buffers

• Shared Memory is System – takes virtual addressspace from system (S0) address space

• Global Buffers in VLM (“large memory isenabled”) – puts global buffers in Rdb’simplementation of extended memory

• Get buffers out of P0 address space – reduceVASFULL errors

• Makes buffers memory resident – less pagefaulting

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Effects of Different FeaturesSMS, GB VLM

0

5

10

15

20

LB XFC GB noXFC

SMS GB VLM

CPU Seconds

0

20

40

60

80

100

120

LB XFC GB noXFC

SMS GB VLM

Elapsed Seconds

320 Buffers

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Effects of Different FeaturesFast Commit

• Don’t write modified pages to disk at theend of every transaction

• If a failure occurs before modified pagesfrom a committed transaction are written todisk then those pages are rebuilt from thejournal

• Can increase page lock conflicts• Typically reduces/eliminates RUJ writes

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Effects of Different FeaturesOPT

• Ownership of “hot” pages that are oftenmodified by many processes are passedbetween processes without first beingwritten to disk

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Effects of Different FeaturesFast Commit and OPT

320 Buffers

0

20

40

60

80

100

120

LB XFC GB OPT LB FC XFC

Elapsed Seconds

0

10000

20000

30000

40000

50000

60000

70000

LB XFC GB OPT LB FC XFC

Data Writes

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Effects of Different FeaturesRow Cache

• Row cache copies referenced rows fromdisk data pages into an in-memory cache

• Once a cache is loaded little read I/O isneeded

• With “snapshots in cache” enabled manywrites can be avoided

• Reduces page lock contention• Requires more expertise to setup

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Effects of Different FeaturesRow Cache

020406080

100120140

LB XFC GB OPT LB FCXFC

LB RC FC

Elapsed Seconds

320 Buffers

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Effects of ContentionLock Conflicts and Deadlocks

• If there is a lock conflict on a table or rowlock then all buffers are flushed anddemoted

• VMS waits DEADLOCK_WAIT secondsbefore checking for deadlocks

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After-Image Journaling(AIJ Writes)

• Always enable ALS• Does double buffering• Less AIJ lock contention• Better tuned for group writes

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Before-Image Journaling(RUJ Writes)

• RUJ buffers must be flushed before databuffers are flushed – synchronous write

• Lock conflicts cause data buffers (and thusRUJ buffers) to be flushed

• If Fast Commit is enabled then RUJ flushesare often avoided

• Larger ABW batches allow larger RUJbatches and thus fewer RUJ writes

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Summary

• XFC works great! May be better than globalbuffers

• XFC and global buffers are redundant• Don’t skimp on buffers• Fast commit can significantly reduce writes• Row cache prevents I/O and lock conflicts• Beware of contention• Always enable ALS• Experiment to see what works best for you

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For More Information

• www.oracle.com/rdb• metalink.oracle.com• www.hp.com/products/openvms• Magnus.Weiman@oracle.com• Paul.Mead@oracle.com

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Q U E S T I O N S

A N S W E R S&