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DB2 for z/OS Buffer Pool Tuning: Win by divide andconquer or lose by multiply and surrender

John CampbellDB2 for z/OS Development

Session Code: 7008

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© Copyright IBM Corporation 2014. All rights reserved.U.S. Government Users Restricted Rights - Use, duplication or disclosure restricted by GSA ADP Schedule Contract with IBMCorp.

The information contained in this presentation is provided for informational purposes only. While efforts were made to verifythe completeness and accuracy of the information contained in this presentation, it is provided “as is” without warranty ofany kind, express or implied. In addition, this information is based on IBM’s current product plans strategy, which are subjectto change by IBM without notice. IBM shall not be responsible for damages arising out of the use of, or otherwise related to,this presentation or any other documentation. Nothing contain in this presentation is intended to, nor shall have the effect of,creating any warranties or representations from IBM (or its suppliers or licensors), or altering the terms and conditions of anyagreement or license governing the use of IBM products and/or software.

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Disclaimer2

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Agenda

• Page size selection

• Multiple buffer pools

• Page management

• Tuning buffer pool size

• Benefit from larger buffer pools

• Buffer pool simulation

• Deferred writes and castout processing

• Other miscellaneous topics

• Summary

• Appendix – General Recommendations

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Page size selection

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Page size selection

• 4K page size usually optimize the buffer hit ratio

• But the page needs to be large enough to store the max row size

• DB2 can store at most 255 rows on a page

• A large page size (>8K) provides better sequential performance, but couldaggravate buffer pool hit ratio for random/mixed access

• When row size is large, a large page size helps minimize DASD spaceconsumption

• On average, each page wastes a half-row of space

• e.g., If you average 10 rows per page, you waste 5% of the space

• Index considerations

• A large page size is necessary for index compression

• A large page size minimizes index splits

• A large page size can reduce the number of index levels

• A large page size may increase the frequency of deferred write I/Os

• With DB2 10, a large page size helps enable inline LOBs, which may helpimprove I/O and CPU performance significantly

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LOB table space page size considerations

• A page in a LOB table space contains only one LOB (i.e., one row)

• A small page size always provides the most space efficiency, especially whenLOBs are small

• If a LOB fits in one page, then it only takes one I/O to read the LOB

• Otherwise it takes a minimum of two I/Os and the second I/O will read up to 128KB

• DB2 10: Special tuning considerations for inline LOBs

• See “DB2 for z/OS Best Practices” web site

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Multiple Buffer Pools

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Multiple buffer pools

• Multiple buffer pools are recommended

• Ease of monitoring and tuning

• Online monitoring via Omegamon, –DISPLAY BPOOL, DISPLAY GBPOOL

• Post processing via DB2PM, RMF CF Activity post processing

• Useful for access path monitoring

• Potential reduction of DB2 latch contention, CFCC latch contention

• Moderation required as too many buffer pools can

• Fragment your total buffer pool space too much

• Create administrative overhead

• Dynamic tuning

• Full exploitation of buffer pool tuning parameters for customized tuning

• -ALTER BPOOL is synchronous and effective immediately, except for buffer poolcontraction because of waiting for updated pages to be written out

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Multiple buffer pools

• Catalog/Directory is in BP0, BP8K0, BP16K0 and BP32K

• Recommendation to not use these buffer pools for user objects

• Can enlarge buffer pool size for release migration and mass data recovery

• Best Insert performance practice (see Info APAR II14743)

• Recommendation to isolate LOB/XML table spaces out into their own separatebuffer pools away from other objects to avoid read LSN impact during insert

• Recommendation to separate out into different buffer pools: table spaces withoutLOB columns vs. UTS table spaces with LOB columns

• Avoid being impacted by read claim tracking for UTS table spaces with LOBcolumns because V10 NFM added tracking by read Log Sequence Number(LSN)for UTS table spaces that have LOB or XML column

• Separate large objects with potentially unstable access path out spaces out intotheir own separate buffer pools away from other objects

• Avoid CF performance impact when table space scan is accidentally chosen

• z/OS APAR OA41203 (fair queuing support) to mitigate the CF performance risk

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Example of buffer pool separation for 4K page size pool

• BP0 DB2 Catalog/Directory

• BP1 Table spaces

• BP2 Index

• BP3 In-memory heavy read (data and index)

• VPSEQT=0

• Optionally PGSTEAL(FIFO or NONE)

• BP4 In-memory heavy update (data and index)

• VPSEQT=0

• DWQT=VDWQT=90%

• Optionally PGSTEAL(FIFO or NONE)

• BP5 (optionally) Large table spaces with unstable access path

• BP6 LOB/XML

• BP7 Work files

• VPSEQT 90%

10

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Page Management

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LRU Processing

• DB2 has a mechanism to prevent sequentially accessed data frommonopolising a buffer pool and pushing out useful random pages

RS

R

R

R

R

SS

SS

Oldest sequential buffer

Oldest buffer

• If the length of the Sequential LRU chain (SLRU) exceeds VPSEQT, buffer manager willsteal the oldest sequential buffer

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GetPage and Buffer classification

• Buffers in a BP are classified as either random or sequential

• Classification is done at getpage level

• A buffer that is allocated for prefetch always goes on the Sequential LRU chain

• Prefetched pages are always classified as sequential, but then may be reclassifiedby the random getpage and removed from the SLRU chain

• A buffer is never reclassified from random to sequential

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VPSEQT recommendations

• General recommendations

• Use the default value of 80% (or range of 50-80%) for most buffer pools

• Should consider reducing VPSEQT when increasing VPSIZE to reduce sync I/Os

• However, ensure that VPSEQT x VPSIZE is greater than 320 MB in order toensure that the SQL/utilities use the best prefetch quantity and format writequantity

• Set VPSEQT to 99% for the sort only work file buffer pool, and 90% for otherworkfile usage

• If mixed, use 90%

• Set VPSEQT to 0% for in-memory buffer pool to avoid the unnecessary overhead ofwasteful scheduling of prefetch engines when data is already in buffer pool

• With DB2 10, use PGSTEAL=NONE

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Lowering VPSEQT with DB2 11

• If the buffer pool tuning goal is to protect random pages avoid synchronousI/O, as opposed to avoiding prefetch I/O, then ...

• Lower VPSEQT to protect random pages and improve the random Getpage hit ratio

• However, ensure that VPSEQT x VPSIZE is greater than 320 MB in order to ensurethat the utilities use a 512K quantity for prefetch and format write operations

• Doing so minimizes the number of utility I/Os

• But lowering VPSEQT might be a problem if list prefetch I/O is running slow

• Indicated by high Other Read I/O Suspension Time

• In this case caching the data in the buffer pool to avoid I/O is more beneficial

• Use IBM DS8000 storage with the combination of flash storage (HPFE or SSD)and zHPF to optimize list prefetch I/O

• Use of Easy Tier will automate the storing of "hot" data on flash or SSD, and is very cost effective

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DISPLAY BUFFERPOOL(bpid) DETAIL

• New statistics that were added in DB2 11, and in APAR PM70981 for DB2 10

• SEQUENTIAL

• The length of the SLRU chain

• DB2 statistics IFCID 2 record contains the minimum and maximum of this statisticduring the statistics interval

• Use can use these statistics to track the mixture of random and sequential bufferpool activity

• VPSEQT HITS

• Number of times that the length of the SLRU changed and became equal to VPSEQT

• RECLASSIFY

• The number of times that a random Getpage found the page was on the SLRU chain

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PGSTEAL - LRU, FIFO and NONE

• LRU (Least Recently Used) Default

• Maintains LRU chain to keep the frequently used page remained long

• FIFO (First In First Out)

• Remove the oldest pages out, recommend to use when you are seeing 100% bufferpool hit for frequently accessed objects

• Less maintenance for buffer pool chain

• NONE (no PGSTEAL) In-memory buffer pool

• Pre-load table or index at the first access using sequential prefetch

• Assumption is the object fit into the buffer pools

• If not, FIFO is used to manage PGSTEAL

• Less maintenance for buffer pool chain

• MRU (Most Recently Used), internal, not a user option

• Remove newest pages out when DB2 knows that the pages are not re-used

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MRU usage

• DB2 uses MRU for all “format write” Getpages

• These buffers are eligible to be stolen immediately after they are written

• Applicable to LOAD, REBUILD INDEX (load phase), REORG (reload phase) andRECOVER

• Prior to DB2 9, DB2 did not use MRU for sequential reads

• The COPY utility began using MRU in DB2 9

• DB2 11 adds MRU usage for most of the other utilities:

• UNLOAD, RUNSTATS

• REORG TABLESPACE and INDEX (unload phase)

• REBUILD INDEX (unload phase)

• CHECK INDEX and DATA (unload phase)

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Tuning Buffer Pool Size

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Buffer Pool Allocation

• Both buffer pools and associated control blocks are allocated in 64 bit privatestorage (thread storage is allocated in 64 bit shared)

• DB2 10 allocates as needed for both virtual and real

• DB2 11 allocates virtual storage immediately according to VPSIZE, butallocates real storage as needed

• Exception – Buffer pools with PGFIX(YES) and FRAMESIZE 1MB or 2GB

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Long-Term Page Fix for BPs with Frequent I/Os

• It has always been strongly recommended that DB2 buffer pools should bebacked up 100% by real memory to avoid paging to AUX (or Flash Express)

• Given that there is sufficient real memory and no paging, might as well pagefix each buffer pool just once to avoid the repetitive CPU cost of page fix andfree for each and every I/O

• ALTER BPOOL(name) PGFIX(YES|NO)

• Requires the buffer pool to go through reallocation before it becomesoperative

• A DB2 restart is necessary to change PGFIX for BP0, BP8K0, etc

• Observed 0 to 8% reduction in overall IRWW transaction CPU time

• Recommendation to use PGFIX(YES) for buffer pools and to never over-sizeyour buffer pools

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Buffer I/O Intensity

• Obtain (Total Page I/Os) as SUM of

• Pages read from DASD synchronously (sync read I/O)

• Pages read from DASD asynchronously (List, Dynamic, Sequential prefetch read)

• Pages written to DASD (mostly asynchronously)

• Pages read from GBP synchronously

• SYNCREAD (XI) DATA RETURNED + SYNCREAD(NF) DATA RETURNED

• Pages read from GBP asynchronously

• Pages written to GBP (CHANGED pages written to GBP)

• I/O Intensity = (TOTAL Page I/Os) divided by VPSIZE

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Getpage Intensity

• Getpage Intensity = number of getpages divided by VPSIZE23

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1 MB and 2 GB Size Page Frames

• 1 MB size page frames requires z10 or above and use LFAREA defined inIEASYSxx

• The advantage of 1 MB pages is to improve TLB (Translation Lookaside Buffers)performance during getpage

• Use 1MB page frames for buffer pools which have high getpage intensity

• DB2 10 attempts to allocate 1MB frames for buffer pools with PGFIX(YES)

• DB2 11 supports FRAMESIZE parameter (4K, 1M, 2G) for flexibility

• 2 GB size page frames requires DB2 11, zEC12 or zBC12, PGFIX(YES)

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PGFIX (YES) and 1MB Page Frames

SIZE #Get Page

#Pages Read

Sync

#Pages Read

Prefetch

#Pages

Written

Hit

Ratio

I/O

Intensity

Getpage

Intensity PGFIX 1MB

BP0 3K 138 0 0 0.06 100% 0 5

BP1 524.3K 1496.3K 0.03 0 589 100% 0 285 Y Y

BP2 2097K 160.4K 404 0 402 100% 0 8

BP3 524.3K 93.6K 2101 35300 197 98% 7 18 Y Y

BP4 2097K 40.9K 9873 2530 433 76% 1 2 Y

• PGFIX (YES): recommended for buffer pools with high I/O intensity, such as BP3 andBP4

• 1MB page frames: recommended for buffer pools with high getpage intensity, such asBP1 & BP3

• DB2 10: PGFIX YES - preferred frame size is always 1MB, DB2 11 can use FRAMESIZE

• Option 1 - use PGFIX (YES) for BP3 and BP4 but do not configure 1MB LFAREA

• Option 2 - use PGFIX (YES) and 1MB (LFAREA) for BP1, BP3 and BP4

• DB2 11 - use PGFIX(YES) and 1M frames for BP1, BP3, and use PFGIX(YES) and 4Kframes for BP4

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Evaluating buffer pool performance

• Use the random buffer pool hit ratio, better still use page residency time

• Unlike buffer pool hit ratio, page residency time is not affected by“extraneous” Getpage buffer hits

• e.g., If a thread does two rapid Getpages and page releases of the same page, thesecond Getpage is an extraneous buffer hit, because it has no risk of incurring async I/O

• Random page residency time

• Maximum of the following two formulae:

• Ratio of VPSIZE to total # pages read/sec (including prefetched pages)

• Ratio of VPSIZE * (1-VPSEQT/100) to random synch IO/sec

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Large memory

• “Memory is cheap. CPUs are expensive”

• For every I/O that you save, you avoid the software charge for the CPU that ittook to otherwise do that I/O

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CPU Cost Saving by Reducing DB2 Syc I/Os

• Banking (60M account) workload with 2 way data sharing :

• 11 % response and 6 % CPU reduction from 52 GB GBP to 398 GB for bothmembers with same LBP size (60GB)

• 40% response and 11% CPU reduction from 30GB LBP to 236GB LBP for bothmembers with same reasonable GBP size (60GB)

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Findings Summary

• Sync I/O reduction ties closely with CPU reduction

• This was done in z/OS 2.1, zEC12, DB2 11

• Expect larger saving with z/OS 1.13

• For this particular workload, the higher benefit on investing on local bufferpools

• Not all the objects become GBP-dependent

• Large TS defined with Member Cluster

• Most of access pattern is fairly random

• GBP has to be big enough to support enough directory entries

• The best performer was with both large LBP/GBP

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IBM Brokerage Workload – Enlarging Local Buffer Pool

• 10->24GB 14% CPU reduction, 3% Throughput improvement (26->13 sync I/O)

• 24 ->70GB 3% CPU reduction,15% Throughput improvement (13->8 sync I/O)

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DB2 Buffer Pool Simulation - Why?

• Larger local buffer pools can potentially reduce CPU usage by reducing syncI/Os

• The benefit depends on the size of active workload and access pattern

• May not see any benefit if working set size is very small and already fit in thebuffer pools today

• May not see any benefit if working set is too large and increment is not largeenough

• Pages have to be re-referenced – not for one time sequential read

• Try and validate may not work well with customer workload with highvariations

• Existing available tooling requires expensive set of traces and intensiveanalysis

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Buffer Pool Simulation

• Simulation provides accurate benefit of increasing buffer pool size fromproduction environment

• -ALTER BUFFERPOOL command will support

• SPSIZE (simulated pool size)

• SPSEQT (sequential threshold for simulated pool)

• -DISPLAY BPOOL DETAIL and Statistics Trace will include

• # Sync and Async DASD I/Os that could have been avoided

• Sync I/O delay that could have avoided

• Cost of simulation

• CPU cost: approximate 1-2% per buffer pool

• Real storage cost: approximate 2% of simulating pool size for 4K pages (1% for 8K,so on…)

• For example, to simulate SPSIZE(1000K) 4K pools requires approx. 78MBadditional real storage

• V11 APAR PI22091 for Buffer Pool Simulation is now available

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Deferred writes and GBP castout

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Deferred Writes

• VDWQT (Vertical Deferred Write Queue Threshold) based on the dataset levelas a % of VPSIZE or number of buffers

• DB2 schedules a write for up to 128 pages, sorts them in sequence, and writesthem out in at least 4 I/Os A page distance of 180 pages is applied to each writeI/O to avoid high page latch contention, since buffers are latched during I/O

• VDWQT hit when 64 updated pages queued for a GBP dependent object

• DWQT (horizontal Deferred Write Queue) at the BP level

DS1 DS2 DS3 DS4 DS5

DWQT

VDWQT

Buffer Pool

VPSIZE

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Deferred Writes …

• Setting VDWQT and DWQT to 90 is good for objects that reside entirely in thebuffer pool and are updated frequently

• Setting VDWQT to 1-2% is good if the probability of re-writing pages is low

• If VDWQT set to 0, DB2 waits for up to 40 changed pages for 4K BP (24 for 8K, 16for 16K, 12 for 32K) and writes out 32 pages for 4K BP (16 for 8K, 8 for 16K and 4for 32K)

• In other cases, set VDWQT and DWQT low enough to achieve a “trickle” writeeffect in between successive system checkpoints or log switch

• Setting VDWQT and DWQT too low may result in writing the same page out manytimes, short deferred write I/Os, increased DBM1 SRB CPU, and LC23

• If you want to set VDWQT in pages, do not specify anything below 128

• Target to hit VDWQT instead of DWQT to increase the number of pages perI/O and reduce the number of I/Os

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Critical BP Thresholds

Immediate Write ThresholdChecked at page update

>> After update, synchronous write

Data Management ThresholdChecked at page read or update

>> Getpage can be issued for each rowsequentially scanned on the same page –potential large CPU time increase

Prefetch ThresholdChecked before and during prefetch

90% of buffers not available for steal, or runningout of sequential buffers (VPSEQT with 80%default)

>> Disables Prefetch (PREF DISABLED – NOBUFFER)

97.5%

95%

90%

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System Checkpoint interval

• Periodically, the system checkpoint flushes out ‘dirty’ pages out of all of thebuffer pools to DASD, causing a burst of I/Os

• DB2 restart has to read the logs since the start of the prior system checkpointinterval and apply those log records

• Reducing the system checkpoint interval will help avoid the burstiness of thewrites, and reduce the restart time for DB2, but it may cause more write I/O

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Potential causes of poor write I/O performance

• Problems with remote replication

• Network contention

• A poor DASD control unit implementation can cause reads to queue behind thewrites

• Poor local disk performance

• RAID 5 rank is overloaded

• 10K RPM HDDs

• Poor CF performance

• CF links are overloaded

• Low write buffer residency time

• Caused by VDWQT set too low?

• Insufficient buffer pool size

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Potential for poor castout performance

• When castout performs poorly, the GBP can fill up

• When the GBP fills up, bad things happen

• DB2 start pacing for the commit - slower response time

• After repetitive write failures pages will be put on LPL

• Recovery actions are then necessary

• Problems are often precipitated by batch jobs that generate a surge ofupdates pages being written to the GBP

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Overlap GBP reads with castout I/O (New in V11)

Buffer pool

Cast out I/O workarea

GBP

• DB2 10 did not overlap writing to DASD with reading from the GBP

• DB2 11 does overlap

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GBP write-around (New in DB2 11)

Bufferpool

Castout I/Owork area

GBP

GBPOOLT=30%

Start write-around (50%)

Stop write-around (40%)

Castout to DASD

Commits neverwrite-around

GBP

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Miscellaneous topics

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Sequential buffer misses

• Do not use the sequential (or overall) buffer hit ratio, which could legitimatelybe negative in the skip-sequential case

• With DB2 11, look at the absolute number of sequential sync I/Os

• A lot of sequential sync I/Os indicates a problem

• Possible causes:

• Prefetch was disabled for one of two reasons

• You ran out of prefetch engines

• The dirty page count exceeded the prefetch threshold of 90% - check forPrefetch Disabled messages in OMPE

• Prefetch I/O was done, but the prefetched pages were stolen prior to the Getpages

• Detecting that this happened is difficult to do prior to DB2 11

• What should you do if this is happening?• Reduce the number of prefetch streams

• Increase the number of sequential buffers by increasing VPSEQT and/or VPSIZE

• Schedule the work that is using prefetch to a different time of day

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Optimizing list prefetch

• http://www.redbooks.ibm.com/abstracts/redp4862.html?Open

• Up to 3 times faster with spinning disks

• Up to 8 times faster with solid state disks

Use IBM DS8700 or DS8800 storage wth zHPF, which uses ListPrefetch Optimizer

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Optimizing format writes and preformatting

• zHPF increases the throughput for loading a tablespace with 4K pagesby 50% if VPSEQT x VPSIZE >=320MB

• zHPF doubles the throughput of preformatting, used by inserts

• Preformatting is often asynchronous, but not the first 16 cylinders of a newz/OS extent

• zHPF also reduces the number of I/Os per cylinder from 15 to 2, therebysaving CPU and reducing the load on remote networks

Use IBM DS8700 or DS8800 storage wth zHPF in order to optimize theperformance of format writes and preformat writes

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Solid State Disks

• The cost of SSD continues to drop

• SSD is ideal for DB2 list prefetch

• SSD is also ideal for improving synch I/O response time if the DASD cache hitratio is not high

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Summary

• General recommendation to use multiple buffer pools• For dynamic monitoring and tuning• To avoid performance bottlenecks• Improve overall performance in an environment with buffer pool related stress

• For a workload with no buffer pool related performance problems e.g., a workload withnegligible I/O's• Sufficient to provide the minimum of 4 buffer pools, one for each page size• But will be miss the ability to dynamically monitor buffer pool activities such as Getpage which may

help to• Expose inefficient access paths• Identify heavy catalog access due to repeated rebind• etc.

• When I/O activity is significant, multiple buffer pools designed to match the specificcharacteristics of objects assigned can provide a significant improvement in avoidingperformance bottlenecks and contributing to an overall performance improvement

• 32K versus 4K size work file buffer pools• For small sort record size, 4K size work file buffer pool can cut down the number of bytes read and

written by up to 6 times or more, helping to reduce work file I/O time primarily and related CPUtime secondarily

• As discussed in the presentation, in-memory versus not, sequential versus random, andread-intensive versus update-intensive can also have a significant positive performanceimpact by allowing separate buffer pools to provide a customized performanceimprovement

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Appendix

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General Buffer Pool Recommendations

• Recommend simplicity in object-to-buffer pool mapping

• Catalog/Directory only (4K, 8K, 16K, 32K)

• Work files only (4K and 32K)

• Set VPSEQT=90, DWQT= 60, VDWQT=50, PGSTEAL=LRU

• General default for tablespaces (4K, 8K, 16K, 32K)

• General default for indexes (4K, 8K, 16K, 32K)

• General default for LOBs and XML (4K, 8K, 16K, 32K)

• Set VDWQT=DWQT=0 to force dribble write of pages to DASD and/or CF

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General Buffer Pool Recommendations …

• Recommend simplicity in object-to-buffer pool mapping …

• Additional ‘specialised' bufferpools - after proper justification

• In-memory buffer pool, supersized to eliminate read I/Os• Once achieved, can set PGSTEAL=NONE

• Potential for significant CPU savings

• Journals - Heavy inserts/low reference• Low deferred write thresholds

• Sequential scan• Large enough to sustain optimised prefetch quantity (VPSIZE*VPSEQT >= 160MB/320MB)

• Very large objects accessed randomly and unlikely to get page re-reference• Do not waste an excessive number of buffers

• Small LOBs that are heavily re-referenced (e.g. LOBs for MQ shared queues)

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General Buffer Pool Recommendations …

• Good value to use a separate set of isolated buffer pools when introducing anew application for informational value, but it should not persist

• Not a scalable solution - finite number of buffer pools

• Provides some isolation but fragment real storage

• More complex management

• Clean up any pollution in the existing object-to-bufferpool mapping

• Work files should be in their own isolated set of buffer pools (e.g. BP7 and BP32K7)away from BP0 and BP32K

• Separate out table spaces and indexes in default general set of bufferpools

• Default bufferpools for tablespaces: BP2, BP8K2, BP16K2, BP32K2

• Default bufferpools for indexes: BP3, BP8K3, BP16K3, BP32K3

• Align ZPARM settings with the buffer pool strategy

• Use Statistics Trace Class 8 to collect dataset level I/O statistics to help with objectclassification and identify tuning opportunities

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John CampbellDB2 for z/OS Developmentcampbelj@uk.ibm.com

Session 7008DB2 for z/OS Buffer Pool Tuning: Win by divide and conquer or lose by multiply and surrender