DB2 Data Sharing Performance

© 2013 IBM Corporation
DB2 Data Sharing Performance
Martin Packer, IBM
martin_packer@uk.ibm.com

© 2013 IBM Corporation2
Abstract
 This presentation provides a view of how to look at the DB2
Data Sharing performance numbers
– from both a z/OS / RMF and a DB2 perspective

 Performance topics include:
– XCF
– Coupling Facility
– Data Sharing Structures
– and the application's perspective.

Agenda
 XCF
 Coupling Facility
– CPU
– Links
 DB2 Data Sharing Structures
 Application Perspective
 Some Parting Thoughts

Major DB2 Data Sharing Components
z/OS Image 1 z/OS Image 2
Coupling
Facility
XCF Structures
GBPs
LOCK1
IRLM 1 IRLM 2XCF XCFDB2 1 DB2 2
Shared Data
SCA

Major DB2 Data Sharing Components
 Locking
– DB2 Subsystems in a group in one z/OS image share an IRLM address space
– IRLMs communicate through their LOCK1 structure
• groupname_LOCK1
– IRLMs also communicate via XCF
• DXRnnn groups
• XES locking services also use XCF
– IXCLOnnn groups
 Group Buffer pools
– 1 CF structure per GBP
• groupname_GBPn
• Members connect directly to GBP structures
 Shared Communications Area (SCA)
– Status sharing
• groupname_SCA
• Much lower activity than LOCK1 and GBPs
– Not usually considered for tuning
• Members connect directly to SCA structure

XCF
 General signalling mechanism
– Introduced before the other Parallel Sysplex functions
 Traffic divided into transport classes
– These use either Coupling Facility structures or CTCs to pass
messages
• Dynamically routed based on XCF observing performance
• Dedicated CTCs
– Originally were faster than CF structures
– Must define a pair of paths for each connection
– Definition can get quite complex
– Transport classes have a maximum message size
• Fitting messages to classes is a significant tuning item
 Applications use specific XCF group names
– Example: IXCLOxxx is XES lock resolution
– Application address spaces connect as members of the group

XCF Tuning
 Aim to reduce transfer times
– “Mean Transfer Time” MXFER TIME in RMF
 Aim to minimise traffic
– Rates at all levels in RMF
– Eg Minimise Locking False Contention
– Eg Set up GRS Star in a way that minimises ENQs
 Optimise the use of links
– More modern CF-based links tend to outperform CTCs
• But CTCs still better for small messages
• CTCs drive SAP utilisation
– RMF counts the number of times each path was chosen
• Understand why signals use the paths in the ratio they do
 Transport Class buffer sizes
– Buffers that are too big waste memory
– Buffers that are too small have to be expanded
– RMF has counts of “Fit”, “Small”, “Big” and “Big With Overhead” messages
– RMF lists transport classes and their maximum buffer size values

 To manage XCF traffic down it's useful to know
who talks to whom
– 74-2 just says how many messages a member receives
and sends
• Not who their partners are
The XCF Traffic “Guessing Game”

DB2 Data Sharing XCF Groups – Between 2 Systems

Interesting “Hourly Spike” Behaviour

Coupling Facility

Parallel Sysplex SMF Records
 XCF - 74-2 (RMF XCF Activity Report)
– Applications
– Groups
– Paths
> CTCs are treated like real devices so SMF 74-1, 73 and 78-3 can be
useful
– Members and Job Names
 Coupling Facility - 74-4 (RMF Coupling Facility Activity Report)
– Usage Summary Section – Structure sizes and CPU usage
– Structure Activity Section
– Subchannel Activity Section – Path / Subchannel information
– CF to CF Section – Duplexing traffic at the CF level

SMF 74-4 Field: R744SETM “Structure Execution Time”
 Always 100% Capture Ratio
– Adds up to R744PBSY
 Multiple uses:
– Capacity planning for changing request rates
– Examine which structures are large consumers
– Compute CPU cost of a request & compare to service time
• Interesting number is “non-CPU” element of
– Understand whether CPU per request has degraded
– Estimating Structure Duplexing CPU cost
 NOTE:
– Need to collect 74-4 data from all z/OS systems sharing to get total request rate
• Otherwise “CPU per request” calculation will overestimate
Structure Level CPU

ISGLOCK Requests
0
2
4
6
8
10
12
14
16
0 10 20 30 40 50 60 70
Requests / Second
Microseconds
CPU Time Service Time
3us?

Structure CPU Time – By Time Of Day

Coupling Facility Path Information
 Dramatically improved in CFLEVEL 18 (zEC12)
– RMF APAR OA37826
• SMF 74-4
• Coupling Facility Activity Report
– Configuration:
• Detailed adapter and link type, PCHID, CHPID
– OA37826 gives CHPID even without CFLEVEL 18
• Infiniband and ISC only
– Performance:
• “Degraded” flag
• Channel Path Latency Time (R744HLAT)
– Divide by 10 us to give distance estimate in Postprocessor Report
– Would be interesting if it degraded

R744HOPM - Channel path operation mode
Value Meaning
X'01' CFP path supporting a 1.0625 Gbit/s data rate
X'02' CFP path supporting a 2.125 Gbit/s data rate
X'10' CIB path operating at 1x bandwidth using the IFB protocol,
   adapter type HCA2O LR
      adapter type HCA2O
   adapter type HCA3O LR
      adapter type HCA3O
X'30' CIB path operating at 12x bandwidth using the IFB3 protocol,
      adapter type HCA3O

DB2 Data Sharing Structures

Synchronous and Asynchronous CF Requests
 Synchronous (Sync)
– z/OS engine waits for completion
• Each microsecond of request service time is a microsecond of lost engine capacity
– e.g GRS Star
 Asynchronous (Async)
– z/OS engine does not wait for completion
– Response times usually longer than for Sync requests
– e.g XCF signalling
 Automatic Sync to Async Conversion
– Algorithm introduced by z/OS Release 2
– Requests converted wholesale
– With conversion an occasional request is tried as Sync
• Governs whether conversion is the right thing to do
• Factors
– Larger data transfer
– Longer / slower links
– Processor speed
– Duplexing
– Thresholds recently refined

Structure Duplexing
 2 copies of the same structure in different CFs
– Maintained in sync
– Higher resilience to component failures
• Loss of z/OS images and ICF on same footprint less likely to cause an outage
• Faster than structure rebuild
 Bidirectional links required between the two CFs
– Preferably more than one
 User-Managed
– “User” is DB2
– DB2 writes data to both structures
• Async write to primary
• Then Sync write to secondary
• Completion when both writes succeed
• Reads only from the Primary
• In event of failure reads from Secondary
 System-Managed
– XES writes to primary and secondary
• Both CFs communicate through a separate link to ensure status is shared
• Request only completes when both structures have been accessed

Locking - LOCK1 Structure
 Locks must be known and respected between members
– Data Sharing uses global locks to achieve this
 But not all locks need to be propagated
– Only the most restrictive state needs be
 Locking is propagated from IRLM, via XES to the LOCK1 CF
structure
– IRLM knows about locking states that XES doesn’t
• XES only knows about “shared” and “exclusive” locks
• DB2 had many more states, even before Data Sharing

LOCK1 Requests
0
2
4
6
8
10
12
750 800 850 900
Requests / Second
Microseconds
CPU Time Service Time
3.5us?

Locking - LOCK1 Structure
 Contention types:
– Real Contention
• Different members really do need to use the same resource at the
same time
• Real application delay inherent while the holder retains the lock
– XES Contention
• When XES believes there is contention but IRLM knows there isn’t
– because of its more comprehensive view of locking
– IRLMs have to talk via XCF to resolve this - DXRnnn
– False Contention
• When the hashing algorithm for the lock table provides the same
hash value for two different resources
– XESs have to talk via XCF to resolve this - IXCLOnnn

Group Buffer Pool Tuning
 GBP tuning has similarities to local pool tuning
– But some twists
 Important to minimise traffic
– Application GETPAGEs in general
– Traffic to GBPs
 Also important to minimise response times
– Which is mainly a matter of tuning the underlying CF access
 Minimising the amount of data actually shared may be practical
– For many designs it isn’t
 Important to avoid invalidations due to too few directory entries
– GBP space divided into Directory entries and Data elements
• Directory entry reclaims if too few entries
– Causes invalidations of local buffers
• Installation can alter the balance
• Installation can increase the size of the group buffer pool

25us?

Group Buffer Pool Tuning - Traffic
 Reads:
– Cross invalidation reads
• Data returned i.e. is in GBP
• Data not returned i.e. is known to be down level but page not in the GBP
– Requires disk read
– Buffer pool miss reads
• Data returned i.e not in the local pool but is in the GBP
• Data not returned i.e is in neither the local pool nor the GBP
– A bigger GBP ought to provide more hits and fewer misses
• But I rarely see high GBP hit ratios
 Writes:
– Avoid GBPCACHE(NONE) as writes are SYNCHRONOUS TO DISK at Commit time
• Harmful to the Committing unit of work’s performance
– Writes can also be caused by the LOCAL pool’s Deferred Write thresholds being hit
• In this case Commits aren’t waited for
 Castouts:
– Dribbling out a good idea
• Just like for local pools

Locking Tuning
 It’s important to reduce locking traffic at all levels
– Application
– DB2 subsystem
 It’s also important to reduce False Contention
– Usually by increasing the Lock Table portion of the LOCK1 structure
• Number of entries will be a power of 2
– 4-byte lock table entry means fewer entries for same size than 2-byte
 It’s nice that in DB2 Version 8 there’s a remapping of IRLM lock
states to XES ones
– May reduce XES lock contention
 CF Request response times also important

Application Perspective

Data Sharing Instrumentation
 Accounting Trace
– Generally provides a time breakdown for each application
• Plan, Correlation ID and Package level
• Excellent tuning instrumentation for applications
– Timings:
• Global Lock Wait
• Time to retrieve pages from GBP
– Subsumed within Sync DB Wait and Async Read
• Time for commits
– Can involve GBP traffic
– Activities
• Group Buffer Pool
• Global Locking
 Statistics Trace
– Activities
• Group Buffer Pool
– Note: MXG change 25.075 required to support incompatibly-changed DB2 Version 8 GBP
statistics
• Global Locking

Some Parting Thoughts
 Parallel Sysplex has many benefits
– More fully realised with Data Sharing
 Need to manage carefully performance and cost
 Configuration choices make an enormous difference
 Avoid shared coupling facilities for Production
 Good monitoring tools for both z/OS / Hardware, and DB2
 Tune not only DB2 structure and XCF Performance
– But also other structures and users of XCF

DB2 Data Sharing Performance

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