DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

8.1 CF request batching
8.1.1 Performance
z/OS 1.4 and CF Level 12 bring two new coupling facility “commands”, Write And Register
Multiple (WARM) and Read For Castout Multiple (RFCOM).
DB2 V8 data sharing will use these new coupling facility commands, if available, to reduce the
amount of traffic to and from the coupling facility for writes to group buffer pools and reads for
castout processing, thus reducing the data sharing overhead for insert, update and
delete-intensive workloads.
When the group buffer pool is allocated in a coupling facility with CFLEVEL= 12 or higher,
DB2 can register a list of pages that are being prefetched with one request to the coupling
facility. This can be used for sequential prefetch (including sequential detection) and list
prefetch.
For those pages that are cached as “changed” in the group buffer pool, or those that are
locked for castout, DB2 still retrieves the changed page from the group buffer pool one at a
time. For pages that are cached as “clean” in the group buffer pool, DB2 can get the pages
from the group buffer pool (one page at a time), or can include the pages in the DASD read
I/O request, depending on which is most efficient.
z/OS 1.4 and CF level 12 introduce two new CF “commands” to:
► Write And Register Multiple (WARM) pages to a group buffer pool with a single command.
► Read multiple pages from a group buffer pool for castout processing with a single CF read
request. The actual command is called Read For Castout Multiple (RFCOM).
CF level 13 also introduces performance improvements related to DB2 castout processing.
See the Web page:
http://www.ibm.com/servers/eserver/zseries/pso/coupling.html
DB2 V8 uses the CF request batching commands to read and write pages to and from the
group buffer pool if more than one page needs to be read or written.
DB2 instrumentation (statistics IFCID 2 and accounting 3 and 148) records are enhanced to
reflect the usage of these new commands. The DB2 PE accounting and statistics reports are
enhanced to externalize the new counters that indicate the number of WARM and RFCOM
requests in the GBP section:
► WRITE AND REGISTER MULTI
The number of Write and Register Multiple (WARM) requests.
► PAGES WRITE & REG MULTI
The number of pages written using Write and Register Multiple (WARM) requests.
► READ FOR CASTOUT MULTI
The number of Read For Castout Multiple (RFCOM) requests.
A number of performance studies have been done to understand the impact of using CF
request batching. For these studies, a two member data sharing group was used, running the
IRWW workload. z/OS 1.3 was compared to z/OS 1.4, to isolate the impact of CF request
batching. CF request batching is not available in z/OS 1.3.
Chapter 8. Data sharing enhancements 321