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

RESTORE SYSTEM measurements
• RESTORE SYSTEM
Invokes ‘FRRECOV
COPYPOOL(DSN$location-name$DB)'
Followed by LOGAPPLY phase
• RESTORE SYSTEM
LOGONLY
Enters LOGAPPLY phase
directly
Figure 5-5 RESTORE SYSTEM measurement
Remember that RESTORE SYSTEM only restores the database copy pool. The logs are
applied after that restore.
Make sure all of the background copies have completed from an invocation of the BACKUP
SYSTEM utility before starting the restore utility.
The log apply performance can be improved by activating fast log apply (FLA).
Table 5-2 presents the log apply phase measurement results.
Table 5-2 LOGAPPLY phase measurement
IRWW non-data sharing Without FLA With FLA (100 MB) With FLA (500 MB)
Log records processed (million) 3.800 3.642 3.772
Elapsed time (sec.) 1,320 325 262
Rate (log records/sec.) 2,900 11,200 14,400
A FLA buffer size of 100 MB resulted in a 4 times improvement in elapsed time. With APAR
PQ95164, the DSNZPARM parameter LOGAPSTG will not determine the FLA buffer size for
the RESTORE utility, only if it is used or not. DB2 will first try to allocate a 500 MB FLA buffer;
if not successful DB2 will reduce the memory request to 250 MB, then 125 MB. During restore
the RESTORE SYSTEM utility is the only process running and there should be no problem
obtaining 500 MB of storage for the FLA feature at this time. The 500 MB FLA buffer further
decreased the elapsed time by 28% compared to a 100 MB FLA buffer. Compared to the test
case without FLA a 500 MB buffer resulted in nearly 5 times faster log apply phase.
List prefetch is the major contributor to FLA improvements, so the more random the logging
had been, the more advantage FLA will bring.
sec.
60
50
40
30
20
10
0
Elapsed Time for
FRRECOV
31
52
67 195
# of Vols
DB Pool
Chapter 5. Availability and capacity enhancements 225