Tuning Recovery - Expert Oracle RAC Performance Diagnostics and Tuning - page 320

SMON[INST-TXN-RECO]:about to recover undo segment 7 status:3 inst:1
SMON[INST-TXN-RECO]:about to recover undo segment 8 status:3 inst:1
SMON[INST-TXN-RECO]:about to recover undo segment 9 status:3 inst:1
SMON[INST-TXN-RECO]:about to recover undo segment 10 status:3 inst:1
Wed Aug 27 10:08:15 2014

FaSt-Start paraLLeL rOLLBaCK

Fast-start parallel rollback is performed by SMON , which acts as a coordinator and rolls back transactions using
parallel processing across multiple server processes. the parallel execution option is useful when transactions
run for a longer duration before committing. When using this feature, each node spawns a recovery coordinator
and recovery process to assist with parallel rollback operations.

Setting the parameter FAST_START_PARALLEL_ROLLBACK enables the fast-start parallel rollback feature. this
setting indicates the number of processes to be involved in the rollback operation. the valid values are FALSE ,
LOW , and HIGH . the default value is LOW .
• LOW —Dop is set to 2 * CPU_COUNT
• HIGH —Dop is set to 4 * CPU_COUNT
in most cases, transaction recovery can be monitored by querying the V$FAST_START_TRANSACTIONS view.
however, if the recovery data is less than 10 undo blocks, oracle fails to report data through this view, in which
case querying the X$KTUXE view could be helpful:

SELECT usn,
state,
undoblockstotal "Total",
undoblocksdone "Done",
undoblockstotal - undoblocksdone "ToDo",
decode(cputime, 0, 'unknown',
SYSDATE + ( ( ( undoblockstotal - undoblocksdone ) / (
undoblocksdone / cputime ) ) /
86400 ))
"Estimated time to complete"
FROM v$fast_start_transactions;

■ in oracle Database release 12.1.0, there are improvements in the overall performance of the raC environment.
if in a raC environment, multiple nodes in the cluster fail and only one surviving node remains, the rDBMS kernel module
will shut down all LMS background processes and treat the cluster database as a single instance. the alert log shows
“ Decreasing number of real time LMS from 1 to 0 ” illustrating this.

Note