'redo synch time overhead ...' statistics

redo synch time (usec)
redo synch time overhead (usec)

redo synch time overhead count (<128 msec)
redo synch time overhead count (<2 msec)
redo synch time overhead count (<32 msec)
redo synch time overhead count (<8 msec)
redo synch time overhead count (>=128 msec)

Oracle Support note "Bug 12614085 - Diagnostic enhancement to add new statistics for investigating "log file sync" and "log file parallel write" relationship (Doc ID 12614085.8)" talks of an enhancement that enables this collection but I am not sure I understand it.

Can anyone interpret that note and help me understand that statistics ? If I see the overhead time being reported by these statistics, what really (what overhead) does it represent ? What if we have DataGuard in the picture ? Does it also reflect DataGuard LGWR SYNC overheads ?

Hemant K Chitale

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