From MOS 372557.1

Alert Log Messages: Private Strand Flush Not Complete

SYMPTOMS

"Private strand flush not complete" messages are being populated to the alert log, example:

Mon Jan 23 16:09:36 2012
Thread 1 cannot allocate new log, sequence 18358
Private strand flush not complete
Current log# 7 seq# 18357 mem# 0: /u03/oradata/bitst/redo07.log
Thread 1 advanced to log sequence 18358
Current log# 8 seq# 18358 mem# 0: /u03/oradata/bitst/redo08.log

CHANGES

When you switch logs all private strands have to be flushed to the current log before the switch is allowed to proceed.

CAUSE

The message means that we haven't completed writing all the redo information to the log when we are trying to switch. It is similar in nature to a "checkpoint not complete" except that is only involves the redo being written to the log. The log switch can not occur until all of the redo has been written.

A "strand" is new terminology for 10g and it deals with latches for redo .

Strands are a mechanism to allow multiple allocation latches for processes to write redo more efficiently in the redo buffer and is related to the log_parallelism parameter present in 9i.

The concept of a strand is to ensure that the redo generation rate for an instance is optimal and that when there is some kind of redo contention then the number of strands is dynamically adjusted to compensate.

The initial allocation for the number of strands depends on the number of CPU's and is started with 2 strands with one strand for active redo generation.

For large scale enterprise systems the amount of redo generation is large and hence these strands are *made active* as and when the foregrounds encounter this redo contention (allocated latch related contention) when this concept of dynamic strands comes into play.

There is always shared strands and a number of private strands .

Oracle 10g has some major changes in the mechanisms for redo (and undo), which seem to be aimed at reducing contention.

Instead of redo being recorded in real time, it can be recorded 'privately' and pumped into the redo log buffer on commit.

Similarly the undo can be generated as 'in memory undo' and applied in bulk. This affect the memory used for redo management and the possibility to flush it in pieces. The message you get is related to internal Cache Redo File management.

...You can disregard these messages as normal messages.

SOLUTION

These messages are not a cause for concern unless there is a significant time gap between the "cannot allocate new log" message and the "advanced to log sequence" message.

These messages have also been seen when there are issues with the storage side or network for the archive log destination, as this leads to delay or hang in LGWR switch.