Error messages in alert log:
Thread cannot allocate new log, sequence Checkpoint not complete
  OR/AND
ORACLE Instance - Cannot allocate log, archival required. Thread cannot allocate new log, sequence
From the AWR Report we can see what issue is more frequent:
a) DBWR has not finished checkpointing (log file switch (checkpoint incomplete) )
b) ARCH has not finished copying the redo log file to the archive destination (Archival required)
c) Wait event in session is log file switch (checkpoint incomplete).

Add REDO log files to groups or enlarge the existing REDO log files.
Redo log switching should occur approximately(大约) every 20-30 minutes to get better database performance.

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.

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.