The Flash Cache Insert Skip: DBWR Overloaded alarm is raised when the ratio of insert skips from the DBWR being overloaded exceeds a threshold.
Typically, this happens when Oracle has tried to insert a block, being aged out of the buffer cache, into flash cache but the insert has been skipped. There are various reasons as to why a block will be skipped.
If a database is experiencing excessive insert skips from the DBWR being overloaded, this suggests the DBWR is writing dirty buffers to disk and cannot write to the flash cache. In systems with a large amount of writes, the flash cache may not be useful.