B4 Images         (Overview)

DESCRIPTION
The number of "B4 Images" represents the number of before-journal writes a
transaction has issued since it was initiated.  When recovery is active for a 
given database, a database update requires a before-journal write. If the 
before image is already on the journal for the current transaction, it is not 
rewritten to the journal.  Only the before image of the record is written to the
Before Journal, not the entire control interval.


VALUES
The value of "B4 Images" is a numeric quantity, initialized to zero at 
transaction initiation.


INTERPRETATION
It should be noted that in some cases it may appear that some transactions issue
more before images than other transactions, even though the transactions may be
the same.  This occurs because of Sun MTP's optimization of physical I/O.  When
a before image is being written out on behalf of a transaction, Sun MTP deter-
mines if another before image from another transaction is also ready to write.
If so, both images are written out at the same time.


CONCLUSIONS
An excessively large number of before images (relative to the transaction 
profile) indicates that the transaction is writing too many records to the 
database without performing a checkpoint. When this occurs, other transactions
may end up waiting for the same records. This can degrade system performance. 
The transactions should be analyzed for problems.
