DB2 archive log issues
IBM recommends the creation of dual archive log data sets. Because of the cost and availability of the DASD space necessary to store the amount of archive log data that DB2 might generate, many DBAs choose tape as the medium for storing their DB2 archive log data sets, thus sacrificing the improved access for recovery and application backout provided by DASD. In large installations, DB2 may create hundreds of archive log data sets per week. Storing so many archive log data sets on tape can be costly because of the number of tapes required and the physical space required to store them.
Another issue regarding the DB2 archive log is that much of the data within the archive log data set is completely unnecessary for any DB2 forward recovery process. A considerable amount of log data recorded by DB2 is there only to allow any transaction backouts that may be required. After the data has been committed, up to 80 percent of the data recorded in the log is not needed by any DB2 forward recovery process. Also, some DB2 objects may exist within your environment that will never require a DB2 forward recovery involving access to log data. These objects can always be recovered in a recovery to a specified image copy or be re-created by batch processes. However, DB2 does not give you the option to omit logging of the information on such objects.
Data compression of the archive log data sets does allow you to reduce the storage media required to contain the log data. However, IBM recommends that hardware data compression not be used when you store archive log data sets on tape due to the adverse effect on performance when you perform transaction backouts (in cases where archive log data is needed to perform the backout).
Even without data compression, when an application is terminated and archive log data residing on tape is required to allow DB2 to back out the application, the backout process can impose a heavy burden on DB2 resources. DBAs, therefore, must choose between the following two options for storing archive log data sets:
Storing them on DASD, which allows faster backouts but is more costly
Storing them uncompressed on tape, which is less expensive but provides slower access for backouts
In an attempt to compromise, many DBAs keep as much data as possible in the active logs, thus increasing the amount of DASD dedicated to DB2 log data.
The PACLOG utility provides solutions to DB2 archive log issues. The product provides impressive savings of archive log media along with a set of archive log management functions that include an interactive modeling tool that helps you optimize the number of active logs and their size.
PACLOG achieves media savings in the following ways:
Replaces log data records not needed by any DB2 recovery process with easily compressible data
Removes log data for user-specified DB2 objects
Compresses the log data
Allows reductions in the size of active log data sets stored on DASD if the archive log data sets reside on DASD instead of tape
PACLOG also offers the following functionality:
Stores the processed primary and backup archive logs on different device types
Provides statistical information about the logs and the bootstrap data set (BSDS)
Provides recovery site primary and backup archive logs in addition to the archive logs that were created by DB2