Tuesday, September 04, 2007

MODIFY RECOVERY [DB2 9 for z/OS]

Another IBM DB2 utility that has received some attention under Version 9 is MODIFY RECOVERY. For those not sure what this utility is, MODIFY RECOVERY is used to remove records from the SYSIBM.SYSCOPY table in the DB2 Catalog, along with related log records from the SYSIBM.SYSLGRNX directory table and related information from the DBD.

Before the new V9 changed, when you ran MODIFY RECOVERY you had to specify the deletion criteria in one of two ways:

  • Given a specific date, delete all recovery information before that data
  • Or given an age in days, delete all recovery information older than the age

But as of V9 we can alternately specify what is to be retained instead of what is to be deleted. This way, we tell the utility what we want to keep and it will remove the rest.

So, instead of coding the DELETE parameter (with a DATE or AGE) we would instead code a RETAIN parameter. The RETAIN parameter can specify one of five different options:

LAST (integer) – Using this option informs DB2 to choose a date such that the number of full image copies kept is not less than the integer specified. So, if we specify LAST(5), then we will delete all entries that exist prior to the date on which the fifth oldest copy was taken. If more than one copy was taken on that date the result could be that more than 5 copies are retained.

LOGLIMIT – Specifying this option will delete all copies where DB2 no longer has log records to recover forward from. DB2 uses the BSDS to establish the oldest archive log timestamp.

GDGLIMIT – You can use this option to enable the number of copies to be retained to be linked to the corresponding GDG base limit.

GDGLIMIT LAST (integer) – It is possible also to combine the GDGLIMIT and LAST options. DB2 will then use the GDG base limit, if the last primary copy is a GDG, if not it uses the integer specified.

GDGLIMIT LOGLIMIT – And finally, we can combine the GDGLIMIT and LOGLIMIT options, too. If the most recent primary full copy is a GDG then the GDG limit is used. If the last copy is not a GDG then the BSDS is used to determine the date prior to which deletions will occur.

The last two options (the ones that combine two parameters) are particularly useful if you have a mix of GDGs and non-GDGs for your image copies. They are also helpful if you use the LISTDEF option when you make image copy backups.

It is important to understand that for all these options MODIFY RECOVERY will cause DB2 to determine the most recent date that satisfies the retention requirement. After ascertaining the correct date to use, all entries before that data will be deleted. In effect DB2 chooses the most recent date that would satisfy the retention criteria. So, under some circumstances there may be more than one image copy on the specific date that is established, and therefore DB2 will keep those additional copies, too.

As an example, say we take 7 image copies on a single day. Then we run MODIFY RECOVERY … RETAIN LAST (3). In this case, all 7 will be retained as they were made on the same day, even though we indicated that we wanted to retain only the last 3.

Finally, there is one more significant change made to the MODIFY RECOVERY utility. Prior to V9, information was only deleted when copies were removed from SYSIBM.SYSCOPY. But as of V9, MODIFY RECOVERY will deletes SYSIBM.SYSLGRNX entries even if no SYSIBM.SYSCOPY records are deleted.

Why is this important? Well, some shops have migrated over to running BACKUP SYSTEM instead of individual image copies. In this case, there will be no individual image copies to remove in SYSIBM.SYSCOPY, but the SYSIBM.SYSLGRNX records will continue to grow. After migrating to V9 you can use MODIFY RECOVERY to trim the size of SYSIBM.SYSLGRNX which can improve performance for processes that need to access log range information.

1 comment:

Gerry D. said...

Craig,
Suppose a db created in 1993 has been regularly dropping a table so often the dbd size is 8 million. will modify recovery reduce the dbd size? is a reorg necessary ? it will eliminate the -497 abend but is the large dbd size a detriment to concurrency and performance or do we really just care that that the syslgrnx and syscopy entries are cleared out? Is there an upper threshold limit as to what a DBD can be? My recommendation was to dop the db and start from scratch also emlinating the need for contiguous space when a dbd is loaded in.
Thanks,
Gerry