Monday, October 29, 2007

Improvements to the REORG Utility [DB2 9 for z/OS]

As we continue to wind our way through the multiple improvements that DB2 V9 brings to the IBM utilities, it is time to address the enhancements made to the REORG utility.

Performance

First up, let’s talk about performance improvements. In DB2 9 for z/OS, the REORG utility can unload and reload partitions in parallel. This should result in a nice reduction in elapsed time when you are reorganizing partitioned table spaces. To enable this improvement you will need to code the NOSYSREC keyword or the UNLDDN keyword with a template. Note that NOSYSREC is always used for SHRLEVEL change.

Parallelism will not be enabled if any of the following conditions apply:

  • DATAWKnn ddnames are specified in the JCL
  • SORTDEVT keyword is not specified
  • UTPRINT is allocated to anything other than SYSOUT
  • The REBALANCE keyword is used.

Also consider that REORG SHRLEVEL CHANGE can use subtasks during the LOG phase to speed up the processing of log records. Subtasks will be used when unload and reload have been done by partition and there are log records to apply for the partition.

BUILD2 Phase Eliminated in Online REORG

As you probably know, an Online REORG uses shadow data sets. The reorganization is done to the shadows while the data remains available in the primary data sets. When the REORG is done, the shadow data sets are switched to be the primary data sets.

As most DBA know, reorganization by partition can create a long outage when non-partitioned indexes (NPIs) exist on the table. Why is this so? Well, prior to V9, during Online REORG, NPIs had to be updated with the new RIDs for the data of the partition being reorganized during the BUILD2 phase. And that is what typically caused the sometimes significant outage.

With DB2 V9, the BUILD2 phase is eliminated. Instead, the entire NPI will be reorganized and then switched in the SWITCH phase along with all of the other data sets. The only outage is during the SWITCH phase, which will be much, much shorter than the BUILD2 phase outage.

As a result of the elimination of the BUILD2 phase, DB2 will need additional temporary storage for the shadow data sets for each NPI. Additionally, the cost of the REORG will increase because building an entire NPI will consume more CPU than the previous method of updating RIDs in the BUILD2 phase. Finally, you will need to modify any REORG jobs against different partitions that ran in parallel because the entire NPI will be built and then switched.

What about running REORG SHRLEVEL REFERENCE by partition? Well, REORG SHRLEVEL REFERENCE by partition will also rebuild any associated NPIs using shadows. So the same type of concerns apply (more disk storage, additional CPU, no separate jobs in parallel).

Reorganization and LOBs

Finally, as I blogged about before in the posting on LOBS, prior to V9 you could not access LOB data during a REORG. And a REORG did not reclaim physical space from the LOB data set because LOBs were moved within the existing LOB table space. V9 fixes these problems. During a REORG (in V9), the original LOB table space is drained of writers. All LOBs are then extracted from the original data set and inserted into a shadow data set. When this operation is complete, all access to the LOB table space is stopped (the readers are drained) while the original data set is switched with the shadow data set. At this point, full access to the new data set is enabled, and an inline copy is taken to ensure recoverability of data.

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Wednesday, October 24, 2007

Miscellaneous Utility Enhancements [DB2 9 for z/OS]

Today we will continue along with our coverage of new features and functionality oif DB2 9, specifically within the IBM DB2 utilities. Here we will discuss several miscellaneous changes introduced to the IBM DB2 utilities in version 9.

One of these small changes comes to us with the DSN1LOGP utility. The utility will now detect possible erroneous recovery information. For example, consider a situation where you specify a range of log records to print but the entire range is no longer recorded in the BSDS. This can happen if your archive logs have rolled off. Prior to DB2 V9 DSN1LOGP would have just merrily printed the records for you. But because this can be confusing and c ould lead you to recoverying improperly, V9 will detect this situation, produce a RC 4 and the DSN1224I error message Indicating logs could not be found.

Another nice new utility-related enhancement made to DB2 9 for z/OS is that the DB2 Utilities Panel within DB2I adds an option for you to specify the SDSNLOAD library. This is useful when there are more than one DB2 subsystems on an LPAR.

DSN1COPY has been enhanced, too. As of DB2 9 for z/OS, running DSN1COPY RESET to copy a compressed table space from a non-data sharing DB2 subsystem to another non-data sharing subsystem will copy the specified data and reset the PGLOGRBA of each page and the dictionary version. In V7 and V8 the dictionary version was not reset (instead, you had to REORG the copied table space with KEEPDICTIONARY=NO in the target subsystem). This was not an issue for data sharing because the LRSN is used instead of an RBA.

Keep an eye out for future editions of this blog where we will go over the V9 changes for REORG and the BACKUP and RESTORE SYSTEM utilities.

Wednesday, October 10, 2007

RECOVER Improvements [DB2 9 for z/OS]

Our examination of the new utility functionality in DB2 9 for z/OS continues today as we uncover what’s new with the IBM RECOVER utility.

Point-In-Time Consistency

The most significant enhancement centers on recovering to a point-in-time (PIT) with consistency. As any DBA who has ever been charged with recovering an operational database to a prior PIT knows, it can be challenging to accomplish. Of course, if you have a usable QUIESCE point then point in time recovery is easy. But that means you’d have to have planned ahead of time and taken a QUIESCE, which is not always possible with heavy 24x7 workloads.

Well, DB2 V9 offers some relief because the IBM RECOVER utility is enhanced to automatically identify uncommitted work at the PIT to which you are recovering. After detecting the uncommitted work DB2 will rollback any changes made to the database objects that are being recovered. This means that anything that is being recovered will be in a transactionally consistent state. Of course, you still have to make sure that all of the appropriate objects are participating in the recovery.

This works with for PIT recoveries executed with either the TOLOGPOINT or TORBA keywords only. To accomplish this two new phases are added after the LOGAPPLY phase: LOGSCR and LOGUNDO.

The LOGSCR phase runs for each DB2 member with an active unit of recovery (UR). If no UR was active then this phase will be skipped. During the LOGCSR phase, all active and incomplete URs are identified. This includes URs that were:
  • INFLIGHT
  • INABORT
  • INDOUBT
  • POSTPONED ABORT

If any URs were found, the LOGUNDO phase runs. This is where RECOVER backs out
the changes made on recovered objects by active URs. In a data sharing environment, this
will be done one member at a time.

Additional Information on the Progress of Your Recovery

Another improvement made to the IBM RECOVER utility in V9 is the ability to gather more information about the progress of the recovery. Any DBA who has been tasked with sitting around and monitoring recoveries what the DISPLAY command to show accurate, up-to-date, and appropriate information about the status of the recovery.

Well, DB2 V9 enhances the -DISPLAY UTILITY command to provide additional information during the LOGAPPLY phase. DISPLAY now will show the range of the log that needs to be applied, for the database objects that are being recovered. It will also show the progress up to the last commit, as well as the elapsed time since the start of the log apply phase of the recovery.

This information will assist the DBA in determining how long it will take for the recovery job to successfully complete.

Using an Earlier Image Copy

One final enhancement made to the RECOVER utility in V9 gives the DBA control to get DB2 to recover using an earlier image copy. This is accomplished by specifying a point in the log before which DB2 must select an image copy for the recovery.

This technique can be particularly helpful when the most recent image copy is damaged or somehow unusable. You can use an earlier copy by providing an RBA/LRSN value to the new RESTOREBEFORE parameter that is at a point on the log prior to when the unusable image copy was taken.

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Monday, October 08, 2007

LOAD and UNLOAD Enhancements [DB2 9 for z/OS]

This week we continue our exploration of the features and functionality of DB2 9 for z/OS with a look at what’s new in the IBM LOAD and UNLOAD utilities. The first question people seem to ask with regard to LOAD is “Has performance been improved?” And the answer is yes, due mostly to improvements in handling indexes. IBM reports that performance can improve up to 24 percent if a table has mostly variable keys.

Handling DECFLOAT Data

In terms of functionality, the first change we’ll discuss is how to deal with DECFLOAT data type in LOAD (and UNLOAD). If you do not recall what the DECFLOAT data type is, refresh your memory here.

The DECFLOAT data type is compatible with SMALLINT, INTEGER, BIGINT, DECIMAL, FLOAT, DECFLOAT. Of course, you will have to concern yourself with how the data is handled if it is loaded to or from DECFLOAT. You specify how to handle this manipulation using the DECFLOAT_ROUNDMODE parameter, which can be used in both the LOAD and UNLOAD utilities.

The following DECFLOAT_ROUNDMODE specifications are allowed:

  • ROUND_CEILING - Round towards positive infinity.
  • ROUND_DOWN - Round towards zero.
  • ROUND_FLOOR - Round towards negative infinity.
  • ROUND_HALF_DOWN - Round to nearest; if equidistant, round down.
  • ROUND_HALF_EVEN - Round to nearest; if equidistant, round so that the final digit is even.
  • ROUND_HALF_UP - Round to nearest; if equidistant, round up.
  • ROUND_UP - Round away from zero.

If you do not specify DECFLOAT_ROUNDMODE, the LOAD statement uses the DFPDEFDM value in DSNHDECP as the default value.

Loading LOBs

Loading and unloading LOBs has been improved. For LOAD, an input field value can contain the name of the file that contains a LOB column value. The LOB column value will then be loaded from that file. For UNLOAD, you can store the value of a LOB column in a file and record the name of the file in the unloaded record in the base table.

LOAD, LOBs, and SORTKEYS NO

When a LOAD SHRLEVEL NONE of a table with a LOB column, fails, it can only be restarted with RESTART(CURRENT). This has been the case since LOBs were introduced to DB2.

However, when the SORTKEYS keyword was introduced in V5, a restriction with its use was that a LOAD that abended in the RELOAD, SORT, or BUILD phases could only be restarted with RESTART(PHASE). Restart will convert any RESTART option to RESTART(PHASE) when SORTKEYS is specified. This situation limited our options when LOBs were involved.

In V8 SORTKEYS became the default, so SORTKEYS and LOB restart processing were incompatible. In V9 this is fixed:

  • You can turn SORTKEYS off by specifying SORTKEYS NO.
  • And you can now use RESTART(PHASE) for LOAD REPLACE of a table with a LOB.

If you run LOAD REPLACE SHRLEVEL NONE on a table with a LOB, specifying SORTKEYS NO means that you can restart with either RESTART(CURRENT) or RESTART(PHASE). Without the SORTKEYS NO you can only use RESTART(PHASE).

If you run LOAD RESUME YES SHRLEVEL NONE on a table with a LOB, you must specify SORTKEYS NO if you want the job to be restartable; only RESTART(CURRENT) will be available.

Skipping Locked Rows

Recall from an earlier post here that you can skip locked rows in your transactions using the SKIP LOCKED DATA option within your SQL statements. Well, guess what, you can skip locked rows when running an UNLOAD, too.

If you are running an UNLOAD with SHRLEVEL CHANGE ISOLATION CS, you can also specify SKIP LOCKED DATA. This will cause the UNLOAD utility to skip rows on which incompatible locks are held by other transactions. This option only applies to table spaces with row level or page level locking.

Tuesday, October 02, 2007

More Online Utilities [DB2 9 for z/OS]

Today we continue our investigation into the new functionality in DB2 9 for z/OS... and specifically into the improvements made to IBM utilities for version 9. We will turn our attention to improved availability with IBM DB2 utilities. Under DB2 V9, three more utilities can run online – that is, with SHRLEVEL CHANGE.

First up we have CHECK DATA. This utility is executed when you want to make sure that the data in your tables matches your integrity constraints. Prior to V9, access to tables (or table spaces) is read only when CHECK DATA is being run. This means that as long as CHECK DATA is executing you cannot modify data. If you have complex referential sets and/or very large databases you likely will have issues with running CHECK DATA.

You can run CHECK DATA online – that means with SHRLEVEL CHANGE – as of DB2 V9. As with other online utilities, an online CHECK DATA will work with on a shadow copy of the data and indexes. DB2 makes the shadow copy using DFSMS ADRDSSU. This copy can be super fast if you have data set level FlashCopy V2 enabled. Even if you do not have FlashCopy availability will improve because the outage is reduced when you use SHRLEVEL CHANGE (just to make the copy) instead of SHRLEVEL REFERENCE (for the entire utility execution).

DB2 will check data integrity using the shadow copies of the data and indexes. When violations are found DB2 will not set the CHECK pending state. And remember, we are checking shadows, so DB2 will not delete rows in violation. Instead, CHECK DATA will create a PUNCHDDN containing REPAIR LOCATE DELETE input that can be run against the active data.

The CHECK LOB utility has also been enhanced to enable online execution. DB2 V9 allows SHRLEVEL CHANGE for checking LOBs using a shadow copy of the LOB table space. And just like CHECK DATA, DB2 uses the DFSMS ADRDSSU utility to create the shadow. And similarly, the check is executed against the shadow and REPAIR statements are generated that you subsequently can run against the actual LOB table space.

Finally, you can also run the REPAIR LOCATE utility specifying SHRLEVEL CHANGE. This enhancements allows us to execute the REPAIR utility online with LOCATE against indexes, index spaces and table spaces.

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