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Online consistent copies

Online Consistent Copy technology provides the database administrator with an efficient way to make consistent copies of Db2 table spaces and indexes without having to quiesce or cause any other outage to the spaces being copied.

The spaces being copied are continuously available for read and write access while the copy is being made.

One way that Online Consistent Copy does this is by making SHRLEVEL CHANGE Instant Snapshot copies. Online Consistent Copy uses the log to externalize changes to the copies for all complete transactions and to back out changes in the copies for any inflight transactions. This method is a very efficient way to make consistent copies of very large objects.

If your hardware does not support Instant Snapshots, you can still make OCCs using the SNAP VSAM option. SNAP VSAM is not as fast as using Instant Snapshot hardware, but it can make the same type of copy.

Another way that Online Consistent Copy does this is by making SHRLEVEL CHANGE standard image copies. Then Online Consistent Copy reads the standard image copies, merges them with log records to externalize changes for all complete transactions and to back out changes for any inflight transactions, and outputs the merged, consistent copy as a VSAM data set. In this case, Online Consistent Copy requires the use of the OUTSIZE option on the OPTION command and the BIGDDN option on the COPY command. These options, along with an OUTPUT command, specify the name of the output VSAM data set that will contain the consistent copy and work to determine when a data set is sufficiently large to merit using Instant Snapshot copies instead of standard image copies. Only those data sets that pass a user-specified OUTSIZE threshold are copied as Instant Snapshots. All others use standard image copies as input to the merge process. The main advantage of this process over Instant Snapshots or SNAP VSAM is that it may be faster if you are copying a large number of small objects.

The advantages of Online Consistent Copy are as follows:

  • Allows you to make consistent copies of production data at virtually any time, since no outage is required
  • Minimizes the impact on production applications
  • Requires minimal CPU and I/O on the production system

For more information, see Overview-of-Online-Consistent-Copy.

 

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