ZDRLA adds smart incremental to be even smarter.

 Recently version 19.1.1.2 of ZDLRA software was released, and one the features is something called "Smart Incremental".  I will walk through how this feature works, and help you understand why features like this are "ZDLRA Only".

I am going to start by walking through how incremental backups become "virtual full backups", and that will give you a better picture of how "smart incremental" is possible.

The most important thing to understand about these features is that the RMAN catalog itself is within the ZDLRA  AND the ZDLRA has the ability to update the RMAN catalog.

How does a normal backup strategy work ? 

That is probably the best place to start.  What DBAs typically do is perform a WFDI (Weekly Full Daily Incremental) backup.  To keep my example simple, I will use the following assumptions.

• My database contains 3 datafile database. SYSTEM, SYSAUX, USERS, but I will only use the example of backing up datafile users.
• Each of these 3 datafiles are 50 GB in size
• I am only performing a differential backup which creates a backup containing the changes since the last backup (full OR incremental).
• My database is in archivelog  *

* NOTE: With ZDLRA you can back up a nologging database, and still take advantage of virtual fulls. The database needs to be in a MOUNTED state when performing the incremental backup.

If placed in a table the backups for datafile USERS would look this. Checkpoint SCN is the current SCN number of the database at the start of the backup.

If I were to look at what is contained in the RMAN catalog (RC_BACKUP_DATAFILE), I would see the same backup information but I would see the SCN information 2 columns.

• Incremental change # is the oldest SCN contained in the backupset. This is the starting SCN number of the previous backup, this backup is based on.
• Checkpoint Change # is  starting SCN number of the backup.  Everything newer than this SCN (including this SCN) needs to be defuzzied.

Normal backup progression (differential)

When performing an incremental RMAN backup of a datafile, the first thing that RMAN does is decide which blocks needs to be backed up. Because you are performing an incremental backup,  you may be backing up all of the blocks, only some of the blocks, or even none of the blocks if the file has not changed.

This is a decision RMAN makes by querying the RMAN catalog entries (or the controlfile entries if you not using an RMAN catalog).

Now let's walk through this decision process.  Each RMAN incremental differential's starting SCN is based on the beginning SCN of the previous backup (except for the full).

By looking at the RMAN catalog (or controlfile), RMAN determines  which blocks need to be contained in each incremental backup.

Normal backup progression (cumulative differential)

Up to release 19.1.1.2, the recommendation was to perform a Cumulative Differential backup. The cumulative differential backup compares the starting SCN number of the last full backup to determine the starting point of the incremental backup (rather than the last incremental backup) .

The advantage of the cumulative over differential, is that a cumulative backups can be applied to the last full and take the place of applying multiple differential backups.  However, cumulative backups are bigger  every day that passes between full backups because they contain all blocks since the last full.

Below is what a cumulative schedule would look like and you can compare this to the differential above.

You can see that each cumulative backups starts with the Checkpoint SCN of the last full to ensure that all blocks changed since the full backup started are captured.

The RMAN catalog entries would look like this.

If you were astute, you would notice a few things happened with the cumulative differential vs the differential.

• The backup size got bigger every day
• The time it took to perform the incremental backup got longer
• The range of SCNs contained in the incremental is larger for a cumulative backup.

ZDLRA backup progression (cumulative differential)

As  you most likely know, one the most important features of the ZDLRA is the ability to create a "virtual full" from an incremental backup.,

If we look at what happens with a cumulative differential (from above), I will fill in the virtual full RMAN catalog entries by shading them light green.

The process of performing backups on the ZDLRA is exactly the same as it is for the above cumulative, but the RMAN catalog looks like this.

What you will noticed by looking at this compared to the normal cumulative process that

• For every cumulative incremental backup there is a matching virtual full backup  The Virtual full backup appears (from the newly inserted catalog entry) to have beeen taken at the same time, and the same starting SCN number as the cumulative incremental. Virtual full backups, and incremental backups match time, and SCN as catalog entries.
• The size of the virtual full is 0 since it is virtual and does not take up any space.
• The completion time for the cumulative incremental backup is the same as the differential backups.  Because the RMAN logic can see the virtual full entry in the catalog, it executes the cumulative incremental EXACTLY as if it is the first differential incremental following a full backup.

Smart Incremental backups -

Now all of this led us to smart incremental backups. Sometimes the cumulative backup process doesn't work quite right.  A few of the reasons this can happen are.

• You perform a full backup to a backup location other than the ZDLRA. This could be because you are backing up to the ZDLRA for the first time replacing a current backup strategy, or maybe you created a special backup to disk to seed a test environment (Using a keep backup for this will alleviate this issue).  The cumulative incremental backup will compare against the last full regardless of where it was taken (there is exceptions if you always use tags to compare).

• You implement TDE or rekey the database.  Implementing TDE (Transparent Data Encryption) changes the blocks, but does not change the SCN numbers of the blocks. A new full backup is required.

Previously, you would have to perform a special full backup to correct these issues. In the example below you can see what happens (without smart incremental) to the RMAN catalog if you perform a backup on Thursday at 12:00 to disk to refresh a development environment.

Since the cumulative backups are based on the last full backup, the Thursday - Saturday backups contain all the blocks that have changed since the disk backup started on Thursday at 12:00.

And, since it is cumulative, each days backup is larger, and takes longer.

This is when you would typically have to force a new level 0 backup of the datafile.

What the smart incremental does

Since the RMAN catalog is controlled by the ZDLRA it can correct the problem for you. You no longer need to perform cumulative backups as the ZDLRA can fill in any issues that occur.

In the case of the Full backup to disk, it can "hide" that entry, and continue to correctly perform differential backups. It would "hide" the disk backup that occured, and inform the RMAN client that the last full backup as of Thursday night is NOT the disk backup, but it is the previous virtual full backup.

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 In the case of the TDE, it can "hide" all of the Level 0 virtual full backups, and the L1 differential backups (which will force a new level 0).

All of this is done without updating the DB client version. All the magic is done within the RMAN catalog on the ZDLRA.

Now isn't that smart ?