Monthly Archives: August 2020

Oracle Backup Terminology

Oracle backup terminology can be kind of confusing. And, by can be, I mean that it is confusing. There are lots of different terms that sound similar but mean very different things. I’ve seen very experienced DBAs use the following statements synonymously: “I just did a whole backup” and “I just did a full backup”. The correct response to “I just did a whole backup.” is “Thanks! I appreciate it.”, while the correct response to “I just did a full backup.” is “Of what?” or maybe even “Why?”. Because, according to Oracle’s terminology, those two statements mean quite different things.

I’ll be defining the terminology that Oracle uses in their training material and documentation and, in addition, making suggestions on what your backups should most often look like.

Backup Strategy – Whole or Partial

Every Oracle backup is either a whole backup or a partial backup.

A whole backup in Oracle terminology means that you’ve backed up all data in all datafiles and at least one control file. Since the control files (you have configured multiple control files, right?) are copies of each other, you technically only have to backup a single control file.

So what is a partial backup? It’s a backup that isn’t all data in all datafiles and a control file, but instead less than that. If you backup a single datafile, you’ve done a partial backup. If you backup all datafiles but one, you’ve done a partial backup. If you backup all data in every single data file, but don’t back up at least one control file, you’ve done a partial backup.

Backup strategy tells you how much of your database you are backing up. You are either backing up all data in all datafiles and at least one control file, or you are backing up something different than that.

For your backup strategy you should start with a whole backup of your database. After that initial backup you can either do partials forever using the Oracle Suggested Backup Strategy , or you can bounce back and forth between whole and partial backups. To me, the least effective backup strategy is to always do whole backups, but some folks do use this strategy. I much prefer to start with a whole backup and then do partial backups from then on using the Oracle Suggested Backup Strategy.

As an aside, notice that a whole backup doesn’t technically need an spfile to be included in the backup, but I always include an spfile in all my whole backups.

Backup Type – Full or Incremental

The next term that Oracle uses to describe a backup is Backup Type. Every backup is either a Full Backup or an Incremental Backup.

And… Welcome to the start of the confusion! Whole does not equal Full. While you can do a whole full backup, you can also do a partial full backup.

Backup type tells you how the backup can be used relative to other backups. Generally, a full backup stands on it’s own and isn’t used in conjunction with other backups. A full backup backs up all the data in whatever you are backing up. An incremental backup can be used with other incremental backups and, depending on it’s level (which I’ll explain in a moment) either backs up all the data in whatever you are backing up, or all the data that has changed since your previous incremental backup.

Incremental Backup Types – Level 0, Level 1 Differential, Level 1 Cumulative

To further increase backup type complexity, incremental backups have different types themselves. Incremental backups are either a Level 0 backup or a Level 1 backup. And, to make backup types even more confusing, Level 1 backups are also of different types! Incremental Level 1 backups are either Cumulative or Differential backups.

Let’s start with a Level 0 backup. A Level 0 backup is identical to a full backup in that it contains all of the data in whatever you are backing up (remember, it could be either a whole or partial backup that you are doing), but it has one additional property: It can be used as the base backup for later incremental level 1 backups.

A Level 1 backup contains only data that has changed since a previous backup. Which previous backup? Well, it depends on the incremental Level 1 backup type. If it is an incremental Level 1 Cumulative backup, then it will always contain the changes since the last Level 0 backup. An incremental Level 1 Differential backup, on the other hand, will contain the changes since the previous incremental backup whether it was a Level 0, Level 1 Cumulative, or Level 1 Differential.

One might ask: Why do we have these two different Level 1 backup types?

It’s basically a balance between the size and speed of the backup (the first differential or cumulative backup taken after a level 0 backup are the same, but, generally subsequent differentials should be faster and smaller than subsequent cumulative backups), and the time to recover datafiles past the level 0 restore point. A recover process of a single cumulative backup should be faster than recovering 6 differential backups. However, if you have almost all add operations, then the difference could be negligible. If you have many update operations, then the difference could be significant.

Personally I think it is really an artifact from the early days of computing when backups went to tape, and the tapes drives required operators to physically mount and unmount the tapes. If you had 6 incremental differential backups, you might have to physically mount and unmount 6 different tapes if you wanted to restore and recover data in your database. Each time you had to do this, you’d get a prompt from the terminal telling you to go find this particular tape, and each time you did that your restore and recover operations would take a bit longer.

These days we don’t really have to worry about this so much. Most of the time, we just do differential backups when we do level 1 backups because the files that make up the backup are usually located in one place instead of separate physical tapes and we don’t have to mount and unmount tape drives any more, or if we do, the capacity is massive compared to the early days of computing and we are not bouncing from tape to tape like we did in the past.

For backup types, I always start with an incremental level 0 backup. In general, I don’t usually do full backups. Since an incremental level 0 is nothing more than a more flexible full backup, a whole incremental level 0 is usually the way to go for a base backup. Once that is completed, I usually follow that up with incremental level 1 differential backups and then move my level 0 backup forward in time by applying previous level 1 incremental backups to the level 0 backup, effectively moving the level 0 backup forward in time.

Backup File Type – Image Copy or Backup Set

So… Here’s another source of confusion. Backups generate output files, and there are two different types of output files: Image Copies or Backup Sets.

An image copy (which RMAN, the tool Oracle supplies to manage and use database backups, just shortens to COPY) is an exact bit for bit duplicate of the file. It includes all used and unused space in the file. So, if you’ve created a 1 TB datafile and only have 10 MB of data in it and you create an image copy backup of this file, you’re output file will be… 1 TB.

With backup sets, instead of taking an exact copy of the whole file, Oracle just extracts the actual information from the file and then creates a new file (or multiple files if specified and/or necessary) that contains the necessary information. Additionally, this file can also be compressed, so it is often much, much smaller than an image copy backup. That 1 TB datafile with 10 MB of data in it backed up as a backup set could, potentially, be even smaller than 10 MB.

Further more, image copy backups are always either full backups or incremental level 0 backups. If you have an incremental level 1 backup (either kind!), then it can’t be an image copy (it only has information that has changed since a previous backup) and therefore it will always be a backup set.

Since backup sets are so efficient, one might wonder why you’d ever create an image copy. It turns out that the efficiency of a backup set (much less storage used for the backup) is limited to the creation process. When it becomes time to recreate the original file (called a restore in Oracle terminology, which is done from either a full or incremental level 0 backup) all the data in the backup set must be read and the original file is recreated step by step until you have a copy of the file at the point in which it was backed up. With a restore we are going to create a bit for bit copy of the original file at the time of the backup… and that is exactly what an image copy is! So, instead of recreating the original file, if you have an image copy backup, you can actually point to the image copy and tell the database to use it directly. This means that the time to restore the file from an image copy can be effectively instantaneous. The time to restore a file from a backup set depends on the size of the backup set, so as your backups get bigger, your restore time gets longer if you are using backup sets.

The computer science way of saying this is:

  • A restore operation of an image copy backup can be an order 1 operation.
    Using the RMAN switch command, the amount of time to restore a 1 MB datafile is the exact same amount of time to restore a 32 GB datafile.
  • A restore operation of a backup set is always an order N operation.
    We can’t switch to a backup set, so we’ll have to read all the data and recreate the datafile from scratch. So a 1 MB datafile will restore much more quickly than a 32 GB datafile.

Backup File Destination – Disk or Tape

In the earlier days of computing computer storage was broken into two distinct buckets: Disks which were faster but extremely expensive per storage unit, and tapes which were slower but much less expensive per storage unit. These days the lines between disk and tape have become a bit blurred with cloud backups that can appear as either disk or tape and could go (on the cloud) to disk or tape under the covers (under the cloud?). Also, the cost differences generally favor tape.

As far as Oracle backups go, backups are written to one of two different devices: DISK (self explanatory) and SBT (which stands for System Backup to Tape). It’s important to understand that these device types are logical rather than physical. If you configure some AWS or Oracle Cloud Object Storage in the cloud as a local drive mounted to your computer and do a disk backup to it, as far as Oracle is concerned the backup went to disk (even though it went to ‘the cloud’). If you configure a the SBT driver to point to some disks, Oracle will consider the backup written to this device to be a tape backup. The Oracle Database Cloud Backup Module, for example, turns cloud storage into a ‘logical tape drive’.

Backups to the logical device type disk can be either image copies or backup sets. Backups to the logical device type SBT however can only be backup sets.

So, it seems kind of clear: You can backup up to image copies or backup sets to disk, and backup sets can go to tape… but, guess what (remember, this can be a bit confusing). It turns out that there are two different kinds of ‘device type disk’ backups. Oracle gives you the ability to define a special disk location called the fast recovery area. Of course, just to make things fun, when this special location was first introduced it was called the flash recovery area. When Oracle introduced this term it was before the dawn of flash disks (usually called solid state drives now). Since this could be confusing(!), Oracle decided to change the name of the flash recovery area to the fast recovery area. At least the abbreviation for the fast recovery area is the same as it always was: FRA.

So, what’s the difference between a disk backup to the FRA and a disk backup to ‘not the FRA’? Really, nothing. They are the same. There is no ‘different information’ in a backup to the FRA vs. a backup to ‘not the FRA’. However, backups to the FRA are managed differently by Oracle.

The FRA is defined by setting two Oracle database parameters (not RMAN configuration settings as one might expect): DB_RECOVERY_FILE_DEST which points to a location logically on the server, and DB_RECOVERY_FILE_DEST_SIZE which determines how much space is logically allocated for this particular database to use. It’s important to realize that the db_recovery_file_dest_size is logical, not physical. This means that if you point to a mount point (let’s say you set db_recovery_file_dest to /u02/app/oracle/fast_recovery_area) that has 1 TB of storage allocated to it, but you set the db_recovery_file_dest_size to 2 TB, Oracle won’t complain in the least… until you actually write more than 1 TB of information to the FRA. At that point you’ll get some out of space errors from the OS and whatever operation Oracle was trying to do in the FRA will fail. So, obviously, that would be just silly to do. Let’s assume that you are going to set your FRA logical size to be something that makes sense for the system you are running Oracle on. What does using this ‘logically identified and size space’ get you? Well, if you put items into the FRA and they are no longer needed to meet your retention targets (these are defined with RMAN configuration settings) then Oracle will, if it can, automatically remove no longer needed items for you. If you do not have those two database parameters set then your disk backups go to ‘not the FRA’ and you have to manually maintain the space. If you do have those two database parameters set, and you do a disk backup to ‘someplace other than the FRA’ then again, you’ll have to manually maintain the space.

So far we have covered the following Oracle Backup Terminology:

  • Backup Strategy
    • Whole
    • Partial
  • Backup Type
    • Full
    • Incremental
      • Level 0
      • Level 1
        • Differential
        • Cumulative
  • Output File Type
    • Backup Set
    • Image Copy
  • Output File Location
    • Tape (SBT)
    • Disk
      • FRA
      • not FRA

For now, this seems like quite a bit of ground to cover, and this post is getting quite long. At some point in the future I’ll attempt to cover other things like the following:

  • Database Log Mode
    • NOARCHIVELOG (I call this ‘polish your resume mode’.)
  • Backup Mode
    • Consistent/Cold
    • Inconsistent/Hot
  • Using Created Backups
    • Restore
      • SP Files
      • Control Files
      • Data Files
      • Archive Logs
    • Recover
      • Database Data Files
      • Image Copy Backups (Incrementally Updated Image Copies)
    • Creating Database Clones
    • Point In Time Recovery
  • Backup Targets
    • Database
    • Data Files
    • Archivelogs
    • FRA
  • Instance/Database Types
    • Target
    • Auxiliary
    • Catalog

And more?

Happy DBA’ing!