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Forum Discussion
Rsynch backup --0 Differential versus Incremental
Back in the ‘old’ days backups used to have Incremental and Differential methods of backing up information. Historically these differences came about because of devices called tape drives and the associated time to restore data.
Fast forward to today.
Incremental, according to prior art, meant that if you had five Incremental backups each was based on the prior base backup. You had a base and Incremental#1 would be applied then Incremental#2, then Incremental#3 and so forth. Restoration required that all increments would be applied sequentially in the correct order.
Differential, according to prior art, meant that if you had five Differential backups each of the backups were based on changes from the original base. Restoration required only the base and the most recent Differential backup since all changes from the base were in that Differential backup.
Rsync apparently provides a way to implement those basic backup modalities. The OS manual on pages 281 and 284 discuss how to implement these features but seems silent on Differential backups and the associated base.
My question is how do I parameterize the Netgear GUI for a Rsync backup to start a ‘new’ base backup after a given number of differential backups?
The second question is if I switch between Incremental to Differential will that trigger a ‘new’ base level backup process? If not how do I make sure that the next occurrence is a base level backup?
The third question is selecting the backup time. My systems show selected times via a drop-down menu but there is no apparent way to change that time to something other than the drop down value. What am I missing here?
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ReadyNAS backup jobs don't do versioned backups, so your question is moot with regard to the type of versioning. You get one current backup or one backup with everything current plus the last version of anything that has been backed up and then deleted since the backup was purged, depending on options selected. If you want all the old files for a period of time and then purge them at some interval, you can select a time interval in the rsync options to do a full backup at some period and delete all files prior to that.
A much better solution is to have two jobs. In the main one, don't select "Remove deleted files on target" in the Advanced options. In another that is run periodically, do select it. That way, the main one keeps the old files but the periodic one purges files that no longer exist and transfers new and updated ones only, it doesn't erase all files in the backup and have to transfer them all.
Note that in both of these methods, you get a purge of old files without regard to their age. You can't set up a purge for files that are greater than some age.
You can kind of get versioning by insuring a snapshot is taken before the purge. If using the deletion and re-write option, those snapshots will grow quite large, so it's best with the other one.
FWIW, tape drives are still in use - though generally not as the primary backup medium. Tape is cheaper than cloud or disk backup, and has an inherent "air gap" that is useful when protecting from ransomware.
Digital999 wrote:
My question is how do I parameterize the Netgear GUI for a Rsync backup to start a ‘new’ base backup after a given number of differential backups?
The second question is if I switch between Incremental to Differential will that trigger a ‘new’ base level backup process? If not how do I make sure that the next occurrence is a base level backup?
You can't. The GUI doesn't support differential backups at all. It also doesn't support incremental backups as you describe them. So the second question doesn't apply.
In a ReadyNAS backup job, an incremental backup simply updates the destination folder incrementally. Files that are newer or changed are updated, the files that aren't newer and appear unchanged are not updated. At the end of the process, you end up with a complete copy of the source in the destination folder.
The modes are you describing operate very differently. Sometimes these backup modes are called "versioned" backups. You'd have one folder for that last full backup. Then a different folder for each differential backup or incremental backup. Those folders would only include the changes from the full backup (in the case of differential) or the last backup (in the case of increment) - they would not be complete copies of the source folder.
You can get something similar (though not identical) if you enable snapshots on the source share (or the destination share if you are backing up to another ReadyNAS) - then the snapshots let you access previous versions of the files and folders. But to do exactly what you'd want would require that you create your own backup scripts - it's not something you can do with the GUI.
Digital999 wrote:
The third question is selecting the backup time. My systems show selected times via a drop-down menu but there is no apparent way to change that time to something other than the drop down value. What am I missing here?
This is a bit confusing, and IMO the "to" time is a bit broken - it's not really clear how it applies.
But you are limited to the choices in the two pull-down lists.
Hmmm….
Very illustrative – thanks to both of you.
For historical reference, my experience goes way back – tape reels were 2400’ and weighted about 8 pounds. Computer installations for large systems often had 40 to 60 tape drives. Large data installations would have thousands of tapes. Operators would string six to ten tapes on their arms as they changed tapes during a sort or backup operation. In that environment the amount of data recorded was often an important design consideration.
Differential backups required more information and thus used more tape. Incremental operations required less tape but needed to be applied sequentially or the restoration process would be ruined.
So much for history. Unfortunately, that background knowledge tainted my questions – the question probably appeared foolish so I felt obliged to explain.
Back to today…
Turns out that our current workstation backup software uses a very similar metaphor and that also colored my view. Instead of tapes think CDs or DVDs or files on the ReadyNAS system. CDs and DVDs are essentially sequential write devices that produce different versions of the files being backed up.
StephenB indicated that…
You can't. The GUI doesn't support differential backups at all. It also doesn't support incremental backups as you describe them. So the second question doesn't apply.
In a ReadyNAS backup job, an incremental backup simply updates the destination folder incrementally. Files that are newer or changed are updated, the files that aren't newer and appear unchanged are not updated. At the end of the process, you end up with a complete copy of the source in the destination folder.
The key, in my mind, is to recognize that there is only one destination file produced and different versions are not possible without different Rsynch GUI defined jobs.
Now, that it has been explained it is clear -- that said the system documentation on page 281 of my manual uses the term differential which drove me to obvious invalid conclusions. This manual reference is needs clarification in my view, especially since it is different than many backup programs currently being sold.
The system documentation needs to be updated relative to the time selection.
Thanks again.
The terminology in the Netgear's documentation isn't technically correct. Both incremental and differential terms implies that you have versioning of your backups, which isn't the case on the ReadyNAS. So, they really shouldn't use those terms to be honest.
Here is a bit about how rsync works in general: https://serverfault.com/questions/138287/how-does-rsync-do-incremental-backups/138412
It is a simple yet powerful backup tool but it does not do versioning. There are ways around that where people backup into different dirs, and hardlink files, etc. (which is done via the CLI not the ReadyNAS Web UI). I use BTRFS snapshots to do my versioning. I take a snapshot on the destination NAS before the backup, every time. That way I can go "back in time" via my snapshots, if needed. It works well for me.
Cheers
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