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Re: Rsynch backup --0 Differential versus Incremental

Digital999
Luminary

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?

Message 1 of 12
Sandshark
Sensei

Re: Rsynch backup --0 Differential versus Incremental

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.

Message 2 of 12
StephenB
Guru

Re: Rsynch backup --0 Differential versus Incremental

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.

 

Message 3 of 12
Digital999
Luminary

Re: Rsynch backup --0 Differential versus Incremental

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.

 

Message 4 of 12
rn_enthusiast
Virtuoso

Re: Rsynch backup --0 Differential versus Incremental

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

 

Message 5 of 12
StephenB
Guru

Re: Rsynch backup --0 Differential versus Incremental


@rn_enthusiast wrote:

It is a simple yet powerful backup tool but it does not do versioning.

 


Actually when I looked into @Digital999 's question, I discovered the --compare-dest option, which does let you make versioned backups with rsync (both differential and incremental).

 

https://tylersguides.com/guides/rsync-backups/ wrote:

To create incremental and differential backups by use the –compare-dest option. The directory specified by –compare-dest is compared to the source directory. Any files in the –compare-dest that are identical to or newer than the file in the source are not copied to the destination.

...


The guide goes on with more details. 

 

But the GUI doesn't support that option.

 

@Digital999  wrote:

For historical reference, my experience goes way back – tape reels were 2400’ and weighted about 8 pounds.


FWIW, so does mine.  The drives I used were 800 bpi - which when I work it out now means they only held about 23 MB.  Times have certainly changed.

Message 6 of 12
rn_enthusiast
Virtuoso

Re: Rsynch backup --0 Differential versus Incremental

I guess I should have explained myself better. That still isn't a good way of versioning @StephenB as I see that more of a "sparse" kind of backup. I know we are going into nitpicking here and the point is moot since none of this can be done from RN GUI anyway, but IMO proper versioning is where you can see all data in each versioned folder.

 

All my different folders will show how all the data looked at that point in time. Obviously, you don't want to store duplicates of the same files so instead what many do to accomplish that is:

1. Initial full backup, rsync source data into folder "2021-04-04"

2. Copy/clone the "2021-04-04" dir into a new dir "2021-04-05", using hardlinks: cp -al

3. Next day, rsync the source data with delete flag into the new dir: rsync -avh --delete

4. Copy, using hard links dir "2021-04-05" to "2021-04-06"

5. Rsync source data with delete flag into: "2021-04-06"

and so on.

 

What this gives you, is that each folder contains the entire file structure as it looked at that time but not taking any additional space for files that didn't change. Hardlinks are broken only when rsync changes/update/removes files. Files that remain the same are just hardlinked throughout each backup iteration. It is quite similar to snapshots in a way, just less efficient because snapshots store block-changes where as rsync stores the whole file a second time, if it changed

 

That is true versioning, to me.

Message 7 of 12
rn_enthusiast
Virtuoso

Re: Rsynch backup --0 Differential versus Incremental

FWIW, I am sure that was how ReadyNAS Replicate did its versioning and that is in fact the only Netgear solution that ever had actual versioning.

Message 8 of 12
rn_enthusiast
Virtuoso

Re: Rsynch backup --0 Differential versus Incremental

And differential backups like this work just as well. Say, you wanted to do diff backup after 15 days, you simply copy (hardlink: cp -al) the original backup "2020-04-04" into "2020-04-19" and then resync the source data into "2020-04-19". again using: rsync -avh --delete

 

This is a superior way or doing it because it will still be easy to restore individual files from any date you like and if you need to restore lots of data from a major incident, you can just go to any date and copy out all the data from that point in time, without having to mess around finding pieces of data in one dir and then other data in another dir, etc.

Message 9 of 12
StephenB
Guru

Re: Rsynch backup --0 Differential versus Incremental


@rn_enthusiast wrote:

 but IMO proper versioning is where you can see all data in each versioned folder.

I don't know what "proper" versioning actually means.  I do know that the guide I linked to above will let you create the differential and incremental backups as @Digital999 described them.   If you want to call that a "sparse backup" instead, then that's fine with me.

 

How the backups are presented for backup restore isn't necessarily the same as how they are physically stored.  Acronis TrueImage has options for saving incremental backups as deltas, but still will show you the full image when you open the TIB files. 

 

But I think we are getting into the weeds here. 

 

Message 10 of 12
rn_enthusiast
Virtuoso

Re: Rsynch backup --0 Differential versus Incremental

Yea we are just splitting hairs 😄
All which is irrelevant because NAS GUI doesn't do any of it, anyway.

Message 11 of 12
Digital999
Luminary

Re: Rsynch backup --0 Differential versus Incremental

Thanks to everyone who reeplied.  Some of the later replies got into the weeds -- at least for me and my staff.

My personal opinion is that if somebody implemented a NAS GUI on top of RSynch it would be a big hit.  I certainly would  pay for that capability.  What Netgear offers is fairly thin soup -- I am reasonably convinced there would be a reasonable market.  Could be an intern project for Netgear.

 

 

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