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For historical reference, my experience goes way back – tape reels were 2400’ and weighted about 8 pounds.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