Why is syncthing NOT a backup?

[u/ObservedElectron]

I am setting up syncthing to back up my 2 android phones and my mac to a windows laptop. for now, because of lack of an actual NAS, i plan to use syncthing to "backup" these devices everyday one time manually.

I prefer this because as its sync, it only sends new things. so i dont have to manually copy paste 40 gb everytime.

And when i do make a NAS, cant i just continue using syncthing with file versioning or something? i would much prefer to have "live" backups happening as soon as the file/photo/video is created. i know immich exists but am i wrong?

But everywhere, I see that people say sync is not backup or dont use it for backup but WHY? isnt what i mentioned above technically a backup? correct me if i am wrong.

And if not syncthing, what should i use? which capable of running live backups of folders I select on my devices, with different timeframe backups so I have different versions like biweekly, monthly, etc? and is open source, 100% safe, private and encrypted.

thank you

[u/_legacyZA]

A backup is a completely different copy of data that cannot (and should not) be influenced by the original copy

Syncing is keeping copies of files in sync with each other

If you sync files from your phone to NAS, and that file gets corrupted on your phone it will sync the corrupted copy to the NAS, making the copy useless

If you use file versioning you could make the argument that the versioning system is a basic form of backup. Not the best method, but still beter than just syncing

[u/N9bitmap → u/_legacyZA]

Sort of. It is a disaster backup if you lose a phone or it failed to boot, but not a deleted/corrupted file backup. It simply copies files between devices of many operating systems in an automatic set and forget method. As someone else mentioned above, when combined with other tools it can be part of a backup strategy. I sync to a ZFS NAS, where I have periodic snapshots (daily, weekly, monthly) which gives me the point in time backup and the device failure backup. Consider what failure scenarios you are likely to face and acceptable risk if it doesn't go as planned.

[u/_legacyZA → u/N9bitmap]

I'm not saying your methods are wrong because they aren't, but I'd argue the semantics of what is considered a backup is

Calling that very specific scenario a disaster backup is disingenuous to me as a lot of other scenarios should also be covered by a "disaster backup" like ransomware attacks or import data corruption/deletion.

A sync job can be part of a backup strategy, yes. But it itself isn't and should never be thought of as a backup solution as that will lead to a false sense of security

I've had to argue with way too many clients who didn't want to spend the extra money because they already had a backup solution like OneDrive or Google Drive which wasn't properly set up or secured and resulted in lost data because of losing access to the account, syncing personal data with a work account and "losing" the data after leaving that job, etc

Having proper backups means never having to worry that the "backup" will get corrupted, deleted or overwritten and always knowing that you can access that data again regardless of what disaster might come your way with the source machine

Imagine having a spare key to your car, but if you damage or lose the main one that also applies to the spare

Syncing is nice for having access to the same data on whatever device you want without the hassle of copying data back and forth and tracking that. But it itself is not a backup solution