[vladvasiliu]

But if the files are only on the remote storage and not local, chances are they haven't been modified recently, so it shouldn't download them fully, just check the metadata cache for size / modification time and let them be if they didn't change.

So, in practice, you shouldn't have to download the whole remote drive when you do an incremental backup.

[bayindirh]

You can't trust size and modification time all the time, though mdate is a better indicator, it's not foolprooof. The only reliable way will be checksumming.

Interestingly, rclone supports that on many providers, but to be able to backblaze support that, it needs to integrate rclone, connect to the providers via that channel and request checks, which is messy, complicated, and computationally expensive. Even if we consider that you won't be hitting API rate limits on the cloud provider.

[NetMageSCW]

If you can’t trust modification time you are doing something so unusual that you probably need to be handling your backups privately anyway.

[bayindirh]

I don't think so.

Sometimes modification time of a file which is not downloaded on computer A, but modified by computer B is not reflected immediately to computer A.

Henceforth, backup software running on computer A will think that the file has not been modified. This is a known problem in file synchronization. Also, some applications modifying the files revert or protect the mtime of the file for reasons. They are rare, but they're there.