FFS on three systems

Posts: 16 · gcvdberg 19 Jun 2019, 15:30

I have the impression that FFS doesn't work correct when used to synchronize 3 systems. I have two computers and a NAS in between so I synchronize computer 1 with the NAS and computer 2 with the NAS, separately. But it seems that there are problems with decisions on deletion or copy. When there is a file on the left side it seems not clear whether this is a new file, to be copied to the right side, or that the file on the right side has been removed so that also the file on the left side must be removed. I experience that wrong decisions are taken. Mostly files that I had removed were placed back by FFS.
Is this a recognized problem and what can I do about it?

Posts: 2286 · Plerry 20 Jun 2019, 06:42

In your case you should use the Two-way sync variant (in Sync-settings, F8).
That variant should be able to properly handle all critical cases when doing multiple syncs with a central location (your NAS).

Posts: 16 · gcvdberg 20 Jun 2019, 11:40

Thank you for your answer. But I always use already two-way sync. Can you explain me how FFS takes decisions on whether to copy a file or remove it? How does it know that a file has been deleted in the past? Is there maybe some system in the background that detects and remembers file deletion?

Posts: 2286 · Plerry 20 Jun 2019, 14:51

FFS uses a database per location and per sync that stores the status of all files/folders involved in that sync as per the end of the sync. (see e.g. Detect Moved Files in the Synchronization Settings manual section).
At the end of the very first time a specific sync is run, such sync specific database is created at either side.

At the start of a new run (Compare) of that sync, the present status of files and folders in that location is compared to the stored status, and the deltas can be determined (file modifications, added files, deleted files and renames).
After doing this for the left- and right-side location, the correct sync actions can be determined.
If, since last running that sync,

- A file was modified in the left-, and not in the right-location, the right side will be updated
- A file was modified in the right-, and not in the left-location, the left side will be updated
- A file was modified in both the left- and right-location, this is an unresolved conflict
- A file was added to the left side, and not to the right side, it will be copied to the right side
- …
- A file was deleted from the right side, and not from the left side, it will be deleted from the left side
- … (etcetera)

Posts: 16 · gcvdberg 20 Jun 2019, 19:30

Thanks Pierry for your answer. I understand that there are file systems that are not supported for detection of file movements. Maybe I have such a file system. Both computers are on Linux and the NAS is a WD based on Linux. Are there perhaps tests to investigate the proper detection of file movements? Isn't it strange that FFS itself cannot warn for expected problems?

Posts: 3606 · xCSxXenon 20 Jun 2019, 21:15

I have only run into similar issues when I start manually running sync configurations, thus messing with the database file. I would make sure you are using RealTimeSync as well, because syncing is a two stage process for this scenario. First stage is syncing computer 1 and the NAS, then the NAS and computer 2. The only time you would get files back that were previously deleted on computer 2 is if they still exist on computer 1 AND the database got messed up. RTS can mitigate that by syncing much quicker once it detects the storage folder is free to use.

Posts: 16 · gcvdberg 21 Jun 2019, 07:33

I suppose that my problem is caused on computer 2. That is a laptop with Windows 10 (NTFS) AND Linux Mint. Both systems act on the same file system. I prefer Linux, but sometimes I need to use Windows for certain tasks. Backup is always with FFS on Linux. Now I notice that I have a mess. Probably the database is disrupted. I never used RTS but will look how this works.