1. Have a torrent which I selective download files in it.
2. Delete files in 'incomplete pieces' folder after that torrent finished. (data already write to disk and it's really finished. There don't have disk cache problem)
3. Restart tixati and I find that torrent's progress had roll back.
4. Force check torrent and it's still doesn't change anything. torrent's progress don't go 100%
5. recover tixati userdata folder which I backuped before delete files in 'incomplete pieces' folder, and restart tixati, now that torrent's progress become 100%!

This only happen with selective-download torrents.

It's really annoy when I have some selective-download torrents and incomplete pieces folder become bigger and bigger.

It's not a bug. Tixati stores not only incomplete pieces in incomplete-pieces folder but also "dnd_file_crust" files. To understand what these files are you need to know how files are treated by bittorrent program.

Imagine a folder with a lot of files of different size in it. When it is distributed by a bittorrent protocol, a program, instead of dealing with each file separately, treats the whole folder as one large continuous file. To exchange data efficiently, the folder is divided to smaller chunks of data called pieces, which all are of the same size, for example 1MB. Integrity of each of those pieces can be checked, checksums are in the .torrent file. When the whole folder is downloaded, it doesn't matter how it is divided. When you select some files, they are contained in a number of 1MB pieces, which can be verified by the checksums.

Tixati has to download whole pieces, even if the selected file takes only small bit of a piece. For example, a file may be smaller than 1MB, Tixati still needs to get at least 1MB piece to verify and save that file. The rest of the data is saved as "dnd_file_crust" with a corresponding hash value in the incomplete-pieces folder. So to verify a selective download, files saved in download location aren't enough. Tixati needs to save the rest of "boundary" pieces somewhere and this place is the incomplete-pieces folder.

If you delete the folder, files already completed are still intact - 100% complete, however the "boundary" pieces need to be complete too in order to be sent to other peers. Tixati shows the transfer isn't complete because all the pieces, containing files selected to download, aren't complete, so it redownloads the missing pieces. When you force check such a transfer, it obviously shows as not complete, Tixati can only acknowledge whole pieces with the checksums. I believe all bittorrent programs do the same thing.

You should leave incomplete-pieces folder alone. When you remove a transfer from Tixati, corresponding files in that folder will be deleted too.

I don't know if this post makes any sense to you, if not maybe someone could explain it better.

Thanks for explaining

I downloaded a file and completed only half of it. After a computer reboot Tixati showed that "the edge of the file missing", so I stopped it and start it again to no avail. So I deleted the download (but not the file from the download directory) and instead of starting from where was left off...it starts from the beginning ( It was already half way done!) ! Why and is there a way to have Tixati starting from where a file download was left off?

Go to location and change incomplete piece storage location to default