I use Fedora 26 and have a DVD-R with some files. So, my goal is to delete that files and write new ones on the unused before (blank) disk space. How i can delete that old files (i.e. completely physically erase from disk) in order not to write on blank disk space smth. during the process?
Writing to a DVD-R works by telling the DVD drive to open a new "session", writing a number of blocks, and the closing the session, which writes a bit of housekeeping information to the DVD.
The whole process is write-only, and you can only write new blocks that have been unused before. It's not possible to re-use a block that has been written, even if it contains only zeroes (because it will still have non-empty error correction and format field bits, which will be non-zero). So it's physically impossible write blocks more than once.
When you read a DVD, by default the drive starts with the session that has been written last (but you can access earlier sessions with the right software).
Each session contains a "catalog" of the files and directories available on the DVD. This catalog can make use of earlier sessions.
So, you can't delete old files on the DVD and re-use their space. You can simulate deleting them by writing a new "catalog" which no longer contains those file names, but the old files still take up space on the DVD, and you'll be able to access the old files if you access earlier sessions.
But writing this catalog will take up space, so "deleting" those files will actually "use" more space.
To make multi-session DVDs that reflect changes made to a directory hierarchy, you can use the
-old-root option of
man mkisofs for details.
I mainly agree with the answer of dirkt. Some oversized comment, though:
This works only if the DVD-R was kept appendable when burning the previous session. E.g. by using cdrecord/wodim/cdrskin/xorrecord option "-multi", or by not using growisofs option "-dvd-compat" or xorriso command "-close on".
After appending a session with a new ISO 9660 directory tree, most operating systems mount the youngest session by default. But the older sessions are still mountable by e.g. Linux mount option "-o sbsector=0" and will then show their old files.
I talk of ISO 9660 filesystems here, because mkisofs is unable to add sessions to UDF filesystems and xorriso does no UDF at all.
It is of course possible to physically destroy old DVD-R content. But not by a normal burner drive and probably not without making that part of the DVD completely unreadable.
If you expect the need to partially erase data from a DVD, use DVD+RW media. They can be overwritten by normal programs like "dd" with granularity of 2 KiB. (xorriso could tell where a file's content is located on the DVD.)
Note that no DVD type is really suitable for read-write filesystems, where you can easily overwrite a file. DVD-RAM claims to be, but that's not feasible in practice.