How can I completely defragment ext4 filesystem

I want all my files in an ext4 filesystem not fragmented because of reasons. Sadly e4defrag (advised here: How to defrag an ext4 filesystem) fails to defragment several files. What are my alternatives?

The filesystem has all files in it already (they are not to be changed in any way) and it is almost full. There are several free blocks (according to df -h: 434M available of 85G and 80G used) which can be used as buffer. I do not need the filesystem mounted while defragmenting. Moreover, I have other filesystem available with enough space to use as a buffer.

One idea I have is to move the files to other filesystem and then copy them back somehow telling the filesystem to store them contiguously.

[EDIT]

I have just found that I cannot rely on e4defrag output. It counts files with more than one extent as fragmented, while it knows that the extents are contiguous:

$ sudo filefrag file.file
file.file: 1 extent found
$ sudo e4defrag -vc file.file
e4defrag 1.45.5 (07-Jan-2020)
<File>
[ext 1]: start 22388736: logical 0: len 32768
[ext 2]: start 22421504: logical 32768: len 32768
[ext 3]: start 22454272: logical 65536: len 32768
[ext 4]: start 22487040: logical 98304: len 27962

 Total/best extents 4/1
 Average size per extent 126266 KB
 Fragmentation score 0
 [0-30 no problem: 31-55 a little bit fragmented: 56- needs defrag]
 This file (file.file) does not need defragmentation.
 Done.

I want all my files in an ext4 filesystem not fragmented because of reasons.

While there are legitimate reasons to defrag, none require every single file to be defragged and contiguous. The main reasons anyone might want every file to be defragged are OCPD related, which is a complete waste of time because the file system will become "fragmented" again shortly after being mounted rw.

The filesystem... is almost full...

In that scenario, you probably will not be able to defrag every file because Linux defrag programs tend to work at the file level and you do not necessarily have enough contiguous free space to defrag every file.

One idea I have is to move the files to other filesystem and then copy them back...

That is your most viable option. However, specific file allocation is determined by the file system driver.

To reorder blocks it is enough to have one free block as a buffer.

Linux file system devs have not given defrag the same priority that Windows devs have. So the problem is not so much that it is technically impossible, but that no one has bothered to write any programs to do so.

The fs may be set to ro after defragmentation.

Then use a filesystem designed for ro use, like squashfs. All files will be defragged, contiguous, and even compressed.

Back before DOS 6, the usual advice for defragmenting a FAT partition was:

1. Copy the files from the partition to something else;
2. Wipe the partition;
3. Re-create the directory structure on the empty partition; and
4. Copy the files back.

I never tried this because MS-DOS 6 came out (with its included defrag utility) before defragmenting came to be an issue for me.