how to create a file that the device blocks are non-contiguous?

[asweisun]

The blocks allocation is decided by the kernel. When writing new data to a file, if the block is not enough, the kernel will select a new block from the device that is contiguous with file's last block.
If the selected block has been used by other file. The kernel can only select a non-contiguous block.

My questions is that:How can we create a file, the device block are non-contguous?

[Rubberman]

Please clarify. Are you asking how to create a file that consists totally (or mostly) of non-contiguous blocks?

[asweisun]

Please clarify. Are you asking how to create a file that consists totally (or mostly) of non-contiguous blocks?

Yes. How to create it?

[Rubberman]

I don't think there is a way to guarantee this. The only reliable (sort of) way to do this is to create 2 files on the same device, and then delete the one you don't want. The algorithm, in pseudo-code, goes something like this:

while not done
    open file 1 to append
    write 1 block to file
    close file
    sync file systems
    open file 2 to append
    write 1 block to file
    close file
    sync file systems
    if total size written >= requested size
        unlink file 2
        set done flag
    endif
endwhile
# at this point, file 1 (target file) should consist of non-contiguous blocks
# however, there is no guarantee of that