Require sudo to delete files, but still allow writing

I've got all my files on a server raid, and it has a reasonably excessive amount of space so I'm not often going to have to delete anything.

I still have things writing to the raid all the time so I don't want to have to use sudo to do that. But is it possible to set permissions so that writing is allowed but deleting files is not?

I assume this will also require me to use sudo for mv commands, but that isn't a problem

Extra info:

Raid uses btrfs raid1

OS is on a separate drive (Ubuntu 14.04 server)

You need directory write permissions to create or delete files. You need file write permissions to change the file.

Considering this tree:

FolderA: (no write permissions for you)
   - FileX (write permissions)
   - FileY (write permissions)

You can now change FileX and FileY but you can't delete them. Nor can you create a FileZ in FolderA.

I don't think it's possible. File creation and deletion in Unix are controlled by the ability to write to the directory --- basically the same flag.

Now what you can do is create two directories --- one with write permission and the other one restricted to root.

[romano:~/tmp/test] % ls -l
total 8
drwxrwxr-x 2 romano romano 4096 Oct  8 18:06 normal
drwxrwxr-x 2 root   root   4096 Oct  8 18:06 onlyroot

You normally work on normal. Suppose you have in it:

[romano:~/tmp/test/normal] % ls
one.txt  three.txt  two.txt

You can clearly delete files and create new ones; suppose you want to protect "one.txt" from deleting. What you can do is creating a hard link to it in onlyroot:

[romano:~/tmp/test/normal] 1 % sudo ln one.txt ../onlyroot

This will create another name for one.txt in onlyroot (using a negligible amount of space; the file is not copied). Now as a normal user you can delete one.txt in the normal folder, but you will have an untouchable version under the onlyroot one.

[romano:~/tmp/test/normal] % ls
one.txt  three.txt  two.txt
[romano:~/tmp/test/normal] % rm one.txt
rm: remove regular empty file ‘one.txt’? y
[romano:~/tmp/test/normal] % cd ..
[romano:~/tmp/test] % cd onlyroot 
[romano:~/tmp/test/onlyroot] % ls
one.txt
[romano:~/tmp/test/onlyroot] % rm one.txt 
rm: remove regular empty file ‘one.txt’? y
rm: cannot remove ‘one.txt’: Permission denied