But I cannot use an editor like pico to create a file on a JFFS-filesystem. It leads to an serious error. Other operations like copying or moving files are no problem.

So I've written a small program which uses file-mapping to change given files. And the results were very strange: When I run this application it should open a file called map.txt and change the first bytes into the sequence "abcdef...xyz". But when I read the file after I've run that application, the file seems to be unchanged. Changes appear first when I unmount that fs and mount in again. Then the file looks like a concatenation of the unchanged and the changed version.