Hi Henning,
you are mixing up several things.
> According to your explanation we shouldn't experience a performance break-down because this new O_CIOR allows to combine the benefits of performant access of Oracle using CIO and other programs can access the same files without performance break-down.
Yes, you do not experience a performance break-down due to cio issues like the read ahead or an I/O error. But you will notice a performance break-down based on file system cache usage.
> Why do we experience a decreased performance when changing from ASYNCH to SETALL?
Because of the file pages (like pages for data files) are not cached by the file system cache anymore, if the database accesses the data files with cio. So the file pages need to be read from disk, if you access the data files with third party tools (like backint with old fashion backup). If you set the filesystemio_options parameter to ASYNC, the file pages are placed in the OS file system cache (while accessing them by usual Oracle operations) and so you can access them faster by backup as they are already there. Otherwise you need to read the pages from disk (with SETALL option).
Regards
Stefan
