Re: statfs() / statvfs() syscall ballsup...

[Jamie Lokier <jamie@shareable.org>]

Joel Becker wrote:
> 	Where I work doesn't change the need for O_DIRECT.  If your Big
> App has it's own cache, why copy the cache in the kernel?  That just
> wastes RAM.

Why don't you _share_ the App's cache with the kernel's?  That's what
mmap() and remap_file_pages() are for.

>  If your app is sharing data, whether physical disk, logical
> disk, or via some network filesystem or storage device, you must
> absolutely guarantee that reads and writes hit the storage, not the
> kernel cache which has no idea whether another node wrote an update or
> needs a cache flush.

That's tough to guarantee at the platter level regardless of O_DIRECT,
but otherwise: you have fdatasync() and msync().

> If Linux came up with a better, cleaner method, Oracle might change.

Take a look at remap_file_pages() and write a note here to say if it
fits the bill.  I thought remap_file_pages() was added for Oracle, but
perhaps it was for a more modern database ;)

Thanks,
-- Jamie