[Pvfs2-developers] Re: pvfs fuse interface

[John Chandy]

On Aug 17, 2007, at 1:02 PM, Murali Vilayannur wrote:

> Hi John,
>
>>> Using direct_io avoids the page cache.  Basicaly, direct_io calls
>>> back almost immediately into the user space code rather than going
>>> through the generic_file_read/write path.  What that also means is
>>> that some attrs may not get updated properly if they are getting
>>> cached in the kernel.
>
> I did not think that using O_DIRECT causes coherence issues in the  
> VFS caches.
> that would be disastrous for databases that rely on O_DIRECT, no?
> What attrs don't get updated?

Actually, I'm talking about using the direct_io option in Fuse not  
the O_DIRECT option.  When you use direct_io, Fuse skips the kernel  
entirely and lets the user space code handle everything.  On MacFUSE  
which is what I was using the most, the attributes dont get  
invalidated all the time.  So, if you did a write that changed the  
size of the file and then did a getattr, the old size would be  
returned from the kernel's cached attributes.  I had to submit a  
patch to MacFUSE to get it partly fixed.

I assumed the same problem existed with Linux, but I just checked and  
the Linux Fuse kernel implementation invalidates all the attributes  
whenever you do a read or write, so I guess its not a problem.

John.