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.
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