Re: O_DIRECT on tmpfs (again)

["Richard W.M. Jones" <rjones@redhat.com>]

On Thu, Nov 29, 2012 at 05:32:14PM -0800, Hugh Dickins wrote:
> Like you, I'm really hoping someone will join in and say they'd been
> disadvantaged by lack of O_DIRECT on tmpfs: no strong feeling myself.

Not disadvantaged as such, but we have had a workaround in libguestfs
for a very long time.

If you use certain qemu caching modes, then qemu will open the backing
disk file using O_DIRECT.  This breaks if the backing file happens to
be on a tmpfs, which for libguestfs would not be unusual -- we often
make or use temporary disk images for various reasons, and people
sometimes have /tmp on a tmpfs.

In 2009 I added code to libguestfs so that if the underlying
filesystem doesn't support O_DIRECT, then we avoid the troublesome
qemu caching modes.  The code is here:

  https://github.com/libguestfs/libguestfs/blob/master/src/launch.c#L147

Since the workaround exists and has been in use for years, we don't
need tmpfs to change.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

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