Re: [libvirt PATCH 1/4] util: add a helper method for controlling the COW flag on btrfs

On Thu, Jul 23, 2020 at 14:00:40 +0100, Daniel Berrange wrote: > On Thu, Jul 23, 2020 at 02:57:32PM +0200, Peter Krempa wrote: > > On Mon, Jul 20, 2020 at 18:33:19 +0100, Daniel Berrange wrote: > > > btrfs defaults to performing copy-on-write for files. This is often > > > undesirable for VM images, so we need to be able to control whether this > > > behaviour is used. > > > > > > The virFileSetCOW() will allow for this. We use a tristate, since out of > > > the box, we want the default behaviour attempt to disable cow, but only > > > on btrfs, silently do nothing on non-btrfs. If someone explicitly asks > > > to disable/enable cow, then we want to raise a hard error on non-btrfs. > > > > > > Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; > > > --- > > > src/libvirt_private.syms | 1 + > > > src/util/virfile.c | 76 ++++++++++++++++++++++++++++++++++++++++ > > > src/util/virfile.h | 3 ++ > > > 3 files changed, 80 insertions(+) [...] > > > + if (buf.f_type != BTRFS_SUPER_MAGIC) { > > > + if (state == VIR_TRISTATE_BOOL_ABSENT) { > > > > Can't we handle the _ABSENT case before even attempting to open the > > file? > > This would require us to use statfs() instad of fstatfs() in order to > check the super magic. I'm not seeing that improves things. I definitely agree. But adding a function which does non-obvious things without any comment pointing to the non-obvious behaviour isn't good practice either.