On Thu, Jul 23, 2020 at 03:08:41PM +0200, Peter Krempa wrote:
> 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@xxxxxxxxxx>
> > > > ---
> > > > 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.
I'll add this
/**
* virFileSetCow:
* @path: file or directory to control the COW flag on
* @state: the desired state of the COW flag
*
* When @state is VIR_TRISTATE_BOOL_ABSENT, some helpful
* default logic will be used. Specifically if the filesystem
* containing @path is 'btrfs', then it will attempt to
* disable the COW flag, but errors will be ignored. For
* any other filesystem no change will be made.
*
* When @state is VIR_TRISTATE_BOOL_YES or VIR_TRISTATE_BOOL_NO,
* it will attempt to set the COW flag state to that explicit
* value, and always return an error if it fails. Note this
* means it will always return error if the filesystem is not
* 'btrfs'.
*/
Regards,
Daniel
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