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

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(+) > > diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms > index 73b72c9e10..eea31a736d 100644 > --- a/src/libvirt_private.syms > +++ b/src/libvirt_private.syms > @@ -2125,6 +2125,7 @@ virFileRewrite; > virFileRewriteStr; > virFileSanitizePath; > virFileSetACLs; > +virFileSetCOW; > virFileSetupDev; > virFileSetXAttr; > virFileTouch; > diff --git a/src/util/virfile.c b/src/util/virfile.c > index 213acdbcaa..5b169b3d11 100644 > --- a/src/util/virfile.c > +++ b/src/util/virfile.c > @@ -71,6 +71,7 @@ > # endif > # include <sys/ioctl.h> > # include <linux/cdrom.h> > +# include <linux/fs.h> > #endif > > #if HAVE_LIBATTR > @@ -4504,3 +4505,78 @@ virFileDataSync(int fd) > return fdatasync(fd); > #endif > } > + > + > +int > +virFileSetCOW(const char *path, > + virTristateBool state) > +{ > +#if __linux__ > + int val = 0; > + struct statfs buf; > + VIR_AUTOCLOSE fd = -1; > + > + VIR_DEBUG("Setting COW flag on '%s' to '%s'", > + path, virTristateBoolTypeToString(state)); > + > + fd = open(path, O_RDONLY|O_NONBLOCK|O_LARGEFILE); > + if (fd < 0) { > + virReportSystemError(errno, _("unable to open '%s'"), > + path); > + return -1; > + } > + > + if (fstatfs(fd, &buf) < 0) { > + virReportSystemError(errno, _("unable query filesystem type on '%s'"), > + path); > + return -1; > + } > + > + 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?