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@redhat.com> --- 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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|