[Libguestfs] Guestmount seems not to sync changes

Hi all, It seems guestmount always fails to flush changes to the mounted qcow2 snapshot. To reproduce this problem: 1. take an external snapshot for a qcow2 2. guestmount the snapshot 3. do some changes, for example: `echo 123 > /test; cat /test`, right now /test can be seen created and containing the right content 4. fusermount -u {mount-point} 5. virt-cat changed file, for the example above, sometimes /test doesn't exist, sometimes /test does exist but has no content My libguestfs version is 1.20.11, and it's running on CentOS 6.6. I tried several ways to work this around, but none of them worked. I suspect it's a cache problem, so I tried to wait a while before umounting the image, sometimes this way works, sometimes it doesn't. I also looked into the documentation for syncing and caching options, I tried to mount the image with fuse options like: guestmount -o direct_io -o sync -o sync_read -o big_writes -w no luck either. What could be the cause? Is mounting a snapshot a problem? Thanks.