How to find out if the filesystem supports reflink copies?

Question

I try to create a snapshot copy of a directory recursively, but it returns errors for every single file if the filesystem (custom shfs) does not support reflink copies:

# cp -a --reflink=always /mnt/user/libvirt /mnt/user/libvirt_copy
cp: failed to clone '/mnt/user/libvirt_copy/libvirt/libvirt.img' from '/mnt/user/libvirt/libvirt.img': Operation not supported
cp: failed to clone '/mnt/user/libvirt_copy/libvirt/test.txt' from '/mnt/user/libvirt/test.txt': Operation not supported
...

As it seems to be impossible to stop cp on the first error (?), which means useless looping through probably thousands of files, I instead like to pre-check if the filesystem supports reflinks.

One idea is to search for a single file and do a testcopy:

if ! find /mnt/user/libvirt -type f -print -quit | xargs -I {} sh -c 'dst_path=$(dirname $(echo {} | sed "s#^/mnt/user/libvirt#/mnt/user/libvirt_copy#")); mkdir -vp $dst_path && cp -a --reflink=always {} "$dst_path"'; then
    echo "Error: Filesystem does not support reflink"
    exit
fi

But this leaves the dir and an empty file which needs to be cleaned up afterwards:

# find /mnt/user/libvirt_copy/
/mnt/user/libvirt_copy/
/mnt/user/libvirt_copy/libvirt.img
# ls -lah /mnt/user/libvirt_copy/libvirt.img
-rw------- 1 root root 0 Nov  1 08:58 /mnt/user/libvirt_copy/libvirt.img

Isn't there an better way? Or maybe there is a solution to stop cp on the first error?

Answer 1

I'm not sure about the "custom shfs" filesystem you mention you are using, but for a normal filesystem on disk, there should be a filesystem-specific command you can run to get information about it. For example, for the XFS filesystem, a sh script could test whether it supports reflink functionality like this:

if xfs_info /yourXfsMount | grep reflink=1; then
   echo "Filesystem supports reflink"
else
   echo>&2 "ERROR: Filesystem does not support reflink"
   exit 1
fi

Answer 2

If you need a generic answer that isn't dependent on the specific filesystem, I think simply attempting to do a test copy is likely your best bet. You don't need to use your existing data for this test, you just need to know what filesystem you are attempting to do the relinking on. Rather than involving any of your real data in the test, I think it is simplest to just create an empty file with a unique name, attempt to copy it, and then clean it up. Maybe something like this:

filesystem=/yourFilesystem
filename=${filesystem}/reflinktest-$(uuid)
filename2=${filename}-copy
touch ${filename}
cp --reflink=always ${filename} ${filename2}
result=$?
rm -f ${filename} ${filename2}
if [ ${result} -ne 0 ]; then
  echo "ERROR: Filesystem ${filesystem} does not support reflink."
  exit 1
fi
echo "Reflink supported"
# Proceed with your reflink copying...