Linux supports a special block device called the loop device, which maps a normal file onto a virtual block device. This allows for the file to be used as a "virtual file system". Let's create such a file:

$ fallocate -l 1G disk.img
$ du -hs disk.img
1001M	disk.img

Now let's create a loop device with this file:

$ sudo losetup -f disk.img
$ losetup -a
/dev/loop0: []: (/home/user/disk.img)

The option -f finds an unused loop device, and losetup -a shows the name of the loop device that was created.

Let's create an XFS filesystem on it, with the flag reflink=1 and the label test:

$ mkfs.xfs -m reflink=1 -L test disk.img
meta-data=disk.img               isize=512    agcount=4, agsize=64000 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=0, rmapbt=0, reflink=1
data     =                       bsize=4096   blocks=256000, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal log           bsize=4096   blocks=1850, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

Now we can mount it on a directory:

$ mkdir mnt
$ sudo mount /dev/loop0 mnt
$ df -h mnt/
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      993M   40M  954M   4% /home/user/mnt

Let's create for testing a file of size 100 MB (with random data):

$ sudo chown mnt: user
$ cd mnt/
$ dd if=/dev/urandom of=test bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB, 100 MiB) copied, 0,445498 s, 235 MB/s

The command df -h . shows us 140M used:

$ df -h .
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      993M  140M  854M  15% /home/user/mnt

Let’s copy the file with reflinks enabled:

$ cp -v --reflink=always test test1
'test' -> 'test1'

$ ls -hsl
total 200M
100M -rw-rw-r-- 1 user user 100M Aug 30 10:44 test
100M -rw-rw-r-- 1 user user 100M Aug 30 10:49 test1

$ df -h .
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      993M  140M  854M  15% /home/user/mnt

So, each copy of the file is 100M, but both of them still take on disk the same amout of space as before (140M). This shows the space-saving feature of reflinks. If the file was big enough, we would have noticed as well that the reflink copy takes no time at all, it is done instantly.

If there were already normal (non-reflink) copies of the file, we could deduplicate them with a tool like duperemove:

$ cp test test2
$ cp test test3

$ ls -hsl
total 400M
100M -rw-rw-r-- 1 user user 100M Aug 30 10:44 test
100M -rw-rw-r-- 1 user user 100M Aug 30 10:49 test1
100M -rw-rw-r-- 1 user user 100M Aug 30 11:03 test2
100M -rw-rw-r-- 1 user user 100M Aug 30 11:03 test3

$ df -h .
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      993M  340M  654M  35% /home/user/mnt

Now let's install and run duperemove

$ sudo apt install duperemove

$ duperemove -hdr --hashfile=/tmp/test.hash .

$ df -h .
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      993M  198M  796M  20% /home/user/mnt

So, it has reduced the amount of disk space used from 340M to 198M.

Unmount and delete the test directory mnt/:

$ cd ..
$ umount mnt/
$ rmdir mnt/

Delete the loop device:

$ losetup -a
$ sudo losetup -d /dev/loop0

Remove the file that was used to create the loop device:

$ rm disk.img

• https://www.thegeekdiary.com/how-to-create-virtual-block-device-loop-device-filesystem-in-linux/
• https://hooks.technology/2018/03/xfs-deduplication-with-reflinks/