zfs snapshot - how to get the full data?

[Lack30]

System information

Type	Version/Name
Distribution	CentOS7
Kernel Version	3.10.0-1160.15.2.el7.x86_64
Architecture	amd64
OpenZFS Version	zfs-0.8.3-1

Describe the problem you're observing

Create volume:

zfs create -ps -V 5G tank/vv1
zfs set sync=always tank/vv1
zfs set compression=off tank/vv1

follow the information of volume

# zfs get all tank/vv1
NAME      PROPERTY              VALUE                  SOURCE
tank/vv1  type                  volume                 -
tank/vv1  creation              Wed Mar  9 11:02 2022  -
tank/vv1  used                  131M                   -
tank/vv1  available             158G                   -
tank/vv1  referenced            131M                   -
tank/vv1  compressratio         1.00x                  -
tank/vv1  reservation           none                   default
tank/vv1  volsize               5G                     local
tank/vv1  volblocksize          8K                     default
tank/vv1  checksum              on                     default
tank/vv1  compression           off                    local
tank/vv1  readonly              off                    default
tank/vv1  createtxg             5825081                -
tank/vv1  copies                1                      default
tank/vv1  refreservation        none                   default
tank/vv1  guid                  12809462581060552425   -
tank/vv1  primarycache          all                    default
tank/vv1  secondarycache        all                    default
tank/vv1  usedbysnapshots       0B                     -
tank/vv1  usedbydataset         131M                   -
tank/vv1  usedbychildren        0B                     -
tank/vv1  usedbyrefreservation  0B                     -
tank/vv1  logbias               latency                default
tank/vv1  objsetid              1605                   -
tank/vv1  dedup                 off                    default
tank/vv1  mlslabel              none                   default
tank/vv1  sync                  always                 local
tank/vv1  refcompressratio      1.00x                  -
tank/vv1  written               131M                   -
tank/vv1  logicalused           131M                   -
tank/vv1  logicalreferenced     131M                   -
tank/vv1  volmode               default                default
tank/vv1  snapshot_limit        none                   default
tank/vv1  snapshot_count        none                   default
tank/vv1  snapdev               hidden                 default
tank/vv1  context               none                   default
tank/vv1  fscontext             none                   default
tank/vv1  defcontext            none                   default
tank/vv1  rootcontext           none                   default
tank/vv1  redundant_metadata    all                    default
tank/vv1  encryption            off                    default
tank/vv1  keylocation           none                   default
tank/vv1  keyformat             none                   default
tank/vv1  pbkdf2iters           0                      default

Format and mount to /vv1

mkfs.ext4 /dev/zvol/tank/vv1
mount /dev/zvol/tank/vv1 /vv1

try to write data to the directory

# dd if=/dev/zero of=/vv1/a.img bs=2M count=1024
1024+0 records in
1024+0 records out
2147483648 bytes (2.1 GB) copied, 38.8102 s, 55.3 MB/s

execute the command zfs snapshot

zfs snapshot tank/vv1@s1
zfs snapshot tank/vv1@s2

clone the snapshot

zfs clone tank/vv1@s1 tank/vv1_clone
zfs clone tank/vv1@s2 tank/vv1_clone2
mount /dev/zvol/tank/vv1_clone /vv2
mount /dev/zvol/tank/vv1_clone2 /mnt

the problem arises here

# md5sum /vv1/a.img /vv2/a.img /mnt/a.img
a981130cf2b7e09f4686dc273cf7187e  /vv1/a.img
39ed878929a2b1cf4cb7bf014e109b7b  /vv2/a.img
a981130cf2b7e09f4686dc273cf7187e  /mnt/a.img

my problem

how to get the full data?