What is the most efficient way to move files between different datasets in the same zpool?

[shivabohemian]

Hello,
When I tried to move large files or folders between different datasets in the same zpool, I found it slow to use the mv command. Is there a more efficient way?

[IvanVolosyuk]

The 'mv' command between different datasets behaves like 'cp'. That's the reason why it is slow. On the other hand, in newly released ZFS 2.2 there is reflink support, so technically cp --reflink=auto src dst should be instant and you can delete the source file after that.

[shivabohemian]

Sorry. I used cp --reflink=always src dst in order to always use the shared data.
The feature@block_cloning is enabled and it works very well(cp fast) when using cp --reflink=always in the same dataset.
cp --reflink=always would never work cross dataset indeed because of the Linux hardcodes. So I use cp --reflink=auto src dst to cp files cross dataset, it's much slower than in the same dataset.
It seems like expected behavior, right?

[rincebrain]

cp --reflink=auto should still block clone cross-dataset on the same pool unless encryption is involved, so no slower.

[shivabohemian]

You can take a look at my test below, which is two unencrypted datasets

root@testserver:/test/dataset1# zfs list
NAME            USED  AVAIL  REFER  MOUNTPOINT
test           5.98G  7.12T  4.00G  /test
test/dataset1  1.97G  7.12T  1.97G  /test/dataset1
test/dataset2    96K  7.12T    96K  /test/dataset2
root@testserver:/test/dataset1# du -sh *
2.0G	dataset.zip
root@testserver:/test/dataset1# time cp --reflink=auto dataset.zip dataset2.zip

real	0m0.209s
user	0m0.004s
sys	0m0.202s
root@testserver:/test/dataset1# du -sh *
1.9G	dataset2.zip
2.0G	dataset.zip
root@testserver:/test/dataset1# time cp --reflink=auto dataset.zip /test/dataset2/dataset2.zip

real	0m18.424s
user	0m0.099s
sys	0m8.059s
root@testserver:/test/dataset1# zfs get encryption test
NAME  PROPERTY    VALUE        SOURCE
test  encryption  off          default
root@testserver:/test/dataset1# zfs get encryption test/dataset1
NAME           PROPERTY    VALUE        SOURCE
test/dataset1  encryption  off          default
root@testserver:/test/dataset1# zfs get encryption test/dataset2
NAME           PROPERTY    VALUE        SOURCE
test/dataset2  encryption  off          default

[rincebrain]

I suppose it also depends on the kernel version, if it's too old, it's not going to work either.

[shivabohemian]

All right. The kernel version I use is 5.10.