When I copy something *to* a ZFS volume, there are always long pauses in the copy progress. This is observed in the macOS Finder, watching the progress of how many bytes have been written. My observations about these pauses:
1. They are quite long... sometimes 10 seconds or more, but are of varying lengths.
2. They happen regularly, periodically, during the copy.
3. They only happen when *writing to* the ZFS volume; reading is linear.
4. The pool is created using settings recommended on the Wiki.
5. It does not matter what devices are used... even nice SSDs do the same thing.
6. My current test pool is a mirror of two Samsung 850 EVO 4TB SSDs.
7. It doesn't matter how the devices are connected to the computer (directly plugged into SATA on my 2010 Mac Pro or external, no difference).
8. If the same devices are configured as HFS or APFS, no pauses are observed.
9. The data is being copied from a PCIe SSD in the Mac Pro so as to not be a bottleneck.
As for actual performance data, the same two drives were first configured as a mirrored APFS pair, connected via USB3. My test folder is a 31GB folder of iPhone photos and videos. When copying to the APFS volume, it took 167s and there were no pauses observed in the copy progress. Then the APFS volume was destroyed and a ZFS pool/volume made in its place on the same disks. The copy then took 272s with frequent pauses observed. Although I will say that the drive lights on the enclosure never stopped flashing during writing. I moved the ZFS pool to the internal bays in the Mac Pro and the time dropped to 188s (not sure how APFS would have fared) but the long pauses were still there. In that configuration, moving the data the other direction (reading from ZFS, writing to boot PCIe SSD) the copy took 113s and showed no pauses.
Is there something I'm missing? Are the pauses for the checksums to be generated? I would have thought the machine should be pretty capable (12x 3.46GHz cores, 48GB).
Is this normal behavior?