If a zvol has the default setting for the "volblocksize" property, it is
8KB. However, it is displayed as "-" (not present), rather than "8K".
The problem was introduced by:
commit 25228e830e86924a41243343b1de9daf2d7dd43a
    Author: Matthew Ahrens <mahrens@delphix.com>
    Date:   Thu Nov 17 14:37:24 2016 -0800
7571 non-present readonly numeric ZFS props do not have default value
which changed changed get_numeric_property() to indicate that readonly
default properties are not present. However, zfs_prop_readonly() returns
TRUE for both readonly and set-once properties (e.g. volblocksize).
Amusingly, that commit essentially reverted:
6900484 default volblocksize is no longer being reported correctly
from November 2009. However, that change was not correct either; the
correct solution is to only do this check for "truly readonly" (i.e. not
setonce) properties.
$ zfs list -t volume -o name,volblocksize
    NAME
VOLBLOCK
    domain0/group-100/appdata_container-101/appdata_windows_timeflow-102/
archive            -
    domain0/group-100/appdata_container-101/appdata_windows_timeflow-102/
datafile           -
    domain0/group-100/appdata_container-101/appdata_windows_timeflow-102/
external           -
    rpool/dump
128K
    rpool/swap
4K
    rpool/swap1
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