Pack our DDT ZAPs a bit denser.

The DDT is really inefficient on 4k and up vdevs, because it always
allocates 4k blocks, and while compression could save us somewhat
at ashift 9, that stops being true.

So let's change the default to 32 KiB, which seems like a reasonable
compromise between improved space savings and inflated write sizes
for DDT updates.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #14654

man/man4/zfs.4

@@ -239,6 +239,16 @@ relative to the pool.
 Make some blocks above a certain size be gang blocks.
 This option is used by the test suite to facilitate testing.
 .
+.It Sy zfs_ddt_zap_default_bs Ns = Ns Sy 15 Po 32 KiB Pc Pq int
+Default DDT ZAP data block size as a power of 2. Note that changing this after
+creating a DDT on the pool will not affect existing DDTs, only newly created
+ones.
+.
+.It Sy zfs_ddt_zap_default_ibs Ns = Ns Sy 15 Po 32 KiB Pc Pq int
+Default DDT ZAP indirect block size as a power of 2. Note that changing this
+after creating a DDT on the pool will not affect existing DDTs, only newly
+created ones.
+.
 .It Sy zfs_default_bs Ns = Ns Sy 9 Po 512 B Pc Pq int
 Default dnode block size as a power of 2.
 .