Add a new zfs property, "redundant_metadata" which can have values
"all" or "most".  The default will be "all", which is the current
behavior.  Setting to "most" will cause us to only store 1 copy of
level-1 indirect blocks of user data files.

The new man page section for this property states

    "The exact behavior of which metadata blocks
     are stored redundantly may change in future releases."

and:

    "When set to most, ZFS stores an extra copy of most types of
     metadata. This can improve performance of random writes,
     because less metadata must be written."

The current implementation is as described above in Matt's blog.
It is controlled by a new global integer
"zfs_redundant_metadata_most_ditto_level", currently initialized
to 2. When "redundant_metadata" is set to "most", only indirect
blocks of the specified level and higher will have additional ditto
blocks created.