Formal release notes are available:
https://subversion.apache.org/docs/release-notes/1.9.html
Of particular note, the client checkout format has *not* changed so
upgrades should *not* be required.
When reading a repository (file:// or running as a local server), an
improved fsfs version 7 is available with significant performance
improvements. An optional upgrade is possible to use the new features.
Without the upgrade, this is fully read/write compatible with the
version 6 fsfs as in svn-1.8.
Relnotes: yes
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