svnsync initialize (init) — Initialize a mirror repository for synchronization from the source repository.
svnsync initialize verifies that a
            repository meets the basic requirements of a new mirror
            repository and records the initial administrative
            information that associates the mirror repository with the
            source repository (specified
            by SOURCE_URL).  This is the
            first svnsync operation you run on a
            would-be mirror repository.
Ordinarily, SOURCE_URL is
            the URL of the root directory of the Subversion repository
            you wish to mirror.  Subversion 1.5 and newer allow you to
            use svnsync for partial repository
            mirroring, though — simply specify the URL of the
            source repository subdirectory you wish to mirror
            as SOURCE_URL.
By default, the aforementioned basic requirements of a
            mirror are that it allows revision property modifications
            and that it contains no version history.  However, as of
            Subversion 1.7, you may now optionally disable the
            verification that the target repository is empty using
            the --allow-non-empty option.  While the
            use of this option should not become habitual (as it
            bypasses a valuable safeguard mechanism), it does aid in
            one very common use-case: initializing a copy of a
            repository as a mirror of the original.  This is
            especially handy when setting up new mirrors of
            repositories which contain a large amount of version
            history.  Rather than initialize a brand new repository as
            a mirror and then syncronize all of the history into it,
            administrators will find it significantly
            faster to first make a copy of the mature repository
            (perhaps using svnadmin hotcopy) and
            then use svnsync initialize
            --allow-non-empty to initialize that copy as a
            mirror which is now already up-to-date with the
            original.
Fail to initialize a mirror repository due to inability to modify revision properties:
$ svnsync initialize file:///var/svn/repos-mirror \
                     http://svn.example.com/repos
svnsync: Repository has not been enabled to accept revision propchanges;
ask the administrator to create a pre-revprop-change hook
$
        Initialize a repository as a mirror, having already
            created a pre-revprop-change hook that
            permits all revision property changes:
$ svnsync initialize file:///var/svn/repos-mirror \
                     http://svn.example.com/repos
Copied properties for revision 0.
$