+dit(bf(--partial-dir=DIR)) Turns on --partial mode, but tells rsync to
+put a partially transferred file into DIR instead of writing out the
+file to the destination dir. Rsync will also use a file found in this
+dir as data to speed up the transfer (i.e. when you redo the send after
+rsync creates a partial file) and delete such a file after it has served
+its purpose. Note that if --whole-file is specified (or implied) that an
+existing partial-dir file will not be used to speedup the transfer (since
+rsync is sending files without using the incremental rsync algorithm).
+
+Rsync will create the dir if it is missing (just the last dir -- not the
+whole path). This makes it easy to use a relative path (such as
+"--partial-dir=.rsync-partial") to have rsync create the partial-directory
+in the destination file's directory (rsync will also try to remove the DIR
+if a partial file was found to exist at the start of the transfer and the
+DIR was specified as a relative path).
+
+If you are deleting files on the destination and your partial-dir is
+inside the destination hierarchy, make sure you specify an exclude to
+prevent the partial file from being deleted (it could get deleted at the
+end of the transfer when using --delete-after, or at the beginning of the
+transfer when using --delete). E.g. "--exclude=.rsync-partial/".
+
+IMPORTANT: the --partial-dir should not be writable by other users or it
+is a security risk. E.g. AVOID "/tmp".
+
+You can also set the partial-dir value the RSYNC_PARTIAL_DIR environment
+variable. Setting this in the environment does not force --partial to be
+enabled, but rather it effects where partial files go when --partial (or
+-P) is used. For instance, instead of specifying --partial-dir=.rsync-tmp
+along with --progress, you could set RSYNC_PARTIAL_DIR=.rsync-tmp in your
+environment and then just use the -P option to turn on the use of the
+.rsync-tmp dir for partial transfers. The only time the --partial option
+does not look for this environment value is when --inplace was also
+specified (since --inplace conflicts with --partial-dir).
+