Added crontab usage example.

README.md

@@ -1,4 +1,38 @@
 zfs_transfer
 ============
 
-Maintain a mirror of ZFS pools over the network
+Maintain a mirror of ZFS pools over the network.
+
+Below is a crontab usage example. Here the various datasets
+and snapshots from the host 192.168.1.14 are replicated to our local
+ZFS pool, using SSH (-o Compression=no -c arcfour) and caching (mbuffer)
+optimizations.
+
+Parent datasets are automatically created before the initial synch,
+when you add a dataset.
+
+We use 'zfs hold' to implement locking that prevents destruction of
+remote snapshots, that would put the synchronization process in danger.
+
+This allows for laid-back snapshot expiry management on the remote host,
+since a lot of force (you need to remove the hold tag) is required to
+destroy the latest mutually known snapshot.
+
+The script should be quite tolerant against various failure conditions.
+If you can make it better, please do!
+
+- by jaakkos, on Fri Sep 5 2014
+
+```
+MAILTO="user@host.com"
+PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+
+0 0  1   *   *     zpool scrub offsite
+
+#                                    Hostname            Remote dataset                          Local dataset                          Hold tag       Keep days
+# m h  dom mon dow   command         ------------        --------------------------------------- -------------------------------------- -------------- ------------
+0 */6 *  *   *     zfs_replicate   -h 192.168.1.14    -r backuppool/virt/machines             -l offsite/virt/machines               -t offsite     -k 30
+10 */6 *  *   *     zfs_replicate   -h 192.168.1.14    -r backuppool/backuppc                  -l offsite/backuppc                    -t offsite     -k 30
+20 */6 *  *   *     zfs_replicate   -h 192.168.1.14    -r backuppool/timemachine               -l offsite/timemachine                 -t offsite     -k 30
+50 */6 *  *   *     zfs_replicate   -h 192.168.1.14    -r backuppool/vdp                       -l offsite/vdp                         -t offsite     -k 30
+```