Operating system command failed
Backup directory does not exist.
...
INFO | jvm 1 | 2013/05/21 09:36:47 | Process timed out: false
INFO | jvm 1 | 2013/05/21 09:36:47 | Process exception null
INFO | jvm 1 | 2013/05/21 09:36:47 | Process stderr: Error: »
    The directory '/opt/continuent/backups/xtrabackup' is not writeable
...Backup Retention
          The number of backups retained is set in the backup retention field. To set it at 
          installation time, use the --backup-retention=N option where N is 
          the number of backups to retain.
        
You can check the number of currently retained backups by looking at the replicator.properties file and searching for the following property:
replicator.storage.agent.fs.retention=3
The default is 3 backups retained at any given time.