we have mailbox 1 and Mailbox2 servers.
On mailbox 1 the Zimbra backups are taken on the mount point /backup. But recently /backup is reaching 100% as a result backup fails and the error received is

com.zimbra.common.service.ServiceException: system failure: Unable to create directory /opt/zimbra/backup/tmp/full-20120908.223219.435/sys
ExceptionId:btpool0-939:1347143539545:d5f6ef86445b3cf4
Code:service.FAILURE
at com.zimbra.common.service.ServiceException.FAILURE (ServiceException.java:253)
at com.zimbra.cs.service.backup.Backup.handleNetworkR equest(Backup.java:157)
at com.zimbra.cs.service.NetworkDocumentHandler.handl e(Unknown Source)
at com.zimbra.soap.SoapEngine.dispatchRequest(SoapEng ine.java:411)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:268)
at com.zimbra.soap.SoapEngine.dispatch(SoapEngine.jav a:160)
at com.zimbra.soap.SoapServlet.doPost(SoapServlet.jav a:266)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:727)
at com.zimbra.cs.servlet.ZimbraServlet.service(Zimbra Servlet.java:188)
at javax.servlet.http.HttpServlet.service(HttpServlet .java:820)
at org.mortbay.jetty.servlet.ServletHolder.handle(Ser vletHolder.java:487)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1093)
at org.mortbay.servlet.UserAgentFilter.doFilter(UserA gentFilter.java:81)
at org.mortbay.servlet.GzipFilter.doFilter(GzipFilter .java:148)
at org.mortbay.jetty.servlet.ServletHandler$CachedCha in.doFilter(ServletHandler.java:1084)
at org.mortbay.jetty.servlet.ServletHandler.handle(Se rvletHandler.java:360)
at org.mortbay.jetty.security.SecurityHandler.handle( SecurityHandler.java:216)
at org.mortbay.jetty.servlet.SessionHandler.handle(Se ssionHandler.java:181)
at org.mortbay.jetty.handler.ContextHandler.handle(Co ntextHandler.java:716)
at org.mortbay.jetty.webapp.WebAppContext.handle(WebA ppContext.java:406)
at org.mortbay.jetty.handler.ContextHandlerCollection .handle(ContextHandlerCollection.java:211)
at org.mortbay.jetty.handler.HandlerCollection.handle (HandlerCollection.java:114)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.handler.rewrite.RewriteHandler.h andle(RewriteHandler.java:350)
at org.mortbay.jetty.handler.HandlerWrapper.handle(Ha ndlerWrapper.java:139)
at org.mortbay.jetty.Server.handle(Server.java:313)
at org.mortbay.jetty.HttpConnection.handleRequest(Htt pConnection.java:506)
at org.mortbay.jetty.HttpConnection$RequestHandler.co ntent(HttpConnection.java:844)
at org.mortbay.jetty.HttpParser.parseNext(HttpParser. java:644)
at org.mortbay.jetty.HttpParser.parseAvailable(HttpPa rser.java:211)
at org.mortbay.jetty.HttpConnection.handle(HttpConnec tion.java:381)
at org.mortbay.io.nio.SelectChannelEndPoint.run(Selec tChannelEndPoint.java:396)
at org.mortbay.thread.BoundedThreadPool$PoolThread.ru n(BoundedThreadPool.java:442)
Caused by: java.io.IOException: Unable to create directory /opt/zimbra/backup/tmp/full-20120908.223219.435/sys
at com.zimbra.common.util.FileUtil.ensureDirExists(Fi leUtil.java:320)
at com.zimbra.cs.backup.FileBackupTarget$FileBackupSe t.<init>(FileBackupTarget.java:663)
at com.zimbra.cs.backup.FileBackupTarget.createFullBa ckupSet(FileBackupTarget.java:188)
at com.zimbra.cs.backup.BackupManager.startBackupFull (BackupManager.java:179)
at com.zimbra.cs.backup.BackupManager.backupIncrement al(BackupManager.java:369)
at com.zimbra.cs.service.backup.Backup.handleNetworkR equest(Backup.java:150)
... 31 more


Kindly advice what can be done.

Your valuable responses are highly appreciated.