Error in fast sign

I use a main BIND to transfer data to OpenDNSSEC, and a slave BIND to get the signed RRSIG from OpenDNSSEC. I use 'nsupdate' in the main BIND to update the data, and ten RR is added every time. When the number of RR is added to 9600, OpenDNSSEC cann't sign the RRs. The error is :
Jun 27 23:57:51 localhost ods-signerd: [xfrd] unable to dump packet zone cn: ods_fopen() failed (Too many open files)
Jun 27 23:57:51 localhost ods-signerd: [xfrd] unable to commit xfr zone cn: ods_fopen() failed (Too many open files)
Jun 27 23:57:51 localhost ods-signerd: [xfrd] bad packet: zone cn received bad xfr packet (192.168.130.119)
Jun 27 23:57:51 localhost ods-signerd: [xfrd] bad packet: zone cn received bad xfr packet (192.168.130.119)
Jun 27 23:57:51 localhost ods-signerd: [xfrd] zone cn make request wait retry