understanding memory issues

Hi!

We use NSD 4.1.6 as slave for a large zone (2G zone file). It seems
sometimes the memory is too short:

03:44:19 nsd[16836]: xfrd: zone xx committed "received update to serial
2018063010 at 2018-06-30T03:44:19 from 2a02:850:9::5 TSIG verified with
key rcode0-distribution"
03:45:30 nsd[16837]: rehash of zone xx. with parameters 1 0 5
6a0bf229ad0c7a2a
03:45:39 nsd[16837]: nsec3 xx 1 %
03:46:29 nsd[16837]: zone xx. received update to serial 2018063010 at
2018-06-30T03:44:19 from 2a02:850:9::5 TSIG verified with key rcode0-xxx
of 1717205965 bytes in 324.657 seconds
03:46:29 nsd[16837]: fork failed: Cannot allocate memory
03:46:32 nsd[16836]: process 16837 exited with status 256
03:46:32 nsd[7188]: handle_reload_cmd: reload closed cmd channel
03:46:32 nsd[7188]: Reload process 16837 failed, continuing with old
database
03:46:32 nsd[16836]: zone xx serial 2018063009 is updated to 2018063010.


What confuses me is that since above error all following attemtps to
transfer and activate the new zone fails, but without any reason.


03:51:50 nsd[16836]: xfrd: zone xx committed "received update to serial
2018063011 at 2018-06-30T03:51:50 from 2a02:850:9::5 TSIG verified with
key rcode0-xxx"
03:51:58 nsd[16836]: xfrd: zone xx: soa serial 2018063011 update failed,
restarting transfer (notified zone)
03:56:05 nsd[16836]: xfrd: zone xx committed "received update to serial
2018063011 at 2018-06-30T03:56:05 from 2a02:850:9::5 TSIG verified with
key rcode0-xxx"
03:56:18 nsd[16836]: xfrd: zone xx: soa serial 2018063011 update failed,
restarting transfer (notified zone)


Any idea why NSD is not logging the cause of the "update failed"? I
guess it is also memory related, or does NSD just not recover from the
initial "Cannot allocate memory"?

Thanks
Klaus