Ceph » RADOS

In a thread on ceph-users [1], three different users with fairly large clusters (~600 OSDs, ~3500 OSDs) reported running into a kernel client limit on the size of the front section of the message:

Dec 26 19:28:53 mon5 kernel: libceph: mon0 10.128.150.10:6789 io error
Dec 26 19:28:53 mon5 kernel: libceph: mon0 10.128.150.10:6789 session lost, hunting for new mon
Dec 26 19:28:53 mon5 kernel: libceph: mon2 10.128.150.12:6789 session established
Dec 26 19:28:58 mon5 kernel: libceph: mon2 10.128.150.12:6789 io error
Dec 26 19:28:58 mon5 kernel: libceph: mon2 10.128.150.12:6789 session lost, hunting for new mon
Dec 26 19:28:58 mon5 kernel: libceph: mon1 10.128.150.11:6789 session established

#define CEPH_MSG_MAX_FRONT_LEN    (16*1024*1024)

The default for osd_map_message_max was reduced to 40 in luminous, but still appears to be too high. While CEPH_MSG_MAX_FRONT_LEN is just an arbitrary constant and I can certainly bump it, I'm not sure that's the right thing to do.
Should osd_map_message_max be further reduced to 20 or 10 or better yet expressed in bytes?

[1] https://www.mail-archive.com/ceph-users@lists.ceph.com/msg51522.html