Hello,
I have news on this issue.
It looks like this was triggered by not having added the line “mgmofs.centraldrain true” in /etc/xrd.cf.mgm.
(as stated in : Config in quarkDB for master/slave(s) ).
Now draining is progressing and not (apparently) impacting other activities.
Sorry, this topic (draining vs central draining) was not clear to me.
Thank you Pete for the suggestion about debugging, to me this was last ressort (and I am not much used to do this).
What remains unexplained is how this small configuration mistake could have had the big impact I observed.
JM