I was having an issue with my rsync command:

time rsync --progress --verbose --acls --xattrs --exclude="/lib/mysql/ibdata1" --exclude="*.tmp" --stats --human-readable --recursive --del --force --times --links --hard-links --executability --numeric-ids --owner --group --perms --sparse --compress-level=6 diligence-test:/usr/ /data/temp/2017-11-15-163925/usr/

getting stuck during “receiving incremental file list” then giving up with:

Timeout, server diligence-test not responding.
rsync: connection unexpectedly closed (16384 bytes received so far) [receiver]
rsync error: error in rsync protocol data stream (code 12) at io.c(235) [receiver=3.1.2]
rsync: connection unexpectedly closed (32569 bytes received so far) [generator]
rsync error: unexplained error (code 255) at io.c(235) [generator=3.1.2]
Command exited with non-zero status 255

I could see the server-side process hang in select() with:

root@diligence-test:/home/jj5# ps aux | grep rsync
root      5421  0.0  0.1  15636  2660 ?        Ss   18:55   0:00 rsync --server --sender -vlHogtpAXrSe.iLsfxC --numeric-ids . /usr/

root@diligence-test:/home/jj5# strace -p 5421
strace: Process 5421 attached
select(1, [0], [], [0], {42, 979828})   = 0 (Timeout)

Anyway I figured out how to dodge the problem by nominating –delete-before instead of –del, e.g.:

time rsync --progress --verbose --acls --xattrs --exclude="/lib/mysql/ibdata1" --exclude="*.tmp" --stats --human-readable --recursive --delete-before --force --times --links --hard-links --executability --numeric-ids --owner --group --perms --sparse --compress-level=6 diligence-test:/usr/ /data/temp/2017-11-15-163925/usr/