Recently, I noticed that changed files were not picked up by the Nextcloud client as fast as before. As a result I sometimes missed a file (or changes in a file) on my laptop that had been created on my desktop PC.

Today, I tried to run tail and got the following message: 

tail: inotify resources exhausted
tail: inotify cannot be used, reverting to polling

This made me realise that the problem with the delayed client sync could be related to the inotify system for monitoring file changes.

It turns out there is a maximum to the number of inotify file watches: 

sysctl fs.inotify

fs.inotify.max_queued_events = 16384
fs.inotify.max_user_instances = 128
fs.inotify.max_user_watches = 524288

Given that on my desktop the Nextcloud client syncs about 235 GB with my personal Nextcloud server and about 10 GB to two servers for work (including several Git repositories), I could imagine the 65536 watches is not enough. Indeed, manually increasing that number made file syncs more or less instantaneous again: 

sysctl -n -w fs.inotify.max_user_watches=524288
sysctl fs.inotify

524288
fs.inotify.max_queued_events = 16384
fs.inotify.max_user_instances = 128
fs.inotify.max_user_watches = 524288

To make these changes permanent I added the following lines at the bottom of /etc/sysctl.conf: 

grep -A1 inotify /etc/sysctl.conf

# Increase the number of inotify watches so that the Nextcloud client
# keeps syncing without delay
fs.inotify.max_user_watches=524288

Side note: to apply the changes in the /etc/sysctl.conf file use sudo sysctl -p.

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