After deleting files or/and decreasing DiskSpace variables Jenkins master
stays offline!
The offline information is permanently stored in config.xml in jenkins
install dir.
It's like
<temporaryOfflineCause
class="hudson.node_monitors.DiskSpaceMonitorDescriptor$DiskSpace">
<size>1011720192</size>
<triggered>true</triggered>
</temporaryOfflineCause>
<disabledAdministrativeMonitors>
<string>hudson.node_monitors.MonitorMarkedNodeOffline</string>
</disabledAdministrativeMonitors>
...
Delete those lines from the file an start jenkins again, everything is fine!