When Jenkins performs log-rolling, it compresses the completed log file leaving a date-stamped, ".gz"-suffixed equivalent while moving on to a new "jenkins.log" file. It also unlinks the old plain-text (non-compressed) log file. However, it does not close its descriptor to that log file and thus its disk space is not freed until Jenkins itself is restarted.

On a Linux system, such ghost files can be discovered via entries in /proc/NNN/fd/*:

1. ll /proc/[0-9][0-9]/fd |egrep '/var.(deleted)'