This is a Perl script that can be used by system administrators to scan directory trees in cron jobs over night and view the result with QDirStat whenever it is convenient - without creating I/O load on the machine you are scanning. You can also use that script to scan directories on a server and view the result on any machine that has an X11 desktop running. The server doesn't need any more infrastructure than a normal Perl installation (i.e., no X11 / KDE / Gnome required).

Using those cache files considerably speeds up QDirStat's reading process. To give some rough numbers, on my laptop it takes QDirStat about 3 minutes to scan /usr. Reading the same information from a cache file takes 3-5 seconds.

On the downside, the disk content may have changed in the meantime. A cache file is outdated by definition. But it may still give you some rough ideas. And there are large directory trees that hardly ever change.

Or you may be a system administrator with a NFS server that houses home directories, and every now and then you have to check exactly who of your users again managed to fill up that filesystem to 95%. One thing you cannot do (or your users will hate you for it) is start QDirStat during working hours to scan all those home directories. So do that with the qdirstat-cache-writer Perl script in a cron job running in the middle of the night and view the result with QDirStat during your normal office hours.

For large directories (archives etc.) that don't change that much, you can also generate a QDirStat cache file (either with the Perl script or with QDirStat itself) and save it to that corresponding directory.

If QDirStat finds a file .qdirstat.cache.gz in a directory, it checks if the toplevel directory in that cache file is the same as the current directory, and if it is, it uses the cache file for that directory rather than reading all subdirectories from disk. If you or the users of that machine use QDirStat often, this might take a lot of I/O load from the server.

There is currently no indication that cached values are displayed. This will have to change.

When you click the "Reload" button, the directory tree is really scanned (just in case you might have thought the cache file is read again). This is intentional.

Tip: If you generate cache files with "qdirstat-cache-writer -l", they will become somewhat (~20%) larger, but you can also use them as a replacement for "locate". Simply use "zgrep" in such a file and ignore the size, mtime etc. fields.

See also QDirStat-for-Servers.md