Skip to main content

Reclaiming VM Hard Disk Space from VirtualBox

Compress Image

Creating a virtual machine requires specifying the amount of space to allocate for the virtual hard disk as one of the first steps. Dreading the notorious "not enough disk space" message, one usually aims to be on the safe side and reserves a generous amount at this stage. If the assignment is done in a static fashion then the allocated space is immediately claimed from the underlying file system and one thus has to come up with a pretty good guess in order not to waste space. Thankfully most VM solutions allow dynamic allocations which can grow on demand limited only by an upper bound making this a very attractive choice.

A newly created VM with such an dynamic disk image indeed starts out nice and space saving but over time the image will eventually take up all the committed underlying disk space even though the VM reports to use only a subset of it. Reflecting on why this is the case leads us to an elegant setup reclaiming unused storage space on the host system.

Read more… (6 min remaining to read)

Undertime - Matching Up Time Zones

Multiple Timezone Clocks

Occasionally I need to setup teleconferences with people in different time zones. For two timezones I am still able to do the calculations and assessment of the possibilities in my head. However, once three or more timezones are involved, I find myself requiring some assistance.

Already a while ago I found the neat little Python script undertime which is very easy to use.

Read more… (2 min remaining to read)

DEM data for QMapShack

As noted in my previous post GPX Downloads from I still need to rework my workflows from QLandkarteGT to the new QMapShack tool as only the latter is available for a Debian Buster system.

For my use cases, knowing the height of a map location is very important and so providing QMapShack with digital elevation data (DEM for short) is an important step. I cannot remember exactly how I did this for QLandkarteGT in the past, but I remember a lot of web browsing to find and download the correct Shuttle Radar Topology Mission (SRTM) followed by some magic script invocations to package it up for QLandkarteGT. Switching to QMapShack seems like a good time to reevaluate how this can be done most easily.

SRTM Tile Grabber

Read more… (3 min remaining to read)

BPF Superpowers for Linux

A powerful set of tools is helpful not only for developers. On GNU/Linux strace is one such tool and helped me already with a wide variety of problems. However it is always confined to one process or a process tree and cannot help with system wide problems / questions. Although there have been many attempts in the past, the Linux developers seem to have found a common underlying infrastructure in the form of BPF for such tools, allowing flexible and high-performance probing.

Already back in 2016, the Netflix developer Brendan Gregg showed what is possible in his talk Linux BPF Superpowers.

Linux bcc/BPF Tracing Tools

Read more… (3 min remaining to read)