After decades of using Linux, I often imagine that I know my way around. Yet, humblingly, sometimes it can still surprise me. The latest example came with the sudden discovery that I could no longer install or delete packages, although applications continued to run normally. My search for a solution eventually led me to a full apt-cache archives, the directory in which removed packages are stored in case they are wanted. But let me start with the symptoms first so others can save time by going directly to ways to fix the problem.

I had never had such a problem before, so I was unprepared when it struck. In the last eight months, however, I had been exploring and deleting packages more often than usual, unknowingly setting up the condition that would cause the problem to strike. The first sign was a Flatpak package that suddenly refused to start. There was no notification of a newer version of the package, and at first I put the problem down to some vagary of Flatpak. To my bewilderment, I could not delete the package. Stranger still, I could not delete any DEBs, either. Instead, running apt-get dist-upgrade, alone of all the basic actions attempted, produced a "no space left" message. Yet typical productivity apps such as Firefox and LibreOffice continued to run as usual.

That was a clue, had I realized it at the time, but the message had misled me to the conclusion that the system as a whole had a memory problem. Yet managing packages is far from the most intensive action on a Debian installation, and the free command showed that the installed RAM had barely been touched, let alone the swap partition. Perhaps my use of six filesystems was the problem? Either /tmp or /var, the two most changeable partitions, could have filled without me noticing? I was quite prepared to crop a few gigabytes from the /home partition, the largest of my filesystems, but GNU Parted showed plenty of space remained on all of them. Clearly, the problem was not system-wide, but what was it then? I began searching through /var for deletable files, and there I discovered Apt's cache.

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