Roadmap

1. test if this still happens in current Tails.

The problem

When activating the APT related persistence presets in read only mode the parent folder (and sub-folders) of where the directories are mounted get hit by some aufs bug (or limitation). For instance, the preset making /var/lib/apt/lists persistent produces the following error when trying to create/modify/delete a file in /var/lib/apt and /var/lib/apt/mirrors etc.:

touch: setting times of `/var/lib/apt/test': Input/output error

and in dmesg:

aufs au_lkup_neg:326:touch[4924]: I/O Error, apt should be negative on b0.

The error does not occur when dealing with the mounted directory like /var/lib/apt/lists. Unmounting the preset doesn’t fix it, but since the underlying fs (/) is also aufs, perhaps the outer aufs is corrupted for some reason?

Interestingly (and obiously) other presets are not affected, so a fine question is what makes the APT presets different. Here’s a list:

• They’re owned by root:root
• ???

There seems to have been a discussion on aufs mailing list, with good explanations on the reason why that kind of bugs happens, and a possible fix by using the udba=inotify option. See http://sourceforge.net/mailarchive/forum.php?thread_name=47B94A72.7030406%40slax.org&forum_name=aufs-users

However, reading aufs manpage, it seems that the udba=inotify option might have a lot of downsides. But it provides more outputs on the reason of that kind of bugs.