The goal of this document is to give some tips and tricks about the BLKID
configuration for systems installed on external media.


1. /etc/blkid.tab
-----------------

This file is updated each time the blkid command is run without '-c' or '-p'
options. If it already exists, then the existing table is saved in the same
directory with the name 'blkid.tab.old', after what the table is updated.
Maybe it should be placed somewhere in /var instead. And for systems running
from an external media, maybe this file should be placed somewhere in /run,
to not collect informations that will surely be obsolete after next boot.

Additionally, the blkid tables could be used to track people, by easily
controling that the removable drive hosting the system was plugged on an
already identified computer the last time the system has run: the UUIDs of
the computer's disks match the blkid table on the removable system. Arggh!

Note that /etc/blkid.tab and /etc/blkid.tab.old cannot just be replaced by
symlinks to files located in a temporary directory, because the blkid program
has the following behaviour:
- If the table don't already exists, it is just written. This means if
  /etc/blkid.tab is a broken symlink to /tmp/blkid.tab, the program will
  create /tmp/blkid.tab by writing into /etc/blkid.tab
- If the table already exists, it is removed before to be rewritten. This
  means if /etc/blkid.tab is a symlink to /tmp/blkid.tab, which exists,
  /etc/blkid.tab will be removed and then rewritten: the symlink is replaced
  by a regular file the second time the command is run during a session.

So, use 'CACHE_FILE' or 'BLKID_FILE' settings instead (see below).


2. CACHE_FILE
-------------

/etc/blkid.conf is the blkid configuration file. By default, it don't exist
on Debian systems. This file can contain the 'CACHE_FILE' variable assignment,
allowing the admin to write new and old blkid tables elsewhere. On a bilibop
system, you can have:
CACHE_FILE=/run/bilibop/blkid.tab


3. BLKID_FILE
-------------

If set in /etc/environment, this variable will override the 'CACHE_FILE'
variable, if set in /etc/blkid.conf. On a bilibop system, you can have:
BLKID_FILE=/run/bilibop/blkid.tab


4. IMPORTANT
------------

You should not use /run/bilibop as a location for the blkid tables in the
following cases:
- bilibop-rules package is not installed (and then /run/bilibop is not
  created by the bilibop-rules initramfs script).
- bilibop-lockfs is not enabled (and then /run/bilibop is not created by the
  bilibop-lockfs initramfs script).
- the value of BILIBOP_COMMON_BASENAME has been modified in bilibop.conf(5).
  (and the directory created by one of the initramfs scripts is not
  /run/bilibop, but /run/${BILIBOP_COMMON_BASENAME})