persistence.conf.5

.TH LIVE\-BOOT conf 2012\-12\-16 3.0~b9-1 "Debian Live Project"

.SH NAME
\fBpersistence.conf\fR \- Configuration file for persistence media in
live\-boot

.SH DESCRIPTION
If live-boot probes a persistence volume with the label (or GPT name,
or file name, but from now on we will just say "label") "persistence",
that volume's persistence is fully customizable through the
\fBpersistence.conf\fR file stored on the root of its file system. Any such
labeled volume must have such a file, or it will be ignored.
.PP
The format of \fBpersistence.conf\fR allows empty lines and lines starting
with a "#" (used for comments), both which will be ignored. A so
called "custom mount" has the format:
.PP
.RS
\fIDIR\fR [\fIOPTION\fR]...
.RE
.PP
which roughly translates to "make \fIDIR\fR persistence in the way
described by the list of \fIOPTION\fRs".
.PP
For each custom mount \fIDIR\fR must be an absolute path that cannot
contain white spaces or the special . and .. path components, and
cannot be /live (or any of its sub-directories).
Once activated all changes (file
deletion, creation and modification) to \fIDIR\fR on the live file
system are stored persistently into a path equivalent to \fIDIR\fR on
the persistence media, called the source directory. The default way to
achieve persistence is to simply bind-mount the corresponding source
directory to \fIDIR\fR, but this can be changed through the use of
\fIOPTION\fRs.
.PP
All custom mounts will be done in an order so that no two custom
mounts can "hide" each other. For instance, if we have the two
\fIDIR\fR:s /a and /a/b it would always be the case that /a is mounted
first, then /a/b. This remains true no matter how the lines in
\fBpersistence.conf\fR are ordered, or if several \fBpersistence.conf\fR files
on different persistence media are used at the same time. However, it
is forbidden for custom mounts to have their source directory inside
the source directory of another custom mount, so the source
directories that are auto-created by live-boot does not support
"nested" mounts like /a and /a/b on the same media. In this case you
must use the \fBsource\fR option (see below) to make sure that they
are stored in different source directories.
.PP
When a source directory doesn't exist on the persistence media for a
certain custom mount, it will be created automatically, and
permissions and ownership will be optimistically set according to
\fIDIR\fR. It will also be bootstrapped by copying the contents of the
\fIDIR\fR into its source directory on the persistence media. The
bootstrapping will not happen when the \fBlink\fR or \fBunion\fR
options are used (see below).

.SH OPTIONS
Custom mounts defined in \fBpersistence.conf\fR accept the following
options in a coma-separated list:
.IP "\fBsource\fR=\fIPATH\fR" 4
When given, store the persistence changes into \fIPATH\fR on the
persistence media. \fIPATH\fR must be a relative path (with respect to the
persistence media root) that cannot contain white spaces or the
special . or .. path components, with the exception that it can be
just . which means the persistence media root. This option is mostly
relevant if you want to nest custom mounts, which otherwise would
cause errors, or if you want to make the whole media root available
(similar to the now deprecated \fBhome-rw\fR type of persistence).
.PP
The following options are mutually exclusive (only the last given one
will be in effect):
.IP "\fBbind\fR" 4
Bind-mount the source directory to \fIDIR\fR. This is the default.
.IP "\fBlink\fR" 4
Create the directory structure of the source directory on the
persistence media in \fIDIR\fR and create symbolic links from the
corresponding place in \fIDIR\fR to each file in the source directory.
Existing files or directories with the same name as any link will be
overwritten. Note that deleting the links in \fIDIR\fR will only
remove the link, not the corresponding file in the source; removed
links will reappear after a reboot. To permanently add or delete a
file one must do so directly in the source directory.
.IP
Effectively \fBlink\fR will make only files already in the source
directory persistent, not any other files in \fIDIR\fR. These files
must be manually added to the source directory to make use of this
option, and they will appear in \fIDIR\fR in addition to files already
there. This option is useful when only certain files need to be
persistent, not the whole directory they're in, e.g. some
configuration files in a user's home directory.
.IP "\fBunion\fR" 4
Save the rw branch of a union on the persistence media, so only the
changes are stored persistently. This can potentially reduce disk
usage compared to bind-mounts, and will not hide files added to the
read-only media. One caveat is that the union will use \fIDIR\fR from
the image's read-only file system, not the real file system root, so
files created after boot (e.g. by live-config) will not appear in the
union. This option will use the union file system specified by
live-boot's \fBunion\fR boot parameter, but is not supported with
\fBunion=unionmount\fR.

.SH DIRECTORIES
.IP "\fB/live/persistence\fR" 4
All persistence volumes will be mounted here (in a directory
corresponding to the device name). The \fBpersistence.conf\fR file can
easily be edited through this mount, as well as any source directories
(which is especially practical for custom mounts using the
\fBlink\fR option).

.SH EXAMPLES

Let's say we have a persistence volume \fIVOL\fR with the a
\fBpersistence.conf\fR file containing the following four lines (numbered
for ease of reference):
.TP 7
1.
/home/user1 link,source=config-files/user1
.TP
2.
/home/user2 link,source=config-files/user2
.TP
3.
/home
.TP
4.
/usr union
.PP
The corresponding source directories are:
.TP 7
1.
\fIVOL\fR/config-files/user1 (but it would be \fIVOL\fR/home/user1
without the \fBsource\fR option)
.TP
2.
\fIVOL\fR/config-files/user2 (but it would be \fIVOL\fR/home/user2
without the \fBsource\fR option)
.TP
3.
\fIVOL\fR/home
.TP
4.
\fIVOL\fR/usr
.PP
It was necessary to set the \fBsource\fR options for 1 and 2, since
they otherwise would become nested with 3's source, which is invalid.
.PP
Line 3 will be taken care of before line 1 and 2 in order to prevent
custom mounts 1 and 2 from being hidden by 3. When line 3 is handled,
\fIVOL\fR/home is simply bind-mounted on /home. To illustrate what
happens for lines 1 and 2, let's say that the following files exist:
.TP 7
a.
\fIVOL\fR/config-files/user1/.emacs
.TP
b.
\fIVOL\fR/config-files/user2/.bashrc
.TP
c.
\fIVOL\fR/config-files/user2/.ssh/config
.PP
Then the following links and directories will be created:
.TP 7
Link:
/home/user1/.emacs -> \fIVOL\fR/config-files/user1/.emacs (from a)
.TP
Link:
/home/user2/.bashrc -> \fIVOL\fR/config-files/user2/.bashrc (from b)
.TP
Dir:
/homea/user2/.ssh (from c)
.TP
Link:
/home/user2/.ssh/config -> \fIVOL\fR/config-files/user2/.ssh/config
(from c)
.PP
One could argue, though, that lines 1 and 2 in the example
\fBpersistence.conf\fR file above are unnecessary since line 3 already
would make all of /home persistent. The \fBlink\fR option is
intended for situations where you don't want a complete directory to
be persistent, only certain files in it or its sub-directories.
.PP
Line 4 can be mounted at any time since its \fIDIR\fR (and source
directory) is completely disjoint from all the other custom
mounts. When mounted, \fIVOL\fR/usr will be the rw branch due to the
\fBunion\fR option, and will only contain the difference compared to
the underlying read-only file system. Hence packages could be
installed into /usr with great space-wise efficiency compared to
bind-mounts, since in the latter case all of /usr would have to be
copied into \fIVOL\fR/usr during the initial bootstrap.

.SH SEE ALSO
\fIlive\-boot\fR(7)
.PP
\fIlive\-build\fR(7)
.PP
\fIlive\-config\fR(7)
.PP
\fIlive\-tools\fR(7)

.SH HOMEPAGE
More information about live\-boot and the Debian Live project can be
found on the homepage at <\fIhttp://live.debian.net/\fR> and in the
manual at <\fIhttp://live.debian.net/manual/\fR>.

.SH BUGS
Bugs can be reported by submitting a bugreport for the live\-boot
package in the Debian Bug Tracking System at
<\fIhttp://bugs.debian.org/\fR> or by writing a mail to the Debian
Live mailing list at <\fIdebian-live@lists.debian.org\fR>.

.SH AUTHOR
persistence.conf was written by anonym <\fIanonym@lavabit.com\fR> for the
Debian project.