.TH snap.ro 8 "12 November 2011" "distorted.org.uk backup"
.SH NAME
snap.ro \- fake snapshot handler which remounts filesystems read-only
.SH SYNOPSIS
.B snap.ro
.I device
.IB key = value
\&...
.SH DESCRIPTION
This is a fake snapshot handler.  Rather than produce a snapshot device
containing a consistent view of the origin's contents, it simply
remounts the contained filesystem read-only.  Obviously, this isn't
satisfactory for operations which might alter the block device, but it's
fine for stuff like backups.
.PP
The following options keys are recognized, either with a
.RB ` .ro '
suffix or without.  Other keys are reported as errors, unless the key
contains a
.RB ` . '
character.
.TP
.BI op= op
The
.I op
must be either
.B snap
(the default) to create a `snapshot' \(en i.e., remount read-only \(en or
.B unsnap
to remove it \(en i.e., remount read/write.
.TP
.BI tag= tag
On snapshot creation, record the
.I tag
in the filesystem before remounting readonly; on snapshot removal,
verify that the tag recorded matches
.IR tag .
This prevents a different script from making the filesystem read/write
prematurely.  The default tag is
.BR snap .
.BI dir= mount-pt
The filesystem is mounted on
.IR mount-pt .
If you omit this option,
.B snap.ro
will search
.B /etc/mtab
for a mount point for the given
.IR device .
If exactly one such mount point is found then it will be used;
otherwise, an error is reported.
.SH BUGS
This assumes that the filesystem is mounted locally. If the block device
is exported (e.g., to a virtual guest, or using a network block device)
then this won't work and you'll need to do something more complicated.
.PP
Linux allows the same device to be mounted in several different places
using bind mounts.  Each of these occurrences needs to be made read-only
if the device contents are to be made consistent;
.BR snap.ro
is too stupid to handle this properly: instead, if
.B dir
is omitted, it checks that the device is mentioned exactly once in
.BR /etc/mtab .
.SH SEE ALSO
.BR snap (8),
.BR mount (8).
.SH AUTHOR
Mark Wooding, <mdw@distorted.org.uk>