doc: Explain that 'file-system' must refer to mapped devices by file name.

* doc/guix.texi (File Systems): Clarify what to do when mapped devices
  are used.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 7c059b923a..4edaa922e1 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -3731,6 +3731,13 @@ is interpreted as a partition label name; when it is @code{uuid},
 The @code{label} and @code{uuid} options offer a way to refer to disk
 partitions without having to hard-code their actual device name.
 
+However, when a file system's source is a mapped device (@pxref{Mapped
+Devices}), its @code{device} field @emph{must} refer to the mapped
+device name---e.g., @file{/dev/mapper/root-partition}---and consequently
+@code{title} must be set to @code{'device}.  This is required so that
+the system knows that mounting the file system depends on having the
+corresponding device mapping established.
+
 @item @code{flags} (default: @code{'()})
 This is a list of symbols denoting mount flags.  Recognized flags
 include @code{read-only}, @code{bind-mount}, @code{no-dev} (disallow