doc: Add documentation covering use of utf8 option when mounting vfat (RHBZ#823887).

examples/guestfs-faq.pod

@@ -137,6 +137,83 @@ To get further information about the failure, you have to run:
 If, after using this, you still don't understand the failure, contact
 us (see previous section).
 
+=head1 COMMON PROBLEMS
+
+See also L<guestfs(3)/LIBGUESTFS GOTCHAS> for some "gotchas" with
+using the libguestfs API.
+
+=head2 Non-ASCII characters don't appear on VFAT filesystems.
+
+Typical symptoms of this problem:
+
+=over 4
+
+=item *
+
+You get an error when you create a file where the filename contains
+non-ASCII characters, particularly non 8-bit characters from Asian
+languages (Chinese, Japanese, etc).  The filesystem is VFAT.
+
+=item *
+
+When you list a directory from a VFAT filesystem, filenames appear as
+question marks.
+
+=back
+
+This is a design flaw of the GNU/Linux system.
+
+VFAT stores long filenames as UTF-16 characters.  When opening or
+returning filenames, the Linux kernel has to translate these to some
+form of 8 bit string.  UTF-8 would be the obvious choice, except for
+Linux users who persist in using non-UTF-8 locales (the user's locale
+is not known to the kernel because it's a function of libc).
+
+Therefore you have to tell the kernel what translation you want done
+when you mount the filesystem.  The two methods are the C<iocharset>
+parameter (which is not relevant to libguestfs) and the C<utf8> flag.
+
+So to use a VFAT filesystem you must add the C<utf8> flag when
+mounting.  From guestfish, use:
+
+ ><fs> mount-options utf8 /dev/sda1 /
+
+or on the guestfish command line:
+
+ guestfish [...] -m /dev/sda1:/:utf8
+
+or from the API:
+
+ guestfs_mount_options (g, "utf8", "/dev/sda1", "/");
+
+The kernel will then translate filenames to and from UTF-8 strings.
+
+We considered adding this mount option transparently, but
+unfortunately there are several problems with doing that:
+
+=over 4
+
+=item *
+
+On some Linux systems, the C<utf8> mount option doesn't work.  We
+don't precisely understand what systems or why, but this was reliably
+reported by one user.
+
+=item *
+
+It would prevent you from using the C<iocharset> parameter because it
+is incompatible with C<utf8>.  It is probably not a good idea to use
+this parameter, but we don't want to prevent it.
+
+=back
+
+=head2 Non-ASCII characters appear as underscore (_) on ISO9660 filesystems.
+
+The filesystem was not prepared correctly with mkisofs or genisoimage.
+Make sure the filesystem was created using Joliet and/or Rock Ridge
+extensions.  libguestfs does not require any special mount options to
+handle the filesystem.
+
 =head1 DOWNLOADING, INSTALLING, COMPILING LIBGUESTFS
 
 =begin html