X-Git-Url: http://git.annexia.org/?p=libguestfs.git;a=blobdiff_plain;f=src%2Fguestfs.pod;h=514db030954abf350327e702d0ee0460fd0bc43f;hp=c959d236548dad9a77a78603e0f6a545a2c959f0;hb=699f38f05f2545a2c188370500df17cc52cbdf98;hpb=70c853d67a0cd5e54c821cd08726b91174517221

diff --git a/src/guestfs.pod b/src/guestfs.pod
index c959d23..514db03 100644
--- a/src/guestfs.pod
+++ b/src/guestfs.pod
@@ -33,7 +33,8 @@ schemes, qcow, qcow2, vmdk.
 
 Libguestfs provides ways to enumerate guest storage (eg. partitions,
 LVs, what filesystem is in each LV, etc.).  It can also run commands
-in the context of the guest.  Also you can access filesystems over FTP.
+in the context of the guest.  Also you can access filesystems over
+FUSE.
 
 Libguestfs is a library that can be linked with C and C++ management
 programs (or management programs written in OCaml, Perl, Python, Ruby,
@@ -524,6 +525,93 @@ For documentation see L<guestfish(1)>.
 
 =back
 
+=head2 LIBGUESTFS GOTCHAS
+
+L<http://en.wikipedia.org/wiki/Gotcha_(programming)>: "A feature of a
+system [...] that works in the way it is documented but is
+counterintuitive and almost invites mistakes."
+
+Since we developed libguestfs and the associated tools, there are
+several things we would have designed differently, but are now stuck
+with for backwards compatibility or other reasons.  If there is ever a
+libguestfs 2.0 release, you can expect these to change.  Beware of
+them.
+
+=over 4
+
+=item Autosync / forgetting to sync.
+
+When modifying a filesystem from C or another language, you B<must>
+unmount all filesystems and call L</guestfs_sync> explicitly before
+you close the libguestfs handle.  You can also call:
+
+ guestfs_set_autosync (handle, 1);
+
+to have the unmount/sync done automatically for you when the handle is
+closed.  (This feature is called "autosync", L</guestfs_set_autosync>
+q.v.)
+
+If you forget to do this, then it is entirely possible that your
+changes won't be written out, or will be partially written, or (very
+rarely) that you'll get disk corruption.
+
+Note that in L<guestfish(3)> I<autosync is the default>.  So quick and
+dirty guestfish scripts that forget to sync will work just fine, which
+can make this extra-puzzling if you are trying to debug a problem.
+
+=item Mount option C<-o sync> should not be the default.
+
+If you use C<guestfs_mount>, then C<-o sync,noatime> are added
+implicitly.  However C<-o sync> does not add any reliability benefit,
+but does have a very large performance impact.
+
+The work around is to use C<guestfs_mount_options> and set the mount
+options that you actually want to use.
+
+=item Read-only should be the default.
+
+In L<guestfish(3)>, I<--ro> should be the default, and you should
+have to specify I<--rw> if you want to make changes to the image.
+
+This would reduce the potential to corrupt live VM images.
+
+Note that many filesystems change the disk when you just mount and
+unmount, even if you didn't perform any writes.  You need to use
+C<guestfs_add_drive_ro> to guarantee that the disk is not changed.
+
+=item guestfish command line is hard to use.
+
+C<guestfish disk.img> doesn't do what people expect (open C<disk.img>
+for examination).  It tries to run a guestfish command C<disk.img>
+which doesn't exist, so it fails, and it fails with a strange and
+unintuitive error message.  Like the Bourne shell, we should have used
+C<guestfish -c command> to run commands.
+
+=back
+
+=head2 PROTOCOL LIMITS
+
+Internally libguestfs uses a message-based protocol to pass API calls
+and their responses to and from a small "appliance" (see L</INTERNALS>
+for plenty more detail about this).  The maximum message size used by
+the protocol is slightly less than 4 MB.  For some API calls you may
+need to be aware of this limit.  The API calls which may be affected
+are individually documented, with a link back to this section of the
+documentation.
+
+A simple call such as C<guestfs_cat> returns its result (the file
+data) in a simple string.  Because this string is at some point
+internally encoded as a message, the maximum size that it can return
+is slightly under 4 MB.  If the requested file is larger than this
+then you will get an error.
+
+In order to transfer large files into and out of the guest filesystem,
+you need to use particular calls that support this.  The sections
+L</UPLOADING> and L</DOWNLOADING> document how to do this.
+
+You might also consider mounting the disk image using our FUSE
+filesystem support (L<guestmount(1)>).
+
 =head1 CONNECTION MANAGEMENT
 
 =head2 guestfs_h *