Allow QEMU to connect directly to an NBD server, by Laurent Vivier.

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@4838 c046a42c-6fe2-441c-8c8c-71466251a162

qemu-doc.texi

@@ -1321,6 +1321,7 @@ snapshots.
 * qemu_nbd_invocation::       qemu-nbd Invocation
 * host_drives::               Using host drives
 * disk_images_fat_images::    Virtual FAT disk images
+* disk_images_nbd::           NBD access
 @end menu
 
 @node disk_images_quickstart
@@ -1503,6 +1504,40 @@ What you should @emph{never} do:
 @item write to the FAT directory on the host system while accessing it with the guest system.
 @end itemize
 
+@node disk_images_nbd
+@subsection NBD access
+
+QEMU can access directly to block device exported using the Network Block Device
+protocol.
+
+@example
+qemu linux.img -hdb nbd:my_nbd_server.mydomain.org:1024
+@end example
+
+If the NBD server is located on the same host, you can use an unix socket instead
+of an inet socket:
+
+@example
+qemu linux.img -hdb nbd:unix:/tmp/my_socket
+@end example
+
+In this case, the block device must be exported using qemu-nbd:
+
+@example
+qemu-nbd --socket=/tmp/my_socket my_disk.qcow2
+@end example
+
+The use of qemu-nbd allows to share a disk between several guests:
+@example
+qemu-nbd --socket=/tmp/my_socket --share=2 my_disk.qcow2
+@end example
+
+and then you can use it with two guests:
+@example
+qemu linux1.img -hdb nbd:unix:/tmp/my_socket
+qemu linux2.img -hdb nbd:unix:/tmp/my_socket
+@end example
+
 @node pcsys_network
 @section Network emulation