+.SS ARRAYS WITH EXTERNAL METADATA
+
+From release 2.6.28, the
+.I md
+driver supports arrays with externally managed metadata. That is,
+the metadata is not managed by the kernel by rather by a user-space
+program which is external to the kernel. This allows support for a
+variety of metadata formats without cluttering the kernel with lots of
+details.
+.PP
+.I md
+is able to communicate with the user-space program through various
+sysfs attributes so that it can make appropriate changes to the
+metadata \- for example to make a device as faulty. When necessary,
+.I md
+will wait for the program to acknowledge the event by writing to a
+sysfs attribute.
+The manual page for
+.IR mdmon (8)
+contains more detail about this interaction.
+
+.SS CONTAINERS
+Many metadata formats use a single block of metadata to describe a
+number of different arrays which all use the same set of devices.
+In this case it is helpful for the kernel to know about the full set
+of devices as a whole. This set is known to md as a
+.IR container .
+A container is an
+.I md
+array with externally managed metadata and with device offset and size
+so that it just covers the metadata part of the devices. The
+remainder of each device is available to be incorporated into various
+arrays.
+