[sheepdog] [PATCH 1/9] Doc. "Sheepdog Advanced" add chapter "more concepts"

[Valerio Pachera]

Signed-off-by: Valerio Pachera <sirio81 at gmail.com>
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 doc/more_concepts.rst |   61 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 doc/more_concepts.rst

diff --git a/doc/more_concepts.rst b/doc/more_concepts.rst
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+More Concepts
+===============
+
+Architecture Overview
+*********************
+
+|Architecture Overview|
+
+Qemu, as any other process, is going to forward I/O request to the OS
+(reading and writing to disk).
+Sheepdog is a layer between qemu and the OS and it's scope is not to store data
+on the server were qemu is running, but distribute along the nodes composing the
+cluster.
+
+This layer (sheepdog), is made os several components:
+
+**QEMU block driver**
+
+that sends I/O request to sheepdog insted of the OS.
+
+**Gateway**
+
+The requests of "qemu block driver" are handled by the gateway.
+E.g., when qemu makes a read reqeust, not all the necessary objects are stored
+on the node were qemu is running.
+The gateway will take care of retriving objects from the other nodes to
+accomplish the request.
+
+Viceversa, for a write request, it will calculate were to store the extra copy
+of each single object.
+
+**Object Manager**
+
+The gateway doesn't write or read directly form disk. It only manages the
+requests.
+Tey are forward by the gateway to the Object Manager that will write to local
+disk.
+
+
+
+
+Zones
+*****
+
+Before the implementation of Multy Disk (that allow a single sheep daemon to
+manage more disks), it was necessary to sun sheep N time for N disk in the same
+host.
+Each sheep daemon was consider as a node.
+This way, more than 1 copie of an object might been store on the same host.
+To avoid that (to have then 1 copie per object per host), it was necessary to
+associate a zone all sheep daemons running on the same host.
+E.g.
+
+::
+
+    sheep -z 1 ...
+
+
+
+
+.. |Architecture Overview| image:: https://github.com/sheepdog/sheepdog/wiki/architecture.png
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