[sheepdog] [PATCH] Doc. "Sheepdog Basic" add chapter "Concepts"

[Valerio Pachera]

Signed-off-by: Valerio Pachera <sirio81 at gmail.com>
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 doc/concepts.rst |   37 +++++++++++++++++++++++++++++++++++++
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+Concepts
+========
+
+To put it simply, Sheepdog **splits** guests' disks into pieces (chunks) and
+spreads them all over the hosts of the cluster.
+The loss of a single piece would mean the loss of the whole disk (vdi).
+But sheepdog **keeps more copies** for each chunk (default is 3 copies).
+When a host dies, many chucks are "lost". That means some chunks will have
+"only" 2 copies instead of 3.
+Sheepdog will use the remaining 2 copies "to form" the virtual disk.
+Chunks are called "objects" in the Sheepdog context.
+The cluster is going to replicate the missing copies as soon as possible on
+the active nodes.
+
+For each node, we need 2 running programs:
+
+- zookeeper (or another cluster manager)
+- sheep
+
+Zookeeper will take care of noticing if any host joins or leaves the cluster.
+It has to be run before *sheep*.
+Sheep is going to manage the storage (write objects to disk).
+
+The only mandatory option we have to pass to *sheep* command is a folder path.
+The folder has to be a mount point for a device
+(partition, logical volume, etc.).
+You must make sure that no other programs write any data in that directory.
+That means it has to be empty the first time you run *sheep* the first time.
+
+Note:
+
+When a host loses contact with the other nodes (e.g. cable disconnection),
+neither sheep daemon nor qemu will die on the disconnected node!
+If the node is able to rejoin the cluster (e.g. insert the cable back),
+qemu will issue the pending write/read operation and the guest will contine
+to run.
+This is called auto-reconnect (works with qemu 1.7 or higher).
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