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	.. _deployment-redundant-servers:

	=====================================
	Kolab Deployment on Redundant Servers
	=====================================

	This deployment provides high-availability to a Kolab Groupware
	deployment through redundancy of its underlying storage.

	Please see either of the following sections for details on their
	semantics:

	* :ref:`deployment-redundant-drbd`
	* :ref:`deployment-redundant-shared-storage`
	* :ref:`deployment-redundant-glusterfs`

	.. _deployment-redundant-drbd:

	Using DRBD
	==========

	When used solely for redundancy, `DRBD`_ replicates the disk of a single
	node to another node, implying the replica master is the active server,
	while the slave remains passive. That is to say, that the replica master
	can, while the replica slave can not access the data on the
	replicated volume.

	.. graphviz::

	digraph drbd {
	rankdir=LR;
	subgraph cluster_master {
	rankdir=TB;
	style=filled;
	color=lightgrey;
	node [style=filled];
	"OS Disk 0" [label="OS Disk",color=green];
	"Data Disk 0" [label="Data Disk",color=green];
	label = "Master";
	}

	subgraph cluster_slave {
	rankdir=TB;
	node [style=filled];
	"OS Disk 1" [label="OS Disk",color=green];
	"Data Disk 1" [label="Data Disk",color=red];
	label = "Slave";
	}

	"Data Disk 0" -> "Data Disk 1" [label="One-Way Replication"];
	}

	In this scenario, the master is the active server running all Kolab
	servers, while the slave is completely passive (it cannot write to the
	disk being replicated). A failover would ensure the master is not
	running any longer, and start the services on the slave -- which
	therefore becomes the master.

	In a more complex scenario, two separate volumes could be replicated in
	an cross-link scenario, allowing the resources of both nodes to be used
	in parallel, establishing a limited form of load-balancing.

	.. graphviz::

	digraph drbd {
	rankdir=LR;
	subgraph cluster_master {
	rankdir=TB;
	style=filled;
	color=lightgrey;
	node [style=filled];
	"OS Disk 0" [label="OS Disk",color=green];
	"Data Disk 0" [label="Data Disk",color=green];
	"Data Disk 1" [label="Data Disk",color=red];
	label = "Master";
	}

	subgraph cluster_slave {
	rankdir=TB;
	style=filled;
	color=lightgrey;
	node [style=filled];
	"OS Disk 1" [label="OS Disk",color=green];
	"Data Disk 2" [label="Data Disk",color=red];
	"Data Disk 3" [label="Data Disk",color=green];
	label = "Slave";
	}

	"Data Disk 0" -> "Data Disk 2" [label="One-Way Replication"];
	"Data Disk 3" -> "Data Disk 1" [label="One-Way Replication"];

	"LDAP?" -> "Data Disk 0";

	"Data Disk 3" -> "IMAP?" [dir=back];
	}

	Using DRBD in a so-called dual-primary mode requires the use of a
	shared filesystem such as Global File System (GFS, not to be
	confused with GlusterFS).

	Designing, installing, configuring and maintaining a DRBD cluster is
	beyond the scope of this document, and not commonly a solution the Kolab
	community supports.

	For more assistence, please consider contacting a local IT solutions
	provider, `Kolab Systems AG`_ or any its `Certified Partners`_.

	.. _deployment-redundant-shared-storage:

	Using Shared Storage
	====================

	Shared storage is usually external to node(s),

	SAN
	---

	NAS
	---

	.. _deployment-redundant-glusterfs:

	Using GlusterFS
	===============

	.. graphviz::

	digraph glusterfs {
	compound=true;

	rankdir=TB;

	subgraph cluster_gfs0 {
	label = "GlusterFS";

	rankdir=LR;

	subgraph cluster_gfs1 {
	label = "GlusterFS Node 1";

	rankdir=TB;

	"OS Disk 2" [label="OS Disk"];
	"Data Disk 2" [label="Data Disk"];
	"OS Disk 2" -> "Data Disk 2" [style=invis];
	}

	subgraph cluster_gfs2 {
	label = "GlusterFS Node 2";

	rankdir=TB;

	"OS Disk 3" [label="OS Disk"]
	"Data Disk 3" [label="Data Disk"];
	"OS Disk 3" -> "Data Disk 3" [style=invis];
	"Data Disk 3" -> "Data Disk 2" [dir=both];
	}

	{ rank=same; "OS Disk 2", "OS Disk 3" }
	{ rank=same; "Data Disk 2", "Data Disk 3" }
	}

	subgraph cluster_kolab0 {
	label = "Kolab";

	rankdir=LR;

	subgraph cluster_kolab1 {
	label = "Kolab Server 1";

	rankdir=TB;

	"OS Disk 0" [label="OS Disk"];
	"GFS Mount 0" [label="GFS Mount"];
	"OS Disk 0" -> "GFS Mount 0" [style=invis];
	}

	subgraph cluster_kolab2 {
	label = "Kolab Server 2";

	rankdir=TB;

	"OS Disk 1" [label="OS Disk"]
	"GFS Mount 1" [label="GFS Mount"];
	"OS Disk 1" -> "GFS Mount 1" [style=invis];
	}

	{ rank=same; "OS Disk 0", "OS Disk 1", "OS Disk 2", "OS Disk 3" }
	{ rank=same; "GFS Mount 0", "GFS Mount 1", "Data Disk 2", "Data Disk 3" }

	}

	"GFS Mount 0" -> "Data Disk 3" [lhead=cluster_gfs0];
	"GFS Mount 1" -> "Data Disk 2" [lhead=cluster_gfs0];

	}

	.. _DRBD: http://www.drbd.org/
	.. _Kolab Systems AG: https://kolabsys.com
	.. _Certified Partners: https://kolabsys.com/company/partners

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