Qdisk - Revision history

Digimer at 21:17, 1 May 2011

2011-05-01T21:17:56Z

← Older revision		Revision as of 21:17, 1 May 2011
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	If you have a cluster of <span class="code">2</span> to <span class="code">16</span> nodes, you can use a quorum disk. This is a small partition on shared storage device that the cluster can use to make much better decisions about which nodes should have quorum when a split in the network happens.		If you have a cluster of <span class="code">2</span> to <span class="code">16</span> nodes, you can use a quorum disk. This is a small partition on shared storage device that the cluster can use to make much better decisions about which nodes should have quorum when a split in the network happens.

	~~Sadly~~, <span class="code">qdisk</span> does not work well on [[DRBD]]~~, so we will not be using it in this tutorial~~. It ~~is still worth knowing about though.~~		Unfortunately, <span class="code">qdisk</span> does not work well on [[DRBD]].

			= How It Works =

	The way a <span class="code">qdisk</span> works, at it's most basic, is to have one or more votes in quorum. Generally, but not necessarily always, the <span class="code">qdisk</span> device has one vote less than the total number of nodes (<span class="code">N-1</span>).		The way a <span class="code">qdisk</span> works, at it's most basic, is to have one or more votes in quorum. Generally, but not necessarily always, the <span class="code">qdisk</span> device has one vote less than the total number of nodes (<span class="code">N-1</span>).

			== Two Scenarios ==

	* In a two node cluster, the <span class="code">qdisk</span> would have one vote.		* In a two node cluster, the <span class="code">qdisk</span> would have one vote.
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	Imagine these two scenarios; First without <span class="code">qdisk</span>, the revisited to see how <span class="code">qdisk</span> helps.		Imagine these two scenarios; First without <span class="code">qdisk</span>, the revisited to see how <span class="code">qdisk</span> helps.

	* First ~~Scenarion~~; A two node cluster, which we will implement here.		* First Scenario; A two node cluster, which we will implement here.

	If the network connection on the <span class="code">totem</span> ring(s) breaks, you will enter into a dangerous state called a "'''split-brain'''". Normally, this can't happen because quorum can only be held by one side at a time. In a <span class="code">two_node</span> cluster though, this is allowed.		If the network connection on the <span class="code">totem</span> ring(s) breaks, you will enter into a dangerous state called a "'''split-brain'''". Normally, this can't happen because quorum can only be held by one side at a time. In a <span class="code">two_node</span> cluster though, this is allowed.

	Without a <span class="code">qdisk</span>, either node could potentially start the cluster resources. This is a disastrous possibility and it is avoided by a fence dual. Both nodes will try to fence the other at the same time, but only the fastest one wins. The idea behind this is that one will always live because the other will die before it can get it's fence call out. In theory, this works fine. In practice though, there are cases where fence calls can be "queued", thus, in fact, allow both nodes to die. This defeats the whole "high availability" thing, now doesn't it? Also, this possibility is why the <span class="code">two_node</span> option is the '''''only''''' exception to the quorum rules.		Without a <span class="code">qdisk</span>, either node could potentially start the cluster resources. This is a disastrous possibility and it is avoided by a fence dual. Both nodes will try to fence the other at the same time, but only the fastest one wins. The idea behind this is that one will always live because the other will die before it can get it's fence call out. In theory, this works fine. In practice though, there are cases where fence calls can be "queued", thus, in fact, allow both nodes to die. This defeats the whole "high availability" thing, now doesn't it? Also, this possibility is why the <span class="code">two_node</span> option is the '''''only''''' exception to the quorum rules.

			== How It Helps ==

	So how does a <span class="code">qdisk</span> help?		So how does a <span class="code">qdisk</span> help?
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	A little <span class="code">qdisk</span> goes a long way.		A little <span class="code">qdisk</span> goes a long way.

			= Further Reading =

	[[Red Hat]]'s Rob Kenna has a [http://magazine.redhat.com/2007/12/19/enhancing-cluster-quorum-with-qdisk/ fantastic article] on <span class="code">qdisk</span>. It is somewhat dated, but is still very much worth reading.		[[Red Hat]]'s Rob Kenna has a [http://magazine.redhat.com/2007/12/19/enhancing-cluster-quorum-with-qdisk/ fantastic article] on <span class="code">qdisk</span>. It is somewhat dated, but is still very much worth reading.

Digimer at 21:15, 1 May 2011

2011-05-01T21:15:39Z

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-A Quorum disk, known as a <span class="code">qdisk</span>, in [[RHCS]] is a special, small partition used on [[SAN]] storage to enhance quorum. It generally carries enough votes to allow even a single node to take quorum during a cluster partition. It does this by using configured heuristics, that is custom tests, to decided which which node or partition is best suited for providing clustered services during a cluster reconfiguration. These heuristics can be simple, like testing which partition has access to a given router, or they can be as complex as the administrator wishes using custom scripts.
+Sadly, <span class="code">qdisk</span> does not work well on [[DRBD]], so we will not be using it in this tutorial. It is still worth knowing about though.
-Quorum disks are limited to cluster with no more than 16 nodes. This limitation is due to inherent storage latency with larger clusters.
+The way a <span class="code">qdisk</span> works, at it's most basic, is to have one or more votes in quorum. Generally, but not necessarily always, the <span class="code">qdisk</span> device has one vote less than the total number of nodes (<span class="code">N-1</span>).
 [[Red Hat]]'s Rob Kenna has a [http://magazine.redhat.com/2007/12/19/enhancing-cluster-quorum-with-qdisk/ fantastic article] on <span class="code">qdisk</span>. It is somewhat dated, but is still very much worth reading.
 You can learn more about quorum disk below:

Digimer: Created page with "{{header}} A Quorum disk, known as a qdisk, in RHCS is a special, small partition used on SAN storage to enhance quorum. It generally carries e..."

2011-05-01T20:30:54Z

Created page with "{{header}} A Quorum disk, known as a qdisk, in RHCS is a special, small partition used on SAN storage to enhance quorum. It generally carries e..."

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{{header}}

A Quorum disk, known as a qdisk, in [[RHCS]] is a special, small partition used on [[SAN]] storage to enhance quorum. It generally carries enough votes to allow even a single node to take quorum during a cluster partition. It does this by using configured heuristics, that is custom tests, to decided which which node or partition is best suited for providing clustered services during a cluster reconfiguration. These heuristics can be simple, like testing which partition has access to a given router, or they can be as complex as the administrator wishes using custom scripts.

Quorum disks are limited to cluster with no more than 16 nodes. This limitation is due to inherent storage latency with larger clusters.

[[Red Hat]]'s Rob Kenna has a [http://magazine.redhat.com/2007/12/19/enhancing-cluster-quorum-with-qdisk/ fantastic article] on qdisk. It is somewhat dated, but is still very much worth reading.

You can learn more about quorum disk below:
* [http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/Configuration_Example_-_Oracle_HA_on_Cluster_Suite/qdisk_configuration.html Quorum Disk in EL5]
* [http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Cluster_Administration/s1-qdisk-considerations-CA.html Quorum Disk in EL6]

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