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P4300 regular managers recommendation

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Fred Blum
Regular Advisor

P4300 regular managers recommendation

 

We used to have a two node P4300 with a VMplayer FOM. The SAN has been expanded to four nodes and the Management Console gives as a recommendation thee regular managers or four regular managers and a FOM.

- What is the advantage of running three regular managers in a four node SAN?

- What is against running four regular managers, no FOM?

- What is the advantage of running four regular managers and a FOM?

 

TIA,

 

Fred

2 REPLIES
Bart_Heungens
Honored Contributor
Solution

Re: P4300 regular managers recommendation

Hi,

 

It is best to run an uneven number of managers... If you have 4 nodes and 4 managers running, if 2 nodes go offline (for instance split-brain situation when boxes are spread accross 2 datacenters), you would have no quorum (majority) and so all nodes will stop I/O towards the servers...

Only with 3 or 5 managers you never can have a 50/50 situation...

 

Now in your case, with 4 nodes, the best would be 5 managers, which means that you need taht FOM... Some companies do not want to implement a FOM or whatever so they have to go for 3 managers...

 

 

Kr,

Bart

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Emilo
Trusted Contributor

Re: P4300 regular managers recommendation

From the 9.0 users manual

http://bizsupport1.austin.hp.com/bc/docs/support/SupportManual/c02008676/c02008676.pdf

Page 150

Managers have the following functions:

  • Control data replication. (Note: managers are not directly in the data path.)
  • Manage communication between storage nodes in the cluster.
  • Re-synchronize data when storage nodes change states.
  • Coordinate reconfigurations as storage nodes are brought up and taken offline.

Managers and quorum

Managers use a voting algorithm to coordinate storage node behavior. In this voting algorithm, a strict majority of managers (a quorum) must be running and communicating with each other in order for the SAN/iQ software to function. An odd number of managers is recommended to ensure that a majority is easily maintained. An even number of managers can get into a state where no majority exists — one-half of the managers do not agree with the other one-half. This state is known as a “split-brain,” and may cause the management group to become unavailable.

 

For optimal fault tolerance in a single site configuration, you should have 3 or 5 managers in your management group to provide the best balance between fault tolerance and performance. The maximum supported number of managers is 5.

 

As you can see,from the above list that Managers can be awfully chatty. So with 4 Nodes probably 3 managers running is best. Although I don’t know if would fail the Best Practices Analyzer if you did run 4 managers plus the FOM, but you wont get it either with only 3 Managers running.