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Exchange 2010 DAC, PAM's and FSW

Posted on 2012-04-07
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We are running Exchange 2010 across two sites (London and Munich). One DAG.

There are 6 servers in each site.

MU-EXCH1 to MU-EXCH6

and

LON-EXCH1 to LON-EXCH6

The active databases in the DAG are spread across both sites.

We also have DAC enabled on the DAG. The FSW is located on a server in MUNICH.

I've read the Technet articles etc, but I'm still not entirely sure of the relationship between the FSW, the PAM and the DAC setting. As far as I know, DAC will prevent multiple copies of a DB being mounted in the event that LON and MUNICH lost connectivity with each other.

Could someone please confirm if I am correct in the following statements -

i. If we lost connecivity between LON and MUNICH, then ALL DB's would be mounted in MUNICH as that is where the FSW is located.

ii. What does the PAM do in terms of deciding where DBs are mounted? Or does it only hold information of WHERE the DB's are mounted?

iii. How can I find out which server has the PAM role and the status of the PAM
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Question by:neil4933
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tigermatt earned 500 total points
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>> If we lost connecivity between LON and MUNICH, then ALL DB's would be mounted in MUNICH as that is where the FSW is located.

Yes. This is accurate. The MUNICH datacentre would obtain quorum (it has a majority of voting nodes), so the databases will be mounted there. LONDON knows the cluster has x number of nodes, but is not in communication with more than half of those nodes, which means the databases in that datacentre cannot be automatically activated. (Which is good, of course).

Note that partial loss of MUNICH, but still with network connectivity to LONDON, could cause issues. This all depends on design and your particular circumstances, of course, so lots of testing is critical to the success of this deployment.

>> What does the PAM do in terms of deciding where DBs are mounted? Or does it only hold information of WHERE the DB's are mounted?

The PAM actually has algorithms built-in to determine which database copies are the best ones to mount in each situation. It makes all the decisions on mounting database copies, drawing on the information you've already configured. So yes, it performs the primary role in determining what is going to be mounted, and where.

>> How can I find out which server has the PAM role and the status of the PAM

The server hosting the PAM is always going to be the one which has control over the cluster quorum resource. You should be able to determine this machine using
Get-DatabaseAvailabilityGroup -Status | fl *Primary*

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Of course, in a failover situation, the cluster resource will change ownership to another DAG member, which will also shift the PAM.

-Matt
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by:neil4933
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Thanks..

I know sometimes, the FSW can be located in a 3rd Datacenter, how does the logic of working out which Data Center holds all the DB's work then?
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by:tigermatt
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The model is essentially the same. Whichever data centre was "up" and able to communicate with the FSW in the third datacentre would have the databases mounted.

Of course, what you do need to be careful of is that the third datacentre is truly isolated. You do not get any FSW resiliency if they use the same Internet or power feeds. I've seen this configuration before and quite honestly, it achieves the same level of resilience but with more complexity. If the DCs are isolated, then by all means consider it as an option, but if they are not, just locating the FSW in one of the other two datacentres gives the same resilience but less complexity.

You should also probably consider the alternate File Share Witness in the datacentre not hosting the FSW. This just saves some configuration if you do have to perform a manual datacentre failover. For an even number of nodes in the secondary DC, you will obviously need an FSW in that site in the case of failover for quorum to be obtained. Configuring this in advance just saves time and effort later when implementing your DR plan. See http://technet.microsoft.com/en-us/library/dd351049.aspx for more explicit details.

-Matt
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