What’s the guidance if you’ve got 3 data centres, you're stretching Exchange 2007 mailbox CCR nodes between two of them and the link speed and latencies are similar between all 3. Should you put the File Share Witness (FSW) in the 3rd data centre?
In my opinion yes I would recommend putting the FSW in the 3rd data centre. One of the potential disadvantages of stretching CCR is that the recovery path when you lose the data centre is always a manual process since you’ve lost 2 of your 3 cluster voters and one which you need to be careful with. ..the failback process is potentially a lot worse especially if you have little control over the servers coming back in the second site (See my blog on You Had Me At EHLO.. here; CCR, Site Resilience and sample decision making processes...). So if you have a 3rd site this does ease the recovery process by ensuring that the FSW is likely to be available give the loss of the first data centre ensuring that failover is an easier proposition.
Only thing I’d be careful with is if the FSW is located on a server that is not managed by the Exchange team that it is well documented that the FSW is located where it is and that maintenance procedures etc.. involve the Exchange team.
Have a look here for some related guidance…
‘Placement of the File Share Witness (FSW) on a Geographically Dispersed CCR Cluster’ @ http://msexchangeteam.com/archive/2007/04/25/438185.aspx and check the updated CNAME record guidance on You Had Me At EHLO.. here; ‘New File Share Witness and Force Quorum Guidance for Exchange 2007 Clusters’ @ http://msexchangeteam.com/archive/2008/04/03/448615.aspx
Also check out another of my blogs here: Why not stretch CCR nodes across 2 Data Centres..?
..and on Technet ‘Planning for Cluster Continuous Replication’