This article is contributed. See the original author and article here.

Hey there….

 

This is Phil here from the FSLogix Team. I recently came across an interesting topic the other day and I thought i would share it with you . 

 

When using VHDlocations and have 2 or more locations set in the string , whether this is a Multi_SZ or Reg_SZ, it may be  perceived that if the first location in the string is not accessible then we will fall back to the second location or the third and so on to create and /or access the VHD(X) container.

 

This is not the correct interpretation.

 




  1. When using VHD locations, only one VHD location can be accessed at any one time

     


  2. This feature is NOT created as a fail-over option; using 2 or more VHD locations does not offer any form of HA or resiliency compared to CCD ( Cloud Cache)

  3. The list of locations in VHDLocations was created to allow a customer to control where the VHD is placed and to be able to control which VHDs are created on which location using Share Permissions

  4. VHDLocations with a list expects all the locations to be available at all times.

  5. Storage locations for user data are not supposed to be unavailable.


So what happens if the first location is not available as a result of the Server being physically unavailable ?  


 


FSlogix has built-in logic to determine that the first location is not available and will not proceed to  create another disk on the next server in the list .


 


If the opposite were true  and the VHD Locations were allowed to proceed in creating new VHDs then we would end up with a scenario of a user having multiple VHDs in different locations ; this would result in  Profile data not be available for that current user session . It would then be an arduous task to merge all that data from the multiple disks to one disk.


 


Caveat: If one of the servers in the list is down then,  it would prevent people from logging on.


 


The only time that a another location is made available is when the share permissions on the server containing the VHD has changed and the share is no  longer accessible. FSlogix will then look at the next server in the list and if the share is accessible then it will create the VHD on that share.


 


To prevent another VHD from being created on another share location, you should ensure that you always specifically set the permissions so that only one location was writeable for specific user(s) or groups and thus preventing any of the other share locations having another  profile and /or ODFC disk for that user.


 


And if its HA /Resiliency you are looking for then Cloud Cache is the solution here.


 


Until next time …….. 


 



 

 

 

 

 

 

 

 

 

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