An Administrator has determined that storage performance to a group of virtual machines is reduced during peak activity. The virtual machines are all located in a VMFS datastore called Production I on an Active/Active storage array. The ESX Host is currently configured with an MRU multipathing policy.
Which of the following achons could be taken to improve the storage performance of these virtual machines (Choose Two)?
A.
Add additional physical storage and add an extent to the VMFS datastore called Productionl
B.
Change the storage multipathing policy to Fixed
C.
Change the storage multipathing policy to Round-Robin
D.
Add additional physical storage, create a new VMFS datastore called Production2 on the new storage, then migrate a portion of the virtual machines from Production I to Production 2
Explanation:Storage performance refers to how to how efficiently, (typically how quickly), storage
operates. By adding a new datastore, and migrate a portion of the virtual machines from
Production1 to Production2 is likely to reduce the load on the original datastore, and
hence improve responsiveness, and hence performance.These pathing policies can be used with VMware ESX 4.x:
Most Recently Used (MRU) — Selects the first working path, discovered at system boot time. If this path becomes unavailable, the ESX host switches to an alternative path and continues to use the new path while it is available. This is the default policy for Logical Unit Numbers (LUNs) presented from an Active/Passive array. ESX does not return to the previous path when if, or when, it returns; it remains on the working path until it, for any reason, fails.
Note: The preferred flag, while sometimes visible, is not applicable to the MRU pathing policy and can be disregarded.
Fixed (Fixed) — Uses the designated preferred path flag, if it has been configured. Otherwise, it uses the first working path discovered at system boot time. If the ESX host cannot use the preferred path or it becomes unavailable, ESX selects an alternative available path. The ESX host automatically returns to the previously-defined preferred path as soon as it becomes available again. This is the default policy for LUNs presented from an Active/Active storage array.
Round Robin (RR) — Uses an automatic path selection rotating through all available paths, enabling the distribution of load across the configured paths.
For Active/Passive storage arrays, only the paths to the active controller will used in the Round Robin policy.
For Active/Active storage arrays, all paths will used in the Round Robin policy.Note: This policy is not currently supported for Logical Units that are part of a Microsoft Cluster Service (MSCS) virtual machine.
Fixed path with Array Preference — The VMW_PSP_FIXED_AP policy was introduced in ESX 4.1. It works for both Active/Active and Active/Passive storage arrays that support ALUA. This policy queries the storage array for the preferred path based on the arrays preference. If no preferred path is specified by the user, the storage array selects the preferred path based on specific criteria.
Notes:These pathing policies apply to VMware’s Native Multipathing (NMP) Path Selection Plugins (PSP). Third party PSPs have their own restrictions.
Switching to Round Robin from MRU or Fixed is safe and supported for all arrays unless otherwise explicitly documented.
Warning: VMware does not recommend changing the LUN policy from Fixed to MRU, as the automatic selection of the pathing policy is based on the array that has been detected by the NMP PSP.
“D” makes sense but why would “B. Change the storage multipathing policy to Fixed” be any better than “A. Change the storage multipathing policy to Round-Robin” in improving storage performance?
Since it is an active-active array, multiple physical paths can be used and I would assume Round-Robin multipathing policy would allow load balancing across these paths to help improve performance.
With a Fixed multipathing policy my understanding is that only the specified path is used unless it becomes unavailable at which point an alternate path is chosen.
Am I missing something?
“Active/Active storage array” is mentioned in the question.
So I think “B.Change the storage multipathing policy to Fixed” is more suitable than C.
Ref: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1011340
Yes, Active/Active storage array is mentioned in the question and Fixed is typically a default path selection policy for an Active/Active array.
I guess I just don’t understand why it would improve storage performance any more than a Round Robin path selection policy since Round Robin is perfectly valid for active/active arrays too.
Only thing I can think of it, is maybe fixed has less overhead, but I’ve yet to read any documentation to support this.
You can read following topic.
http://communities.vmware.com/message/1762124
I happend to have the same question. The only reason I can think of is that maybe there are problems with 1 path. then round robin would just switch every time between the bad path and the good path. fixed should fix this problem.
but the thing is that most san vendors want to have their san put on round robin.
this is truly a very bad question we know from vmware. it just depends on the san.