Refer to the Exhibit.
A vSphere 6.x DRS cluster is configured as shown in the Exhibit.
Based on the exhibit, which statement is true?
A.
A virtual machine can be powered on in the Web Resource Pool with a 3 GHz CPU Reservation.
B.
A virtual machine can be powered on in the Web Resource Pool with a 4 GHz CPU Reservation.
C.
A virtual machine can be powered on in the DB Resource Pool with a 3 GHz CPU Reservation.
D.
A virtual machine can be powered on in the DB Resource Pool with a 4 GHz CPU Reservation.
Explanation:
Explanation/Reference:
C or D, but C is the better answer
Can’t be D:
Production resource pool is limited to 6Ghz. Web Resource pool is consuming 2 GHz of that. DB-VM1 is consuming a further 1Ghz. All that’s left is 3 Ghz.
Actually, Resource Pools don’t “consume” the resource they are reserving like VM’s do. Resource pool Web may have a 2Ghz reservation, unless A second VM is booted (or Web-VM1 goes over its reserved usage), RP “Web” will not grab and hold those 2Ghz.
Granted this is about memory, butFrank Denneman explains it better than I ever could:
Behavior of resource pool memory reservation
Now setting a memory reservation on a resource pool level has its own weaknesses, but it is much fairer and more along the whole idea of consolidation and sharing than virtual machine memory reservations. RP level reservations are immediately active, but are not claimed. This means it will only subtract the specified amount of memory from the unreserved capacity of the cluster.
RP reservations are used when children of the resource pool uses memory and the system is under contention. Reservations are not wasted and the resources can be used by other virtual machines. Be aware, using and reserving are two distinct concepts! Virtual machines can use the resource, but they cannot reserve this as well if it is already reserved by another item.
It appears that resource pool memory reservations work almost similar to CPU reservations, they won’t let any resource go to waste. And to top it off, resource pool reservations don’t flow to virtual machines, they will not influence HA slot sizes. Which unfortunately can lead to (temporary) performance loss if a host failover occurs. When a virtual machine is restarted by HA they are not restarted in the correct resource pool but in the root resource pool, which can lead to starvation. Until DRS is invoked, the virtual machine need to do it without any memory reservations.
http://frankdenneman.nl/2010/05/18/resource-pools-memory-reservations/
I still consider C to be the better answer, as a reservation of a VM says nothing about it’s actual usage, but it “could” be possible to answer D if we had actual usage info.
I’m trying to figure this out as well and have a question.. Can a resource pool run if 100% is used? Does it need wiggle room?
In this scenario based on that I think C is the answer…
6Ghz prod pool – 1Ghz of Web-VM1 – 1 Ghz DB-VM1 = 4 Ghz left
If the answer is D, that means there is no room for growth left.
The only explanation is to why ‘D’ is not the answer is that, as you say, there would be no RAM associated with VM overhead.
I agree with Kevin above that only actual (i.e. actively used) reservation by running VMs do play role here.
Upon further research and test in a lab, turns out reservations work differently on the VM and on the RP level. VM’s reservations are not actually come into play as long as a VM is powered off. In contrast, RP’s reservations are activated / reserved –immediately–. As per VMware Official Cert Guide VCP6-DCV (ISBN-10 0-7897-5648-X, p294), (quote) “… even if a resource pool is empty, its reservations cannot be used by other VMs or resource pools”.
There’s a really good article on this topic by VMware guru Duncan Epping at
http://www.yellow-bricks.com/2010/03/03/cpumem-reservation-behaviour/
So, based on the above, the unreserved capacity in this case is: 6 – 2 (Web RP reservation) – 1 (VM reservation in DB RP) = 3 GHz. So ‘C’ indeed is the only correct answer.
To further illustrate the point, if the question was about starting a VM that is not part of the Web and DB RPs (i.e. sibling to these pools and is right in the parent Production RP), max reservation for it would be 6 – 2 (Web RP) – 2 (DB RP) = 2 GHz.
Not for this question, but worth noting that, as Duncan points, “CPU reservations are friendly. Memory reservations are greedy and hoard memory.” This is to say they work quite differently, which is a rather natural consequence considering the physical nature of these distinct types of resources: CPU cycles (~time / non-material) vs. physical RAM cells.
somehow “Fixed reservation” has been interpreted as “limit” here, but they should be different. https://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.wssdk.pg.doc_50%2FPG_Ch13_Resources.15.5.html
For “Fixed CPU Reservation 6GHz”, it does not means the max/limit CPU usage is 6Ghz, right?
I have had this checked by someone who has taken the exam. This question was on the test exactly as shown here.
C is the answer.
Web-RP only has 1GHz of unreserved capacity which is not expandable, meaning that it has left 1Ghz of capacity to power up further VMs. Thus option A and option B are eliminated.
DB-RP has also 1GHz of unreserved capacity which is expandable, meaning that it can ask its parent RP if the remaining 1GHz is exhausted. The parent RP: Production-RP has 6GHz-2GHz-2Ghz=2GHz of unreserved capacity (Web-RP and DB-RP reservation are substracted automatically from parent no matter of VMs)
So DB-RP has access to 1Ghz (its remaining) + 2GHz (its parents remaining) ressources, a total of 3GHz. —> Answer C