VCAP-CID Study Notes: Objective 2.5

This is Objective 2.5 in the VCAP-CID blueprint Guide 2.8. The rest of the sections/objectives can be found here.

Bold items that have higher importance and copied text is in italic.


  • Compare and contrast vCloud allocation models.
    • I think was covered in Objective 2.4.
  • Identify storage constraints for an Organization Virtual Datacenter (VDC)
    • When configuring storage for a Organization Virtual Datacenter you can set a Storage Limit in GB. This is the storage that is used by VM’s and Catalog items in the organization vDC.
    • When using Fast Provisioning there are constraints regarding the usage of Shadow VM’s and their Linked Clones.
      • When a VM is created with Fast Provisioning a Shadow VM is created. The VM is then created as a linked clone from that Shadow VM.
      • This table (from the vCAT Architecting a VMware vCloud document) shows how placement of Linked Clones behaves:

VCAP CID  2-5-1



Skills and Abilities

  • Determine applicable resource pool, CPU and memory reservations/limits for a vCloud logical design.
    • The allocation pool types do most of that configuration automatically and should not be changed using the vSphere client.
    • But you could create sub resource pools for each type of an allocation pool.
  • Determine the impact of allocation model performance to a vCloud logical design.
    • This is based on the allocation model used:
      • Reservation Pool
      • Allocation Pool
        • Since Allocation Pool is based on resource pool reservation the same concepts apply here as to the Reservation Pools. The only difference is that the users can’t change the limit, reservations and shares on a VM level.
        • Here you don’t have to worry that much as the VM’s will use the CPU/Memory capacity configured, with % of it reserved when VM’s are powered on.
      • Pay-as-you-Go
        •  A default resource pool with no configuration is created, but the VM’s have limits based on the vCPU speed configured and reservation based on the % setting. So a VM with 2 vCPU would have double the limit of the vCPU speed.
        • CPU reservations at a VM level can result in high CPU ready times. Esxtop would show %RDY, and %MLMTD show the percentage of time the VM is ready to run but isn’t because it violates the CPU limit setting. %MLMTD should be 0%.
        • Memory reservation at a VM level is covered in this blog post from Frank Denneman:
        • Here the performane penalty is mostly based on the vCPU speed configured at the creation of the Organization vDC.
          • To small and you’ll have a lot of slow VM’s. You could fix this by adding more vCPU, by that increasing the limit, but the you might end up with VM’s with to many vCPU (create a vCPU scheduling war)
          • If the vCPU speed is set to the MHz number of the actual hosts used means all the VM’s will basically work like they don’t have a limit. But that means you will need to give the Organization a huge quota to work with to be able to turn on all the VM’s on, or leave it at unlimited.
  • Determine the impact to a given billing policy based on a selected allocation model.
    • There is no need for me to write about this as this subject has been covered in this excellent post from Eiad Al-Aqqad.
  • Given service level requirements, determine appropriate allocation model(s).
    • First we need to point out that service levels can be different based on what they should cover:
      • Availability – Are the system running? Based on uptime of the systems in question.
      • Backups – RTO & RPO
      • Serviceability – Initial response – Intital resolution time
      • Performance SLA – Need a certain amount of performance – 15 K disk , SSD disk etc.
      • Compliance – Logging,  ensure infrastructure is compliant to standards (PCI-DSS, etc)
      • Operations – Time when users can be added (if manual)
      • Billing – How long billing information is kept, depends on the local law.
    • Each allocation model has its caveats regarding service levels:
      • Reservation Pool
        • DRS Affinity rules can not be set by users in the default vCloud UI but will need to be “spliced” in as a part of a custom UI perhaps (Objective 2.4 has a link to a great example
        • If the service level are regarding the amount of resources available, Reservation Pool has all their resources reserved.
        • Availability SLA is the same for each cluster of ESXi hosts and has no impact on different allocation models.
      • Allocation Pool
        • If you are using Elastic mode your VM’s might be running two seperate DRS clusters, so you will need to keep that in mind.
        • If the service level are regarding the amount of resources available, Allocation Pool has a part of their resources reserved.
      • Pay-as-you-Go
        • If the service level are regarding the amount of resources available, PAYG has a part of their resources reserved, but it most likely to have performance problems regarding CPUs.
  • Given customer requirements, determine an appropriate storage provisioning model (thin/fast).
    • Thin Provisioning is just that, VM disks use less space on the datastores. So if space efficiency is a requirement than great.
    • Fast Provisioning like I mentioned before uses the Linked Clone technology to create clones that read from a single Shadow VM. Each VM reads that master disk, but writes to their own delta disk. Great for creating multiple VM’s at once as they don’t need to clone the whole disk of the templates.
  • Given a desired customer performance level, determine a resource allocation configuration.
    • Resource allocation configuration are allocation pools and how they are configured.
      • Reservation Pool
        • Customer gets resources reserved and can control how those resources are divided between workloads in that pool.
      • Allocation Pool
        • Customer gets a part of the resource reserved with a chance to burst to a certain amount.
      • Pay-as-you-Go
        • Customer gets a VM based reservation and limited vCPU power. Great for Test/Dev or dynamic workloads (meaning created and then deleted after a short period of time)
    • Performance levels can also mean using different performance Tiers (and perhaps with different level of service)
      • You can create different Provider vDC’s with different CPU speeds, HA configurations, and perhaps adding a SSD caching solution to create different Tiers.
      • Also you can offer storage Tiers, that could be based on different kind of spinning disks, eg. 10K, 15K and 7.2K. All based on the storage array and protocol used.
    • If we create an example
      • 3 Tiers of Provider vDC’s
        • Gold: Reservation Pool – E7 Intel processors – High speed Memory – HA at N+2 – SSD caching enabled.
        • Silver: Allocation Pool- E5 Intel processors – High speed memory – HA at N+1
        • Bronze: PAYG – E5 Intel Processors – High speed memory – HA turned off
      • Storage
        • Gold: 15K HDD + SSD caching in the storage array
        • Silver: 10K HDD
        • Bronze: 7.2K HDD

About larushjartar
VMware Specialist and IBM Technician.

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