• Vmware Slot Size Policy
• Vsphere Slot Size Chart
• Vsphere Ha Slot Size Calculation
• Vsphere 6.5 Slot Size
• Vmware Slot Size Calculation
• Vsphere 6 Slot Size

Recently I have been asked by my customers and also blog readers 🙂 about VMware HA Admission Control. As this topic is often misunderstood, this post covers some information about this important feature of VMware vSphere HA.

To avoid this, you can specify an upper bound for slot sizes; use the configuration editor in the vSphere Client to set the das.slotCpuInMHz and das.slotMemInMB attributes. When you use these attributes, there is a risk that resource fragmentation will cause virtual machines with resource requirements larger than the slot size to be assigned.

In this post you will find answers for the following questions:

• The slot size for Memory is the highest reservation of Memory among all VM’s in the cluster (or 0 MB + Memory Overhead if no per VM reservations exist). If you want to really dive further into HA and slot size calculations, I would highly recommend reading Duncan Epping’s HA Deepdive at Yellow-Bricks.com.
• If VM1 has 2GHZ and 1024GB reserved and VM2 has 1GHZ and 2048GB reserved the slot size for memory will be 2048MB+memory overhead and the slot size for CPU will be 2GHZ. Now how does HA calculate how many slots are available per host? Of course we need to know what the slot size for memory and CPU is first.

1. What is Admission Control?
2. What is and how to check slote size?
3. When and which Admission Control Policy should I use? How to solve Insufficient Resources for HA failover?

Admission control is used to ensure that sufficient resources are available in a cluster to provide failover protection and to ensure that virtual machine resource reservations are respected. There are three Admission control policies:

1. Percentage of Cluster Resources Reserved Admission Control Policy
2. Host Failures Cluster Tolerates Admission Control Policy

It's very important to mention that Percentage of Cluster Resources Reserved and Host Failures Cluster Tolerates are based on CPU and memory reservations at Virtual Machine (VM) level (it ignores resource pools reservation settings). But only? Nope, also overhead reservation is taken into considerations. So the formula is:

Configured Reservation for VM + Overhead Reservation

Overhead Reservation depends on VM configuration and usage (for example: RAM, devices etc). It means that for a large VMs with not configured reservation (equals 0), there is a noticeable Overhead Reservation!

Specify Failover Hosts

The easiest option (it does not mean the best 🙂 ) of Admission Control. vSphere HA attempts to restart its virtual machines on one of the specified failover hosts.

Host Failures Cluster Tolerates

With the Host Failures Cluster Tolerates admission control policy, vSphere HA uses slot to ensure that a specified number of hosts can fail and sufficient resources remain in the cluster to fail over all the virtual machines from those hosts.

Slot is a logical representation of CPU and memory. Depending on vSphere version, the default slot size is 0 MB of RAM and 256 MHz CPU (4.1 and earlier) or 0 MB of RAM and 32Mhz (5.0 and later).

The admission control is performed by VMware HA with the following steps:

1. Calculates the slot size (based on powered-on VMs and selections the largest value).
2. Determines how many slots each host in the cluster can hold.
3. Determines the Current Failover Capacity of the cluster.
4. Determines whether the Current Failover Capacity is less than the Configured Failover Capacity.

So let's make an example. As shown on the above figure, we have three ESXi hosts each with a different amount of available CPU and memory resources:

• ESXi1 - 9GB of RAM and 9Ghz of CPU
• ESXi2 - 6GB of RAM and 9Ghz of CPU
• ESXi3 - 6GB of RAM and 6Ghz of CPU

There are five powered-on VMs:

• VM1 and VM2 - 1GB of RAM and 2GHz of CPU
• VM3 - 2GB of RAM and 1GHz of CPU
• VM4 and VM5 - 1GB of RAM and 1GHz of CPU

As mentioned earlier, the slot size is the largest value so in this example: the slot size is 2GHz of CPU and 2GB of memory.

So how many slots are available per hosts?

• ESXi1 - 3 slots
• ESXi2 - 3 slots
• ESXi3 - 4 slots

If ESXi3 host fails, we have available 6 slots (3+3) so sufficient resources are available in a cluster to provide failover protection (one slot available yet to create).

Since vSphere 5.x, it is possible to configure the default slot size via Web Client:

If you use Standard Client or vSphere 4.x, you need to change some advanced parameters such as das.slotmeminmb and das.slotcpuinmhz. For more information please follow VMware KB here.

Percentage of Cluster Resources Reserved

The Percentage of Cluster Resources Reserved works different - vSphere HA ensures that a specified percentage of aggregate CPU and memory is reserved for failover.

The admission control is performed by VMware HA with the following steps:

1. Calculates the total resource requirement for all powered on machines in the cluster.
2. Calculates the total host resources available for the virtual machines.
3. Calculates the current CPU and Memory failover capacity for the cluster (specified by administrator).
4. Determines if either the current CPU failover or current memory failover is less than the corresponding failover capacity.

As shown on the above figure, there are five VMs and three ESXi hosts and the total VMs requirements for the powered-on VMs is 6GB of RAM and 7GHz of CPU. To calculate the Current CPU Failover Capacity we use the following formula:

(Total host resources - Total VMs requirements) / Total host resources

so CPU: (24-7) / 24 = 70% and Memory: (21-6) / 21 = 71%

If you specify 33% CPU and Memory Failover Capacity, you have around 35% of resource available for new VMs yet 🙂

Insufficient Resources for HA failover

Sometimes you can get the following error during configuring vSphere HA with Admission Control enabled:

Insufficient Resources for HA failover

In most of all cases, the above error can happen when you configure Host Failures Cluster Tolerates Admission Control policy. Why? For example you have a large VM (with large reservation so slot size is also large) and when vSphere HA calculates powered on VMs and available slots on all ESXi hosts, it can be a situation when sufficient resources are not available in a cluster to provide failover protection. To solve this problem you should revise the VMs reservations or reconfigure Admission Control to use the Percentage of Cluster Resources Reserved policy.

When and which Admission Control Policy should I use?

Generally I use the Percentage of Cluster Resources Reserved policy because it's much simpler and more flexible than Host Failures Cluster Tolerates Admission Control policy. Of course, you have to remember that when you add or remove ESXi hosts from your cluster, you need to reconfigure percentages. Also if you have an unbalanced vSphere Cluster you should use the Percentage of Cluster Resources Reserved policy.

Conclusion

I hope that Admission Control is understandable for you now 🙂 For more information (much deeper) please follow a fantastic book written by Duncan Epping: VMware vSphere 5.1 Clustering Deepdive 🙂

Vmware Slot Size Policy

VMware vSphere is VMware's virtualization platform, which transforms data centers into aggregated computing infrastructures that include CPU, storage, and networking resources. vSphere manages these infrastructures as a unified operating environment, and provides you with the tools to administer the data centers that participate in that environment.

The two core components of vSphere are ESXi and vCenter Server. ESXi is the virtualization platform where you create and run virtual machines and virtual appliances. vCenter Server is the service through which you manage multiple hosts connected in a network and pool host resources.

Want to know what is in the current release of vSphere? Look at the latest vSphere release notes.

Learn About Some of Our Features

Beginning in vSphere 7.0, you can only deploy or upgrade to vCenter Server 7.0 using an appliance. The new vCenter Server appliance contains all the Platform Services Controller services from earlier releases, preserving all previous functionality, including authentication, certificate management, and licensing. All Platform Services Controller services are consolidated into vCenter Server, simplifying deployment and administration. As these services are now part of vCenter Server, they are no longer described as a part of Platform Services Controller.

vSphere 7.0 introduces vSphere Lifecycle Manager, a centralized and simplified lifecycle management mechanism for VMware ESXi 7.0 hosts. This new feature includes the functionality that Update Manager provided in previous vSphere releases. With vSphere Lifecycle Manager you can manage ESXi hosts by using images and baselines at the cluster level.

Learn how to use vSphere with Tanzu to transform vSphere into a platform for running Kubernetes workloads natively on the hypervisor layer. With this functionality, you can enable a vSphere cluster to run Kubernetes workloads by configuring it as a Supervisor Cluster. Within the Supervisor Cluster, you can create resource pools, called Supervisor Namespaces, and configure them with dedicated memory, storage, and CPU. You can directly deploy containers natively on ESXi within a Supervisor Namespace. These containers live within a special type of pod called a vSphere Pod. You can also leverage the Tanzu Kubernetes Grid Service to easily provision Kubernetes clusters that run within dedicated Supervisor Namespaces.

Vsphere Slot Size Chart

You can view available vCenter Server updates and upgrades and produce interoperability reports about VMware products associated with vCenter Server using Update Planner. You can also generate pre-update reports that let you make sure your system meets the minimum software and hardware requirements for a successful upgrade of vCenter Server. The report provides information about problems that might prevent the completion of a software upgrade, and actions you can take to remedy those problems.

You can use centralized license management to manage licenses for ESXi hosts, vCenter Server, vSAN clusters, and other VMware solutions. Learn how to use the VMware vSphere Client to manage licenses in your vCenter Server environment.

Learn how to configure networking for vSphere, including how to create vSphere distributed switches and vSphere standard switches, monitor networks to analyze the traffic between virtual machines (VMs) and hosts, and manage network resources. vSphere networking is one of the most critical components in your environment, as it is how your ESXi hosts and VMs communicate.

You can learn about vSphere storage to help you plan a storage strategy for your virtual data center. You can also learn how to configure and use the virtualized and software-defined storage technologies that ESXi and vCenter Server provide. vSphere supports several storage technologies for both traditional and software-defined storage environments.

Learn how to secure your environment using vSphere security features and best practices to safeguard your environment from attack. vSphere provides comprehensive, built-in security, delivering secure applications, infrastructure, data, and access.

You can provide business continuity using vCenter High Availability (vCenter HA) and vSphere Fault Tolerance (FT). vCenter HA provides failover protection against hardware and operating system outages within your virtualized IT environment. If there is a host failure, Fault Tolerance provides continuous protection for a VM.

You can use resource pools, clusters, vSphere Distributed Resource Scheduler (DRS), vSphere Distributed Power Management (DPM), and vSphere Storage I/O Control to manage and allocate resources for ESXi hosts and vCenter Server.

Try Our Deployment and Planning Tools

The following resources are designed to help you plan your vSphere data center deployment, and effectively manage your vSphere environment.

• vSphere Hardware and Guest Operating System Compatibility Guides. An online reference that shows what hardware, converged systems, operating systems, third-party applications, and VMware products are compatible with a specific version of a VMware software product.
• VMware Product Interoperability Matrices. Provides details about the compatibility of current and earlier versions of VMware vSphere components, including ESXi, vCenter Server, and other VMware products.
• VMware Configuration Maximums. When you configure, deploy, and operate your virtual and physical equipment, you must stay at or below the maximums supported by your product. The limits presented in the Configuration Maximums tool are tested limits supported by VMware.

Access Developer and Automation Documentation

VMware {code} is a website dedicated to our developer and automation community. To learn about vSphere APIs, SDKs, and command-line interfaces, visit these VMware {code} resources:

• VMware command-line interfaces under Automation Tools

Explore Our Videos

You can learn about deploying, managing, and administering vSphere by reading the documentation, and by watching videos on the VMware Information Experience video channel.

Learn More About vSphere

Vsphere Ha Slot Size Calculation

To learn about vSphere and data center virtualization, see the following resources.

• Learn more about vSphere by visiting the vSphere Product Page.
• Ask questions about vSphere by visiting the vSphere Community Forum. You can get help, opinions, and feedback from other VMware users by participating in the discussion forums.
• Explore vSphere without having to install it using the VMware vSphere Hands-on Labs environment.
• Learn about the solutions vSphere provides to help you overcome your IT struggles, and create a more efficient digital infrastructure by visiting vSphere White Papers and Technical Notes.
• Read the latest products announcements, technical articles, and operations guidance from VMware on the vSphere Blog.
• Learn about benchmarking, performance architectures, and other performance-focused topics at the blog VMware VROOM!, maintained by VMware's Performance Engineering team.
• Visit the blog virtuallyGhetto by William Lam, a Staff Solutions Architect working at VMware. The blog focuses on automation, integration, and operation of the VMware Software Defined Datacenter (SDDC).

Vsphere 6.5 Slot Size

Use vSphere Documentation

Vmware Slot Size Calculation

The vSphere documents in HTML reflect the latest vSphere update release of each major vSphere version. For example, version 7.0 contains all the updates for 7.0.x releases. All our documentation comes in PDF format, which you can access by selecting the Download PDF icon on any page in the HTML documentation. PDFs for previous releases of vSphere are available for download in a ZIP archive format. The archive can be found under the Archive Packages heading for each major version in the table of contents on the left.

Vsphere 6 Slot Size

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