Slot Size Definition
Posted By admin On 14/04/22Aug 31, 2009 What I'm saying is the 5/8 T-nuts are supposed to be undersize to fit a 5/8 slot but even if they're not it's not a big deal, just file to size. On the indicators, you'll want at least one of each. You want the indicator to fit the job and not to try to fit the job to the indicator; so one ends up collecting a range of devices. A slot is a narrow opening in a machine or container, for example, a hole that you put coins in to make a machine work. He dropped a coin into the slot and dialed. If you slot something into something else, or if it slots into it, you put it into a space where it fits. He was slotting a CD into a CD.
VMware slot sizes are an important topic if you’re concerned with how many ESXi hosts are required to run your environment.
What is a Slot?
To begin this post, we need to understand what a slot is. A slot is the minimum amount of CPU and memory resources required for a single VM in an ESXi cluster. Slot size is an important concept because it affects admission control.
A VMware ESXi cluster needs a way to determine how many resources need to be available in the event of a host failure. This slot calculation gives the cluster a way to reserve the right amount of resources.
How are Slots Sized?
The slot has two parts, the CPU component and the memory component. Each of them has its own calculation. If there are no virtual machine resource reservations in the cluster, then the slot size (for ESXi 5 at least) is 32 Mhz for CPU and 0 MBs + overhead for memory. (I’ve used 80 MBs as my memory overhead in the examples)
On to an incredibly simplistic diagram…
In the example below we have 2 ESXi hosts that have the same amount of resources available for virtual machines. There are different sized VMs, but none of them have a reservation. Doing a quick calculation we can determine that 384 slots are available on each host.
CPU Component: 4 X 3.0 GHz / 32 MHz = 384 slots
Memory Component: 49 GBs / 80 MBs = 627 slots
We take the lower value between the CPU slot size and the memory slot size to determine the number of virtual machines that can be started up under admission control. So therefore we could safely start 384 machines on these ESXi hosts, have one fail, and have the other host start all of them.
(I should mention that it’s unlikely that you could get 384 vms on one of these hosts. That would be a great consolidation ratio.)
Problem Scenario
What if you have a single large VM with a reservation, but the rest of the virtual machines are relatively small.
Let’s look at the same environment, but this time let’s make the larger VM have a reservation on it.
CPU Component: 4 X 3.0 GHz / 2000 MHz = 6 slots
Memory Component: 49 GBs / 4024 MBs = 12 slots
Admission control is going to tell us that only 6 slots are available on host B, so it will only allow 6 VMs on host A to be powered on. Since I’m using a simplistic diagram with only two hosts, we know that these VMs will still fit on the host but since we use the largest slot size to determine how much we can fail over admission control will stop us from powering on VMs.
Option 1 – Don’t use reservations unless their is a good reason to do so.
Option 2 – We can manually configure the slot size on the cluster.
Navigate to the cluster settings and go to the HA Section, Click Edit and you’ll have the option of modifying the slot size. Note that if you do this, some of your VMs will require multiple slots to run. For instance the large VM we used in our example might take more than 1 slot depending on what size you make it. The button below the slot size configuration may help you determine how many VMs will be affected by this change.
If you’re curious about what the slot size is on your system, look at your cluster summary. There will be an item listed for slot size.
If you’re in a situation where you think you need to add extra ESXi hosts to your cluster because you can’t power on virtual machines without exceeding your admission control rules, take a look at your slot sizes first. It may save you some money on a host you don’t really need.
Slot Size Definition Dictionary
Do you want more information on the subject? Take a look at either Frank Denneman or Duncan Epping’s blogs, or their book
A memory slot, memory socket, or RAM slot allows RAM (computer memory) to be inserted into the computer. Most motherboards have two to four memory slots, which determine the type of RAM used with the computer. The most common RAM types are SDRAM and DDR for desktop computers and SODIMM for laptop computers, each having various types and speeds. The picture below is an example of what memory slots may look like inside a desktop computer. In this picture, there are three open and available slots for three memory sticks.
Slot Size Meaning
When buying a new computer or motherboard, pay close attention to the types of RAM the memory slots can accept. Make sure you know exactly the type of RAM to buy for your computer. It is also important to note how many available memory slots are available in your computer. It is not uncommon for computers to have all memory slots occupied. If all slots are full and you want to upgrade the computer memory, you need to remove some or all of the existing memory.
What Is The Slot
Why are the memory slots different colors?
When a motherboard has different colored memory slots, it indicates the memory slots are dual-channel, and pairs of memory should be installed on the same channel (color). For example, a motherboard could have two yellow and two black memory slots. The yellow memory slots could indicate Channel A and Channel B could be indicated by the black slots. If you were only installing two memory sticks, you'd want to install both of them in Channel A (yellow slots) for optimal performance.
Related pages
Slot Size Definition Synonyms
Memory module, Memory terms