Vmware Vsphere Clustering Deepdive - Epping, Duncan - Ebook download as PDF File .pdf), Text File .txt) or read book online. with the vSphere High Availability content as that is what I am most familiar with and will be easiest to update. Clustering Deepdive with and who designed all the great diagrams which you find throughout PDF - ePub - Mobi. The source of . VMware vSphere vCenter Server - Technical Deep Dive. Scott Calvet .. Deployment Types (Basic/Primary/MultiSite Replica Node/HA Backup). ▫ Primary .
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A list of hosts participating in the cluster. Basic design principle Network heartbeating is key for determining the state of a host. Heartbeating We mentioned it a couple of times already in this chapter. The new datastore heartbeat mechanism is only. This has been mitigated by the introduction of the datastore heartbeating mechanism. Heartbeating is the mechanism used by HA to validate whether a host is alive. IP addresses. These heartbeats are sent by default every second.
With the introduction of vSphere 5. Datastore heartbeating enables a master to more correctly determine the state of a host that is not reachable via the management network. MAC addresses and heartbeat datastores. This scenario is described in-depth in Part IV of this book. This however is not a guarantee that vCenter can select datastores which are connected to all hosts. In scenarios where hosts are geographically dispersed it is recommend to manually select heartbeat datastores to ensure each site has one site-local heartbeat datastore at minimum.
It is recommended to manually select site local datastores. If the master determines that the slave is Isolated or Partitioned. Selecting the heartbeat datastores. It should be noted that vCenter is not site-aware.
Let that be clear! Based on the results of checks of both files. Although it is possible to configure an advanced setting das. HA selects 2 heartbeat datastores — it will select datastores that are available on all hosts. If the master determines that a host has failed no datastore heartbeats. If desired. By default. This for instance shows you which datastores are being used for heartbeating and which hosts are using which specific datastore s.
The question now arises: Validating the heartbeat datastores How does this heartbeating mechanism work? HA ensures there is at least one file open on this volume by creating a file specifically for datastore heartbeating. In order to update a datastore heartbeat region. Basic design principle Datastore heartbeating adds a new level of resiliency but is not the be-all end-all. HA will simply check whether the heartbeat region has been updated. The master will simply validate this by checking that the time-stamp of the file changed.
In converged networking environments. Isolated versus Partitioned. HA will detect this and select a new datastore or NFS share to use for the heartbeating mechanism. Heartbeat file On NFS datastores. In other words. Realize that in the case of a converged network environment. On VMFS datastores.
It is possible for multiple hosts to be isolated at the same time. We call the set of hosts that are partitioned but can communicate with each other a management network partition. Two hosts are considered partitioned if they are operational but cannot reach each other over the management network. Figure 14 shows possible ways in which an Isolation or a Partition can occur. Network partitions involving more than two partitions are possible but not likely.
What is this exactly and when is a host Partitioned rather than Isolated? Before we will explain this we want to point out that there is the state as reported by the master and the state as observed by an administrator and the characteristics these have. When any FDMs are not in network contact with a master. In the HA architecture. It should be noted that a master could claim responsibility for a virtual machine that lives in a different partition.
The master cannot alone differentiate between these two states — a host is reported as isolated only if the host informs the master via the datastores that is isolated. If this occurs and the virtual machine happens to fail. When the network partition is corrected. If a cluster is partitioned in multiple segments. When a partition occurs. When the master stops receiving network heartbeats from a slave. If both are negative. When the host is marked as Failed. Before the host is declared failed.
To reiterate. HA will trigger an action based on the state of the host. If the host does not have access to the datastore. If it can. As mentioned earlier. When the host is marked as Isolated. The one thing to keep in mind when it comes to isolation response is that a virtual machine will only be shut down or powered off when the isolated host knows there is a master out there that has taken ownership for the virtual machine or when the isolated host loses access to the home datastore of the virtual machine.
If no master owns the datastores. We do want to stress that this only applies to protecting virtual machines. When the state of a virtual machine changes. When the power state change of a virtual machine has been committed to disk. If the term isolation response is not clear yet. We have explained this briefly but want to expand on it a bit more to make sure everyone understands the dependency on vCenter when it comes to protecting virtual machines.
Virtual Machine Protection The way virtual machines are protected has changed substantially in vSphere 5. The reason for this. As pointed out earlier. Virtual Machine protection workflow. To clarify the process. Virtual Machine Unprotection workflow. We have documented this workflow in Figure 16 for the situation where the power off is invoked from vCenter.
A good example of an agent virtual machine is a vShield Endpoint virtual machine which offers antivirus services. Chapter 4 Restarting Virtual Machines In the previous chapter. HA will still take the configured priority of the virtual machine into account.
There are multiple scenarios in which HA will respond to a virtual machine failure. HA would take the priority of the virtual machine into account when a restart of multiple virtual machines was required.
Restart Priority and Order Prior to vSphere 5. Agent Virtual Machines. Reliability of HA in this case mostly refers to restarting or resetting virtual machines. HA will respond when the state of a host has changed. Before we dive into the different failure scenarios.
Changing the process results in slightly different recovery timelines. We have shown you that multiple mechanisms were introduced in vSphere 5. These agent virtual machines are considered top priority virtual machines.
These apply to every situation we will describe. There are many different scenarios and there is no point in covering all of them. In the meantime. HA would keep retrying forever which could lead to serious problems. Restart Retries The number of retries is configurable as of vCenter 2. Keep in mind that some virtual machines might be dependent on the agent virtual machines. Prior to vCenter 2. We have listed the full order in which virtual machines will be restarted below: HA also prioritizes FT secondary machines.
The default value is 5. If the restart of a top priority virtual machine fails. Now that we have briefly touched on it. Besides agent virtual machines. Prioritization is done by each host and not globally. Each host that has been requested to initiate restart attempts will attempt to restart all top priority virtual machines before attempting to start any other virtual machines.
Basic design principle Virtual machines can be dependent on the availability of agent virtual machines or other virtual machines. You should document which virtual machines are dependent on which agent virtual machines and document the process to start up these services in the right order in the case the automatic restart of an agent virtual machine fails.
This scenario is described in KB article where multiple virtual machines would be registered on multiple hosts simultaneously. Although HA will do its best to ensure all virtual machines are started in the correct order. Document the proper recovery process. HA will continue powering on the remaining virtual machines.
Note Prior to vSphere 5. There are specific times associated with each of these attempts. As said. HA will try to start the virtual machine on one of your hosts in the affected cluster. The following bullet list will clarify this concept. The elapsed time between the failure of the virtual machine and the restart. Before we go into the exact timeline. This by itself could be 30 seconds after the virtual machine has failed. Meaning that the total amount of restarts was 6.
High Availability restart timeline. As clearly depicted in Figure The 33rd power on attempt will only be initiated when one of those 32 attempts has completed regardless of success or failure of one of those attempts. In theory. When it comes to restarts. T2 could be T2 plus 8 seconds. HA will start the 2-minute wait as soon as it has detected that the initial attempt has failed.
To make that more clear. The master. The restart priority however does guarantee that when a placement is done. Let it be absolutely clear that HA does not wait to restart the low-priority virtual machines until the high-priority virtual machines are started.
Another important fact that we want emphasize is that there is no coordination between masters. Although only one will succeed. If there are 32 low-priority virtual machines to be powered on and a single high-priority virtual machine.
In most environments. Keeping in mind that this is an actual failure of the host. Just in case it happens. T18s — If heartbeat datastores are configured. This is a continuous ping for 5 seconds. We want to emphasize this because the time it takes before a restart attempt is initiated differs between these two scenarios. T3s — Master begins monitoring datastore heartbeats for 15 seconds. Now that we know how virtual machine restart priority and restart retries are handled.
T10s — The host is declared unreachable and the master will ping the management network of the failed host. There is a clear distinction between the failure of a master versus the failure of a slave. T15s — If no heartbeat datastores are configured. Basic design principle Configuring restart priority of a virtual machine is not a guarantee that virtual machines will actually be restarted in this order.
Part of this complexity comes from the introduction of a new heartbeat mechanism. Ensure proper operational procedures are in place for restarting services or virtual machines in the appropriate order in the event of a failure. If heartbeat datastores have been configured. The master will also start pinging the management network of the failed host at the 10th second and it will do so for 5 seconds.
On the 10th second T10s. Restart timeline slave failure. When the slave fails. The master monitors the network heartbeats of a slave. If no heartbeat datastores were configured. We realize that this can be confusing and hope the timeline depicted in Figure 18 makes it easier to digest. We have defined this as T0. After 3 seconds T3s.
As an example. If the master did not know the on-disk protection state for the virtual machine. If there is a network partition multiple masters could try to restart the same virtual machine as vCenter Server also provided the necessary details for a restart. The master filters the virtual machines it thinks failed before initiating restarts.
In this. T25s — New master elected and reads the protectedlist. Restart timeline master failure. This change in behavior was introduced to avoid the scenario where a restart of a virtual machine would fail due to insufficient resources in the partition which was responsible for the virtual machine.
The timeline is as follows: At T25s. This means that an election will need to take place amongst the slaves. The reason being that there needs to be a master before any restart can be initiated. The Failure of a Master In the case of a master failure. That leaves us with the question of what happens in the case of the failure of a master. With this change. The election process takes 15s to complete. Slaves receive network heartbeats from their master.
This list contains all the virtual machines which are protected by HA. T10s — Master election process initiated. T35s — New master initiates restarts for all virtual machines on the protectedlist which are not running. As every cluster needs a master. The timeline depicted in Figure 19 hopefully clarifies the process.
At T35s. Isolation Response and Detection Before we will discuss the timeline and the process around the restart of virtual machines after an isolation event. Besides the failure of a host. Keep in mind that these changes are only applicable to newly created clusters.
There was a lot of feedback. Cluster default settings The default setting for the isolation response has changed multiple times over the last couple of years and this has caused some confusion. When upgrading an existing cluster. It is a hard stop. This setting can be changed on the cluster settings under virtual machine options Figure If VMware Tools is not installed. When creating a new cluster.
Shut down — When isolation occurs. This time out value can be adjusted by setting the advanced option das. This does not necessarily mean that the whole network is down. You might wonder why the default has changed once again. Leave powered on — When isolation occurs on the host.
Today there are three isolation responses: This isolation response answers the question. If this is not successful within 5 minutes. The obvious answer applies here. Of course. HA will validate if virtual machines restarts can be attempted — there is no reason to incur any down time unless absolutely necessary. In a converged network environment with iSCSI storage. Basically resulting in the power off or shutdown of every single virtual machine and none being restarted.
It is still difficult to decide which isolation response should be used. We feel that changing the isolation response is most useful in environments where a failure of the management network is likely correlated with a failure of the virtual machine network s.
Basic design principle Before upgrading an environment to later versions. One of the problems that people have experienced in the past is that HA triggered its isolation response when the full management network went down. The question remains. It does this by validating that a master owns the datastore the virtual machine is stored on.
The following table was created to provide some more guidelines. Document them. That is no longer the case with vSphere 5. HA did not care and would always try to restart the virtual machines according to the last known state of the host.
Before the isolation response is triggered. The master will recognize that the virtual machines have disappeared and initiate a restart. When isolation response is triggered. Meaning that if a single ping is successful or the host observes election traffic and is elected a master or slave. When a host has declared itself isolated and observes election traffic it will declare itself no longer isolated.
In this timeline. This delay can be increased using the advanced option: The mechanism is fairly straightforward and works with heartbeats. There are.
The following timeline is the timeline for a vSphere 5. The main difference is the fact that HA triggers a master election process before it will declare a host is isolated. Isolation of a Slave The isolation detection mechanism has changed substantially since previous versions of vSphere.
Isolation Detection We have explained what the options are to respond to an isolation event and what happens when the selected response is triggered. Isolation of a slave timeline Isolation of a Master In the case of the isolation of a master. After the completion of this sequence. These power-off files are deleted when a virtual machine is powered back on or HA is disabled. The power-off file is used to record that HA powered off the virtual machine and so HA should restart it.
A secondary management network will more than likely be on a different subnet and it is recommended to specify an additional isolation address which is part of the subnet. We recommend setting an additional isolation address.
Figure 22 Figure Isolation Address Selecting an Additional Isolation Address A question asked by many people is which address should be specified for this additional isolation verification. Failure Detection Time Those who are familiar with vSphere 4.
HA gives you the option to define one or multiple additional isolation addresses using an advanced setting. Basic design principle Select a reliable secondary isolation address. If required. Another usual suspect would be a router or any other reliable and pingable device on the same subnet. If a secondary management network is configured. This advanced setting is called das. We generally recommend an isolation address close to the hosts to avoid too many network hops and an address that would correlate with the liveness of the virtual machine network.
In many cases. If the master is not in contact with vCenter Server or has not locked the file. When the master node declares the slave node as Partitioned or Isolated. Restarting Virtual Machines The most important procedure has not yet been explained: At this point.
If the host was not Partitioned or Isolated before the failure. We have explained the difference in behavior from a timing perspective for restarting virtual machines in the case of a both master node and slave node failures. The minimum value is These files are asynchronously read approximately every 30s. Now that HA knows which virtual machines it should restart. This setting was completely removed when HA was rewritten. In almost all scenarios 30 seconds should suffice.
Before it will initiate the restart attempts. For now. This validation uses the protection information vCenter Server provides to each master. We do not recommend changing this advanced setting unless there is a specific requirement to do so. We have dedicated a full section to this concept as. However with vSphere 5.
HA will take multiple things in to account: If set to a value less than If this list exceeds 32 virtual machines. A host reports that its unreserved capacity has increased. The master node will report to vCenter the set of virtual machines that were not placed due to insufficient resources. This means that virtual machine restarts will be distributed by the master across multiple hosts to avoid a boot storm.
If DRS is enabled. A failure occurred when failing over a virtual machine. A split brain in this case meaning that a virtual machine would be powered up simultaneously on two different hosts. A host re joins the cluster For instance.
HA will limit the number of concurrent power on attempts to A new failure is detected and virtual machines have to be failed over. The master will then remove the virtual machine from the restart list. This is described more in-depth in Chapter 8. Corner Case Scenario: Split-Brain In the past pre-vSphere 4. But what about DRS? It does. Restart latency refers to the amount of time it takes to initiate virtual machine restarts. If a placement cannot be found. This situation could occur during a full network isolation.
If a virtual machine successfully powers on. If a placement is found. Permanent Device Loss As of vSphere 5. We also recommend increasing heartbeat network resiliency to avoid getting in to this situation. As stated above. A PDL condition. We will discuss the options you have for enhancing Management Network resiliency in the next chapter. This condition indicates that a device LUN has become unavailable and is likely permanently unavailable.
An example scenario in which this condition would be communicated by the array would be when a LUN is set offline.
HA automatically answers the question with Yes. In case it does happen. As just explained. HA will generate an event for this autoanswered question though. This condition is used in nonuniform models during a failure scenario to ensure ESXi takes appropriate action when access to a LUN is revoked.
The question still remains: It should be noted that when a full storage failure occurs it is impossible to generate the Permanent Device Loss condition as there is no communication possible between the array and the ESXi host. It is important to recognize that the following settings only apply to a PDL condition and not to. HA will automatically power off your original virtual machine when it detects a split-brain scenario.
Keep in mind that they truly are corner case scenarios which are very unlikely to occur in most environments. If you set it to true. This setting was introduced because HA cannot differentiate between a virtual machine that was killed due to the PDL state and a virtual machine that has been powered off by an administrator. The second setting is a vSphere HA advanced setting called das.
PDL Advanced Setting. This setting ensures that a virtual machine is killed when the datastore on which it resides enters a PDL state. In the failure scenarios we will demonstrate the difference in behavior for these two conditions. The first setting is configured on a host level and is disk. Note that this can lead to two instances of the same virtual machine being on the virtual machine network. By setting it to true.
This setting allows HA to trigger a restart response for a virtual machine which has been killed automatically due to a PDL condition. This setting was introduced in vSphere 5.
Chapter 5 Adding Resiliency to HA Network Redundancy In the previous chapter we extensively covered both Isolation Detection which triggers the selected Isolation Response and the impact of a false positive. However, this also means that it is possible that, without proper redundancy, the Isolation Response may be unnecessarily triggered. This leads to downtime and should be prevented. To increase resiliency for networking, VMware implemented the concept of NIC teaming in the hypervisor for both VMkernel and virtual machine networking.
When discussing HA, this is especially important for the Management Network. Using this mechanism, it is possible to add redundancy to the Management Network to decrease the chances of an isolation event.
A little understood fact is that if there are multiple VMkernel networks on the same subnet, HA will use all of them for management traffic, even if only one is specified for management traffic!
Although there are many configurations possible and supported, we recommend a simple but highly resilient configuration. We have included the vMotion VMkernel network in our example as combining the Management Network and the vMotion network on a single vSwitch is the most commonly used configuration and an industry accepted best practice.
Management Network active on vmnic0 and standby on vmnic1. Failback set to No. NIC Teaming Tab. Easy to configure. Just a single active path for heartbeats. To increase resiliency, we also recommend implementing the following advanced settings and using NIC ports on different PCI busses — preferably NICs of a different make and model.
When using a different make and model, even a driver failure could be mitigated. The isolation address setting is discussed in more detail in Chapter 4.
In short; it is the IP address that the HA agent pings to identify if the host is completely isolated from the network or just not receiving any heartbeats. If multiple VMkernel networks on different subnets are used, it is recommended to set an isolation address per network to ensure that each of these will be able to validate isolation of the host. Basic design principle. Take advantage of some of the basic features vSphere has to offer like NIC teaming. Combining different physical NICs will increase overall resiliency of your solution.
Link State Tracking This was already briefly mentioned in the list of recommendations, but this feature is something we would like to emphasize. We have noticed that people often forget about this even though many switches offer this capability, especially in blade server environments. Link state tracking will mirror the state of an upstream link to a downstream link. You might wonder why this is. Many features that vSphere offer rely on networking and so do your virtual machines.
In the case where the state is not reflected, some functionality might just fail, for instance network heartbeating could fail if it needs to flow through the core switch. Basic design principle Know your network environment, talk to the network administrators and ensure advanced features like Link State Tracking are used when possible to increase resiliency.
Chapter 6 Admission Control Admission Control is more than likely the most misunderstood concept vSphere holds today and because of this it is often disabled. What is HA Admission Control about? Why does HA contain this concept called Admission Control? Please read that quote again and especially the first two words. Indeed it is vCenter that is responsible for Admission Control, contrary to what many believe. Although this might seem like a trivial fact it is important to understand that this implies that Admission Control will not disallow HA initiated restarts.
HA initiated restarts are done on a host level and not through vCenter. As said, Admission Control guarantees that capacity is available for an HA initiated failover by reserving resources within a cluster. It calculates the capacity required for a failover based on available resources.
In other words, if a host is placed into maintenance mode or disconnected, it is taken out of the equation. This also implies that if a host has failed or is not responding but has not been removed from the cluster, it is still included in the equation. To give an example; VMkernel memory is subtracted from the total amount of memory to obtain the memory available memory for virtual machines.
There is one gotcha with Admission Control that we want to bring to your attention before drilling into the different policies. When Admission Control is enabled, HA will in no way violate availability constraints. This means that it will always ensure multiple hosts are up and running and this applies for manual maintenance mode actions and, for instance, to VMware Distributed Power Management. So, if a host is stuck trying to enter Maintenance Mode, remember that it might be HA which is not allowing Maintenance Mode to proceed as it would violate the Admission Control Policy.
With vSphere 4. When Admission Control was disabled, DPM could place all hosts except for 1 in standby mode to reduce total power consumption. This could lead to issues in the event that this single host would fail.
As of vSphere 4. If by any chance the resources are not available, HA will wait for these resources to be made available by DPM and then attempt the restart of the virtual machines.
In other words, the retry count 5 retries by default is not wasted in scenarios like these.
If you are still using an older version of vSphere or, god forbid, VI3, please understand that you could end up with all but one ESXi host placed in standby mode, which could lead to potential issues when that particular host fails or resources are scarce as there will be no host available to power on your virtual machines.
This situation is described in the following knowledge base article: This section gives a general overview of the available Admission Control Policies. The impact of each policy is described in the following section, including our recommendation.
HA has three mechanisms to guarantee enough capacity is available to respect virtual machine resource reservations. Understanding each of these Admission Control mechanisms is important to appreciate the impact each one has on your cluster design. Below we have listed all three options currently available as the Admission Control Policy. This section will take you on a journey through the trenches of Admission Control Policies and their respective mechanisms and algorithms. It is also historically the least understood Admission Control Policy due to its complex admission control mechanism.
Each option has a different mechanism to ensure resources are available for a failover and each option has its caveats. A new feature only available in the vSphere 5. Although we have already touched on this. This can be very useful in scenarios where the slot size has been explicitly specified. In the case of a 32 host cluster.
The details of this mechanism have changed several times in the past and it is one of the most restrictive policies. HA initiated failovers are not.
Admission Control does not limit HA in restarting virtual machines. This has changed as some felt that MHz was too aggressive. Reservations defined at the Resource Pool level however. If reservations are needed. Even if resources are low and vCenter would complain.
If no reservation of higher than 32 MHz is set.
Now that we know the worst-case scenario is always taken into account when it comes to slot size calculations. Basic design principle Be really careful with reservations.
If no memory reservation is set. Note that this behavior has changed: HA initiated restarts. See the VMware vSphere Resource Management Guide for more details on memory overhead per virtual machine configuration. Admission Control is done by vCenter. It is a combination of the highest reservation of both virtual machines that leads to the total slot size. The question we receive a lot is how do I know what my slot size is? High Availability cluster monitor section Advanced Runtime Info will show the specifics the slot size and more useful details such as the number of slots available as depicted in Figure High Availability advanced runtime info.
The most restrictive number worst-case scenario is the number of slots for this host. This leaves us with a total number of slots for both memory and CPU for a host. When not configured to fully automated user action is required to execute DRS recommendations. Virtual machine spanning multiple HA slots. Figure 30 depicts a scenario where a virtual machine spans multiple slots. If one of these advanced settings is used. In order for this to be successful DRS will need to be enabled and configured to fully automated.
HA will notify DRS that a power-on attempt was unsuccessful and a request will be made to defragment the resources to accommodate the remaining virtual machines that need to be powered on. If you have just one virtual machine with a really high reservation. Notice that because the memory slot size has been manually set to MB. It will take the number of slots this virtual machine will consume into account by subtracting them from the total number of available slots.
Although in total there are enough resources available. Admission Control does not take fragmentation of slots into account when slot sizes are manually defined with advanced settings. If there is a large discrepancy in size and reservations we. Basic design principle Avoid using advanced settings to decrease the slot size as it could lead to more down time and adds an extra layer of complexity. HA will request DRS to defragment the resources.
This is by no means a guarantee of a successful power-on attempt. As you might have noticed. As stated earlier. Unbalanced Configurations and Impact on Slot Calculation It is an industry best practice to create clusters with similar hardware configurations.
However there is a caveat for DRS as well. When the time has come to expand. When you think about it and understand the internal workings of HA. We highly recommend monitoring this section on a regular basis to get a better understand of your environment and to identify those virtual machines that might be problematic to restart in case of a host failure.
What would happen to the total number of slots in a cluster of the following specifications? For HA. The question is will you add the newly bought hosts to the same cluster or create a new cluster? From a DRS perspective. Figure 31 depicts this scenario. High Availability memory slot size When Admission Control is enabled and the number of host failures has been selected as the Admission Control Policy.
A MB memory reservation has been defined on this virtual machine. For the sake of simplicity. As explained earlier. In our example. This will result in: As Admission Control is enabled. Basic design principle When using admission control.
In this case ESXi is taken out of the equation and one of any of the remaining hosts in the cluster is also taken out. As clearly demonstrated. Can you avoid large HA slot sizes due to reservations without resorting to advanced settings?
This makes sense. When a single host failure has been specified. First of all. When you specify a percentage. For those virtual machines that do not have a reservation. In other words: This was a single value for CPU and memory. HA will add up all available resources to see how much it has available virtualization overhead will be subtracted in total.
HA will calculate how much resources are currently reserved by adding up all reservations for memory and for CPU for all powered on virtual machines. VMware introduced the ability to specify a percentage next to a number of host failures and a designated failover host. Even if a reservation has been set.
This would lead to the following calculations: This example also demonstrates how keeping CPU and memory percentage equal could create an imbalance. Admission Control will constantly monitor if the policy has been violated or not.
In order to ensure virtual machines can always be restarted. Admission Control will disallow powering on any additional virtual machines as that could potentially impact availability. When one of the thresholds is reached.
Experience over the years has proven. HA will not be able to initiate a power-on attempt. Instead an event will get generated indicating "not enough resources for failover" for this virtual machine. High Availability summary If you have an unbalanced cluster hosts with different sizes of CPU or memory resources. We recommend selecting the highest restart priority for this virtual machine of course. This way you ensure that all virtual machines residing on this host can be restarted in case of a host failure.
One of those virtual machines has a 4 GB memory reservation. As earlier explained. The following example and diagram Figure 35 will make it more obvious: You have 3 hosts. A host fails and all virtual machines will need to failover.
As such. Available resources. DRS is notified to rebalance the cluster. Failover Hosts The third option one could choose is to select one or multiple designated Failover hosts.
This is commonly referred to as a hot standby. Besides the fact that vSphere 5. Basic design principle Although vSphere 5. Select failover hosts Admission Control Policy. Do the math. Select multiple failover hosts. Although this host was not a designated failover host.
HA will still try to restart the impacted virtual machines on the host that is left. These hosts are literally reserved for failover situations. HA will use it to limit downtime. HA will attempt to use these hosts first to failover the virtual machines. When you designate hosts as failover hosts. Basic design principle Admission control guarantees enough capacity is available for virtual machine failover. It is important.
In the previous section. This impact could be positive but also. We generally recommend enabling Admission Control as it is the only way of guaranteeing your virtual machines will be allowed to restart after a failure. Guarantees failover by calculating slot sizes. The first decision that will need to be made is whether Admission Control will be enabled. As such we recommend enabling it.
Although we already have explained all the mechanisms that are being used by each of the policies in the previous section. This especially goes for HA Admission Control. Slots are based on VM-level reservations and if reservations are not used a default slot size for CPU of 32 MHz is defined and for memory the largest memory overhead of any given virtual machine is used. Decision Making Time As with any decision you make. HA re-calculates how many slots are available.
Although this is a corner-case scenario. Complexity for administrator from calculation perspective. DRS will not migrate virtual machines to these hosts when resources are scarce or the cluster is imbalanced.