A question on the VMware community forum triggered me to validate DRS behavior of applying VM to Host group rules. The scenario describes a stretched HA cluster with overlapping DRS group rules, allowing to run particular VMs on hosts in a single site and a subset of hosts of both sides. How does DRS handle overlapping groups?
The architecture
In this scenario the stretched cluster contains four hosts; ESXi-01 and ESXi-02 are located in Site A, ESXi-03 and ESXi-04 are located in Site B.
A collection of virtual machines are run by the hosts in the cluster, two management virtual machines, vCenter and vCenterDB are a part of this group. Storage housing the virtual machines are available for all hosts. Storage architecture is not the focus of this article, for more information about storage configurations and stretched vSphere clusters, please read the white paper: VMware vSphere Metro Storage Cluster Case Study
Site DRS VM-Host groups
ESXi-01 and ESXi-02 are grouped in Host DRS group Host-Site-A, ESXi-03 and ESXi-04 are grouped in Host DRS group Host-Site-B. All virtual machines running in Site-A are grouped in a VM DRS group VM-Site-A, all virtual machines running in Site-B are grouped in a VM DRS group VM-Site-B. All (Site) rules are configured as preferential rules (Should run on).
Management DRS VM-Host group
In the scenario described on the community forum, the virtual machines vCenter and vCenterDB are placed in an additional VM DRS group; MGMT-VMs.
This group should be run on a select set of hosts of both sides, to simulate similar behavior the Host DRS group configured in my environment contains ESXi-02 of Site-A and ESXi-03 of Site-B. The group is named MGMT-Hosts.
Overlapping rule-set
Because the management virtual machines are a part of the VM-Site-A VM group an overlap of compatible hosts exists. Please note that DRS allows virtual machines to be a member of multiple VM groups. When reviewing the active affinity rules in a DRS cluster, DRS extracts a subset of compatible hosts for each virtual machine and uses this subset for placement and load balancing decisions. A Venn diagram shows the compatible host for the VMs vCenter and vCenterDB and specifically the host(s) listed in the compatibility set.
This means that under normal operation DRS will choose to run the virtual machines on ESXi-02 as it satisfies both rules. With normal condition I want to indicate that there is not excessive load on any of the host and all hosts are configured identically without any hardware failures. Back to the scenario, what if there is a host failure or ESXi-02 is placed into maintenance mode?
Maintenance mode
Now what happens if ESXi-02 is placed into maintenance mode? As previously mentioned, DRS determines the set of compatible hosts. As ESXi-02 is placed into maintenance mode, DRS mark this host as a source host for migration. This way DRS knows which virtual machine to select for migration and excludes ESXi-02 as a valid destination for migrations. DRS must select other hosts listed in the Host DRS group. As ESXi-02 is not a valid destination, DRS needs to select either ESXi-01 or ESXi-03. Which host will it select? This depends on the creation date of the DRS rules, in other words, the newer DRS VM-Host rule is selected first. This is important to understand when applying overlapping VM-Host affinity rules in your environment. The last rule you create is applied first, overruling all other existing rules. In my lab, I created the MGMT rule as last, therefor having the youngest timestamp.
There is no option of showing the timestamp in the user-interface of the web and vSphere client. I have provided this feedback to the engineering team. Hopefully they can include it in future releases.
I placed ESXi-02 into maintenance mode and DRS migrated the virtual machines to ESXi-03, regulated by the MGMT VM-Host affinity rule. After placing ESXi-03 into maintenance mode, the virtual machines were moved to ESXi-01 as there were no compatible hosts available to satisfy the MGMT affinity rule. As ESXi-01 is listed in the Host-Site-A group of the Site-A affinity rule, DRS had no choice other than moving them to ESXi-01. After resetting the lab and destroying all the rules, I created the same set of host groups, VM groups and affinity rules, but I created the Site-A affinity rule as last. This resulted in the behavior that DRS moved the virtual machines to host ESXi-01 after placing ESXi-02 into maintenance mode, as DRS respected the Site-A affinity rule.
Alarms
As DRS supports non-contradicting overlapping affinity rules, no alarm was generated. During the scenario where both ESXi-02 and ESXi-03 were placed into maintenance mode, no alarm was triggered. It was expected to see an alarm that an affinity alarm is violated, however after digging through some code and contacting engineering this appears to be behavior by design. In the current release, the alarm is only triggered when mandatory rules (must run on) are violated.
HA behavior
All rules are configured with preferential rule sets (Should run on) and HA is not aware of DRS constructs. When a mandatory rule set (Must run on) is created, the hosts listed in the rule set are registered in the compatibility list of the virtual machine itself. Only those hosts registered in the compatibility list are viable destinations. During startup, HA checks the compatibility list and attempts to start up the virtual machine on any of these hosts listed. As the virtual machines are a part of a preferential affinity rule, all hosts are listed in the compatibility list and therefor HA could place them on a host outside the DRS Host Group.
Conclusion
If you want certain virtual machines to gravitate to specific hosts or a specific site, please take into account the way DRS sequence the active affinity rules.
Calculating the bandwidth usage and duration of a vMotion process?
Every once in a while I get the question if I have a calculator that can determine the lead-time and the bandwidth consumption of a vMotion process. Unfortunately I haven’t got such a calculator, as there isn’t an easy way to calculate the consumed bandwidth and the duration of a vMotion process.
CPU
vMotion tries to move the used memory blocks as fast as possible. vMotion uses all the available bandwidth depending on the available CPU speed and bandwidth. Depending on the detected line speed, vMotion reserves an X amount of CPU speed at the start of a vMotion process. vMotion computes its desired host vMotion CPU reservation. For every 1GBe vMotion link speed it detects vMotion in vSphere 5.1 reserved 10% of a CPU core with a minimum desired CPU reservation of 30%. This means that if you use a single 1GBe, vMotion reserves 30% of a core, if you use 4 x 1GBe connections, that means vMotion reserves 40% of a core. A 10GBe link is special as vMotion reserves 100% of a single core.
vMotion creates a (system) resource pool and sets the appropriate CPU reservation on the resource pool. It’s important to note that this is being done to the vMotion resource pool, which means that the reservation is shared across all vMotions happening on the host.
Warning: DO NOT CHANGE the default settings of the system vMotion resource pool. This is set dynamically by the kernel depending on its memory state, manually adjusting this setting will likely hurt performance. Please do not attempt to be smarter then the kernel, many have tried, very few have succeeded.
DRS
When DRS is enabled, it can decide to migrate virtual machines as well. It might happen that this occurs at the same time your vMotion process is running. All vMotions will be placed into the vMotion resource pool contesting for the resources acquired by the resource pool of vMotion. If high priority for the manual vMotion is selected (User interface uses the term: Reserve CPU for optimal vMotion performance) then the vMotion process receives a higher priority within the vMotion resource pool. In which case the high priority vMotion will have double the relative CPU shares, and as a result probably complete more quickly than their lower priority counterparts. However it still need to share the resources claimed by the vMotion resource pool. Although it has a higher priority over DRS vMotions, sharing resources still may have an effect on the duration of the vMotion process.
Memory
vMotion copies only the used memory blocks, a virtual machine doesn’t always have to use all of its memory. Therefor its not easy to determine the required bandwidth. To make it more complex, as we are migrating a live virtual machine, the virtual machine can dirty (re-use) memory blocks that are already copied over, those blocks have to be sent again. Prolonging the duration of the process and the used bandwidth.
Swap file
If the swap file is located on a non-shared datastore and pages has been stored in the swap file, those pages are copied over to the new swap file on a location accessible by the destination host. This will increase the demand for bandwidth and increases the duration of the vMotion process. For more information about the impact of non-shared swap files, please read the following articles: (Alternative) VM swap file locations Q&A and (Alternative) VM swap file locations Q&A – part 2.
Conclusion
As you can see, it’s very difficult to determine the duration of a vMotion process and the actual bandwidth it consumes.
How to create a "New Storage DRS recommendation generated" alarm
It is recommended to configure Storage DRS in manual mode when you are new to Storage DRS. This way you become familiar with the decision matrix Storage DRS uses and you are able to review the recommendations it provides. One of the drawbacks of manual mode is the need to monitor the datastore cluster on a regular basis to discover if new recommendations are generated. As Storage DRS is generated every 8 hours and doesn’t provide insights when the next invocation run is scheduled, it’s becomes a bit of a guessing game when the next load balancing operation has occurred.
To solve this problem, it is recommended to create a custom alarm and configure the alarm to send a notification email when new Storage DRS recommendations are generated. Here’s how you do it:
Step 1: Select the object where the alarm object resides
If you want to create a custom rule for a specific datastore cluster, select the datastore cluster otherwise select the Datacenter object to apply this rule to each datastore cluster. In this example, I’m defining the rule on the datastore cluster object.
Step 2: Go to Manage and select Alarm Definitions
Click on the green + icon to open the New Alarm Definition wizard
Step 3: General Alarm options
Provide the name of the alarm as this name will be used by vCenter as the subject of the email. Provide an adequate description so that other administrators understand the purpose of this alarm.
In the Monitor drop-down box select the option “Datastore Cluster” and select the option “specific event occurring on this object, for example VM Power On”. Click on Next.
Step 4: Triggers
Click on the green + icon to select the event this alarm should be triggered by. Select “New Storage DRS recommendation generated”. The other fields can be left blank, as they are not applicable for this alarm. Click on next.
Step 5: Actions
Click on the green plus icon to create a new action. You can select “Run a Command”, “Send a notification email” and “Send a notification trap”. For this exercise I have selected “Send a notification email”. Specify the email address that will receive the messages containing the warning that Storage DRS has generated a migration recommendation. Configure the alarm so that it will send a mail once when the state changes from green to yellow and yellow to red. Click on Finish.
The custom alarm is now listed between the pre-defined alarms. As I chose to define the alarm on this particular datastore cluster, vCenter list that the alarm is defined on “this Object”. This particular alarm is therefor not displayed at Datacenter level and cannot be applied to other datastore clusters in this vCenter Datacenter.
Please note that you must configure a Mail server when using the option “send a notification email” and configure an valid SNMP receiver when using the option “Send a notification trap”. To configure a mail or SNMP server, select the vCenter server option in the inventory list, select manage, settings and click on edit. Go to Mail and provide a valid mail server address and an optional mail sender.
To test the alarm, I moved a couple of files onto a datastore to violate the datastore cluster space utilization threshold. Storage DRS ran and displayed the following notifications on the datastore cluster summary screen and at the “triggered alarm” view:
The moment Storage DRS generated a migration recommendation I received the following email:
As depicted in the screenshot above, the subject of the email generated by vCenter contains the name of the alarm you specified (notice the exclamation mark), the event itself – New Storage DRS recommendation generated” and the datastore cluster in which the event occurred.
How to create a "New Storage DRS recommendation generated" alarm
It is recommended to configure Storage DRS in manual mode when you are new to Storage DRS. This way you become familiar with the decision matrix Storage DRS uses and you are able to review the recommendations it provides. One of the drawbacks of manual mode is the need to monitor the datastore cluster on a regular basis to discover if new recommendations are generated. As Storage DRS is generated every 8 hours and doesn’t provide insights when the next invocation run is scheduled, it’s becomes a bit of a guessing game when the next load balancing operation has occurred.
To solve this problem, it is recommended to create a custom alarm and configure the alarm to send a notification email when new Storage DRS recommendations are generated. Here’s how you do it:
Step 1: Select the object where the alarm object resides
If you want to create a custom rule for a specific datastore cluster, select the datastore cluster otherwise select the Datacenter object to apply this rule to each datastore cluster. In this example, I’m defining the rule on the datastore cluster object.
Step 2: Go to Manage and select Alarm Definitions
Click on the green + icon to open the New Alarm Definition wizard
Step 3: General Alarm options
Provide the name of the alarm as this name will be used by vCenter as the subject of the email. Provide an adequate description so that other administrators understand the purpose of this alarm.
In the Monitor drop-down box select the option “Datastore Cluster” and select the option “specific event occurring on this object, for example VM Power On”. Click on Next.
Step 4: Triggers
Click on the green + icon to select the event this alarm should be triggered by. Select “New Storage DRS recommendation generated”. The other fields can be left blank, as they are not applicable for this alarm. Click on next.
Step 5: Actions
Click on the green plus icon to create a new action. You can select “Run a Command”, “Send a notification email” and “Send a notification trap”. For this exercise I have selected “Send a notification email”. Specify the email address that will receive the messages containing the warning that Storage DRS has generated a migration recommendation. Configure the alarm so that it will send a mail once when the state changes from green to yellow and yellow to red. Click on Finish.
The custom alarm is now listed between the pre-defined alarms. As I chose to define the alarm on this particular datastore cluster, vCenter list that the alarm is defined on “this Object”. This particular alarm is therefor not displayed at Datacenter level and cannot be applied to other datastore clusters in this vCenter Datacenter.
Please note that you must configure a Mail server when using the option “send a notification email” and configure an valid SNMP receiver when using the option “Send a notification trap”. To configure a mail or SNMP server, select the vCenter server option in the inventory list, select manage, settings and click on edit. Go to Mail and provide a valid mail server address and an optional mail sender.
To test the alarm, I moved a couple of files onto a datastore to violate the datastore cluster space utilization threshold. Storage DRS ran and displayed the following notifications on the datastore cluster summary screen and at the “triggered alarm” view:
The moment Storage DRS generated a migration recommendation I received the following email:
As depicted in the screenshot above, the subject of the email generated by vCenter contains the name of the alarm you specified (notice the exclamation mark), the event itself – New Storage DRS recommendation generated” and the datastore cluster in which the event occurred.
vSphere 5.1 Clustering deepdive Cyber Monday deal
We are long time fascinated by the whole Black Friday and Cyber Monday craze in the USA. Unfortunately we do not celebrate Thanksgiving in the Netherlands and none of the shops are participating in something similar as Black Friday.
Similar to last year, we thought it was a great idea to participate in some form and what better than to offer our vSphere 5.1 Clustering Technical Deepdive book for a price you cannot resist. We just changed the price of the vSphere 5.1 Clustering Technical Deepdive to $17.95, Amazon Deutschland is offering the book for 16.00 EURO, while Amazon UK is selling the book for 11.01 Pounds sterling.
The book has some amazing reviews, here is one we like to share with you:
The book contains information critical to VMware administrators. Clustering is a critical technology, and the book covers the underlying concepts as well as the practical issues surrounding implementation. The information contained within will be important far past vSphere 5.1; the principles will apply for decades, and even the details of implementation are unlikely to change dramatically over the next few generations of the product. As with any good “deep dive,” the fundamental concepts discussed will ultimately help you in any clustering situation, even with non-VMware products.
From a reading comprehension standpoint, the book is easy to grok. The information flows quickly and you can read the entire work cover to cover with relative ease. This is a must have for any systems administrator.
What better way than recover from the madness of Black Friday and just sit back and relax reading this amazing piece of work? This is most definitely the deal of the year for all virtualization fanatics!
Duncan and Frank