I read the following in the AZURE214x Azure Fundamentals openedx course:
Each virtual machine in an availability set is placed in one update
domain and two fault domains.
I don't understand why a VM in an availabilty set is placed in two fault domains? I mean a VM can only sit in one fault domain or am I wrong? Can someone explain?
Each virtual machine in an availability set is placed in one update
domain and two fault domains.
No, this is wrong.
Each virtual machine in your availability set is assigned an update domain and a fault domain by the underlying Azure platform.
Also, you could refer to Mistake in Module3, Review question 2 (AZURE202x Microsoft Azure Virtual Machines).
I guess its just a wording thing, VM cannot be in 2 fault domains at the same time.
"Each virtual machine in your availability set is assigned an update domain and a fault domain by the underlying Azure platform."
Reference: https://learn.microsoft.com/en-us/azure/virtual-machines/virtual-machines-windows-manage-availability
Related
I have created two vm's in azure and assigned an availability set.
But in the overview I see that both the vm's are assigned under same Fault Domain.
Should the vm (Vm availability-test-vm2) have been assigned to Fault domain 1 instead of 0 ?
By default, the virtual machines configured within your availability set are separated across up to three fault domains for Resource Manager deployments.
If there are only two VMs within an availability set, each VM should be in a different fault domain. However, If there are more than two VMs(for example, 3 VMs in 2 fault domains) or you have created three VMs(even you delete one of them), it's possible that the remaining 2 VMs are assigned to the same fault domain because the original three VMs should be in fault domain 0, 1, 0.
For example,
You can create and deploy virtual machines in an availability set using Azure PowerShell. Please note that A VM can only be added to an availability set when it is created.
I would like to know some simple explanation to understand about Fault domain and update domain in Azure.
Most of the theory is little confusing to me. So the below picture is what that i understand if we make it as a cluster environment to update the Hypervisor?
enter image description here
Group of VMs that share common hardware are said to be in the same fault domain.
During planned maintenance by Microsoft VMs are required to reboot to complete the update.Group of VMs and underlying Hardware that are rebooted at the same time are said to be in the same update domain.
So I have 2 classes of VM. Lets call them serverA and serverB.
Within my availability set I want to make sure each Fault domain has 1 of each VMs (ServerA and ServerB). Is it possible to have this level of fine control? From what I can tell it looks like this is achieved by adding the Servers in a specific order i.e. serverA, server A, server B server B assuming I had 2 fault domains.
Is this true? Is it the only way?
When you deploy Virtual machines in an Availability Sets, Azure takes care of placing machines in different fault domains and update domain. We do not have granular level control on deciding which VM needs to be in which Fault domain.
In a high level, if you deploy 2 VMs, then Azure will make sure those 2 VMs are part of different Fault domain and update domain.
I provisioned a few Azure VM's in an Availability set using the Azure Java SDK. The VM deployed successfully and is running. However, one of the VM's, the one provisioned first, does not display a fault domain or an update domain.
Why is my VM not being assigned a fault domain or an update domain?
Why is my VM not being assigned a fault domain or an update domain?
No, your VM is in an availability set. Your availability set name is SG-*. When VM in an availability set, it will assign a fault domain and update domain automatic.
You could get your VM's fault domain and update domain value on Azure Portal.
Resource group-->Availability set-->Virtual Machines
Update:
However, one of the VM's, the one provisioned first, does not display
a fault domain or an update domain.
Based on my knowledge, it is a design behavior. The first VM's fault domain and update domain is 0 0. You could all domain on Availability set.
I am reading the explanation of Availability Sets on Microsoft' website but can't 100% understand the concept.
http://www.windowsazure.com/en-us/documentation/articles/manage-availability-virtual-machines/
There are many questions people ask in comments, but there is no technical support from Microsoft is there to answer them.
As I properly understand with availability sets you can duplicate your VM with IIS application and VM with SQL, which means you have to use 4 VM(pay for 4) instead of 2. This means that whenever IIS1 virtual machine is down, website will still be online with help of IIS2 virtual machine and vice versa? Same goes for SQL1 and SQL2 virtual machines?
Am I going to the right direction? If this is the case, how do I keep the data synchronized in SQL1 and SQL2, IIS1 and IIS2 virtual machines at the same time, so website will still be up with latest data and code if one VM is down for updates?
An availability set combines two concepts from the Windows Azure PaaS world - upgrade domains and fault domains - that help to make a service more robust. When several VMs are deployed into an availability set the Windows Azure fabric controller will distribute them among several upgrade domains and fault domains.
A fault domain represents a grouping of VMs which have a single point of failure - a convenient (although not precisely accurate) way to think about it is a rack with a single top or rack router. By deploying the VMs into different fault domains the fabric controller ensures that a single failure will not take the entire service offline.
The fabric controller uses upgrade domains to control the manner in which host OS upgrades (i.e., of the underlying physical server) are performed. The fabric controller performs these upgrades one upgrade domain at a time, only moving onto the next upgrade domain when the upgrade of the preceding upgrade domain has completed. Doing this ensures that the service remains available, although with reduced capacity, during a host OS upgrade. These upgrades appear to happen every month or two, and services in which all VMs are deployed into availability sets receive no warning since they are supposedly resilient towards the upgrade. Microsoft does provide warning about upgrades to subscriptions containing VMs deployed outside availability sets.
Furthermore, there is no SLA for services which have VMs deployed outside availability sets.
As regards SQL Server, you may want to look into the use of SQL Server Availability Groups which sit on top of Windows Server Failover Cluster and use synchronous replication of the data. For IIS, you may want to look at the possibility of deploying your application into a PaaS cloud service since that provides significant advantages over deploying it into an IaaS cloud service. You can create a service topology integrating PaaS and IaaS cloud services through the use of a VNET.
Availability set is combination of these two feature
Fault Domain(you have option to select max 3 when creating new Availability Set)
Update Domains (you have option to select max 20 when creating new Availability Set)
Fault Domain is the physical(like rack, power) set lets you selected 2 fault domain in your availability set and your machine in that availability set will have value 1 and 2 so at least one can be available in case of power failure at any physical set.
Update Domain is set which will be updated by azure system update at once.
if select 4 update domains and your 2 VM have value 2,3 that means they will not be updated together for any planed maintenance
For high availability duplicate VM should not be on same Fault Domain or same Update Domain
Now You can not change availability set after creation of a VM it should be set at the time of creation