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Microsoft Azure and Availability Sets for VMs (EN) |
Introduction
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Behind the scenes, Microsoft Azure is actually racks of servers, network
equipment and storage that have virtualization capabilities. As any physical
equipment in the world might face failures and downtimes, it might happen
that VMs hosted in Microsoft Azure become temporary not available. This article
describes how to minimize the impacts of similar events on your services
running on Microsoft Azure VMs by using the Availability Sets.
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What are Availability Sets and how can they improve the availability of
services running on VMs?
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By running
multiple VMs to provide the same service on Microsoft Azure, you are
certainly trying to introduce a high availability and minimize the downtime
of the service. However, Microsoft will have no idea about what your VM is
used for and this may end up in having your VMs running on the same physical
hardware or using a common power source and network switch. In case of a
planned or unplanned maintenance, this may end with your service fully down
while it was possible to avoid it.
By using
Availability Sets for VMs, you are telling Microsoft that a group of VMs are
used to provide the same service for high availability reasons and should not
be down in the same time. That is very important to have as you cannot choose
where your VM resides but Microsoft can and will provide you with a better
pinning when you provide the required information about your environment.
Availability
Sets for VMs is then a feature provided by Microsoft Azure that is used to
regroup VMs providing the same service to minimize the risk of full
interruption when a planned or an unplanned maintenance is done.
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How are Availability Sets working?
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Availability Sets are using the following
domains:
Update Domain (UD): They define physical hardware
that can be rebooted at the same time. Each Availability Set has five (5)
UDs. When you assign a VM to an Availability Set, it will be part of the next
available UD. If you have more than five (5) VMs then the VM number 6 will
take the same UD as the VM number 1, the VM number 7 will take the same UD as
the VM number 2 and so on. Below is a screen capture about how VMs are
assigned to UDs in case you have seven (7) VMs part of the Availability Set
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Let’s suppose that UD1 will be rebooted. You will
then have VM1 and VM6 not available while the other VMs are still up and
running. Your service will continue running with no interruption.
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How can we know the VMs assignment to UDs and FDs?
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It is true UDs and FDs assignments are not
user-configurable. However, Microsoft Azure allows its administrators to get
the details about the current assignments.
This could be done by using the following method:
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Conclusion
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Availability Sets is a Microsoft Azure feature that provides details to
Microsoft about VMs that are providing the same service and should not be
down at the same time. This makes Microsoft do a better assignment of these
VMs to physical hardware, power source sources and network switches using
Update Domains (UDs) and Fault Domains (FDs). The expected output is that,
when a planned or an unplanned maintenance is done, your service will not be
fully down.
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