April 12, 2018 at 1:22 am
Hi,
My knowledge of Azure is limited, so I am hoping you could shed some light on the matter.
I would like to understand High availability and Disaster Recovery when hosting our SQL Server in Azure. As per SLA, Azure states a 99.5% uptime, which is great.
But what about HA and DR? What if there is a disaster at the data centre? Or corruption of 1/more databases?
I am trying to understand the need for AlwaysOn when hosting at Azure? AlwaysOn is great with on-premise sites for HA and DR - which is the area I have experience with.
But with a hosted platform, where does AO come in, if they can promise a 99% uptime?
April 12, 2018 at 4:31 am
I won't be able to give you a total run down on everything, but let's cover a few of the topics you raise. We'll start just at the data center.
Each database (Azure SQL Database) or instance (Azure Managed Instance) doesn't have one copy in the data center. It has three. These are maintained internally and you'll never see them. Each database (we'll just focus on the one so it's easier to explain) is on three different physical machines within a data center. This is the first aspect of the HA/DR for the system. Not to mention I suppose, the data center itself, located near a power plant, in stable regions outside flood zones with all sorts of protections and secondary power sources and site security. That's also HA/DR. Anyway back to the database. All the databases have backups running. All the databases have consistency checks. It's all managed behind the scenes. If your primary database gets a consistency error, the system automatically fails to one of the two secondaries. All this is managed, so you have, within a data center, HA/DR as part of the product. No added cost or added overhead to you.
Next question is, what happens when a meteor hits a data center. Two things. If you want DR, it is possible to keep copies of your database in redundant storage so you can restore between data centers. You'll be down while the restore happens, but this is part of the system. If you want HA, it's on you to set up, but there is Geo-Replication. This allows you, similar to Availability Groups, to set up secondary copies of your database in other data centers. You just have to pay for that part. It's not a fundamental aspect of the HA/DR setup of Azure.
That's the basics. Hope it's helpful.
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April 12, 2018 at 6:07 am
Thank you for the great info!
Much appreciated
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