Skip to main content

Azure VM Availability SLA

This post is focusing on the availability SLA of Azure Virtual Machines. You might think that there is not so much to say about it, even so, the reality is different.
Let me ask you a simple question when you talk about availability and Azure Virtual Machines:
  1. When do you have 99.9% availability?
  2. When do you have 99.95% availability?

Case 1: Single VM
When there is only one instance of VM, the SLA guaranty us minimum 99.9% availability. There are no actions that you can take to improve this value. Remember that this is the minimum value that is guaranteed. This value means that the real value can be higher than 99.9%.

Case 2: Two or more VMs in the same Availability Set
When there are two or more VMs in the same Availability Set, you have the availability SLA to 99.95%. This value means that:
  • The availability of the two or more VMs combined is at least 99.95%. 
  • On the other hand, this does not mean that for each VM the availability is 99.95%. Per node the availability can be lower but combined, you get 99.95%.

Case 3: Single VM inside a Scale Set
When you have only one VM in a Scale Set, the same SLA is offered as for a Single VM – 99.9%. Having a single, VM inside a VM does not offer additional VM availability.

Case 4: Two or more VMs on a Scale Set with at least Two Fault Domains
Having at least two VMs inside at least two fault domains guaranty us an availability SLA at 99.95%.

Things to consider
There is no increased availability offered if you have only one VM as a single VM or inside a Scale Set (99.9%). Inside a Scale Set, if you have two VMs inside the same Fault Domain the SLA for availability remains the same as for one VM (99.9%). It is required to have multiple VMs inside different Fault Domain or inside an Availability Set to get 99.95% availability.


Popular posts from this blog

ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded

Today blog post will be started with the following error when running DB tests on the CI machine:
threw exception: System.InvalidOperationException: The Entity Framework provider type 'System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer' registered in the application config file for the ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded. Make sure that the assembly-qualified name is used and that the assembly is available to the running application. See for more information. at System.Data.Entity.Infrastructure.DependencyResolution.ProviderServicesFactory.GetInstance(String providerTypeName, String providerInvariantName) This error happened only on the Continuous Integration machine. On the devs machines, everything has fine. The classic problem – on my machine it’s working. The CI has the following configuration:

TeamCity.NET 4.51EF 6.0.2VS2013
It seems that there …

Entity Framework (EF) TransactionScope vs Database.BeginTransaction

In today blog post we will talk a little about a new feature that is available on EF6+ related to Transactions.
Until now, when we had to use transaction we used ‘TransactionScope’. It works great and I would say that is something that is now in our blood.
using (var scope = new TransactionScope(TransactionScopeOption.Required)) { using (SqlConnection conn = new SqlConnection("...")) { conn.Open(); SqlCommand sqlCommand = new SqlCommand(); sqlCommand.Connection = conn; sqlCommand.CommandText = ... sqlCommand.ExecuteNonQuery(); ... } scope.Complete(); } Starting with EF6.0 we have a new way to work with transactions. The new approach is based on Database.BeginTransaction(), Database.Rollback(), Database.Commit(). Yes, no more TransactionScope.
In the followi…

GET call of REST API that contains '/'-slash character in the value of a parameter

Let’s assume that we have the following scenario: I have a public HTTP endpoint and I need to post some content using GET command. One of the parameters contains special characters like “\” and “/”. If the endpoint is an ApiController than you may have problems if you encode the parameter using the http encoder.
using (var httpClient = new HttpClient()) { httpClient.BaseAddress = baseUrl; Task<HttpResponseMessage> response = httpClient.GetAsync(string.Format("api/foo/{0}", "qwert/qwerqwer"))); response.Wait(); response.Result.EnsureSuccessStatusCode(); } One possible solution would be to encode the query parameter using UrlTokenEncode method of HttpServerUtility class and GetBytes method ofUTF8. In this way you would get the array of bytes of the parameter and encode them as a url token.
The following code show to you how you could write the encode and decode methods.