We are thinking about moving our virtual machines (Hyper-V VHDs) to Windows Azure but I haven't found much about what kind of fault tolerance that infrastructure provides. When I run VHD in Azure, I've got two questions:

  1. Is my VHD and all the data in it safe? I think that uploaded VHDs use the "Storage" infrastructure so they should be automatically replicated to multiple disks and geographically distributed but should I still make a full-image backup just to be safe? (Note that of course I will be backing up the actual data inside VMs that I care about; I just want to know if there is a chance greater than 0.0000001% that one day I will receive an email from Microsoft telling me that my VM is gone and that I should create or restore it from scratch).

  2. Do I need to worry about other things regarding the availability of my VMs? I mean, when I have an on-premise server I need to worry about the hardware itself, about the host operating system, what would happen if my router failed, if my Hyper-V's C: drive failed etc. Am I right in thinking that with Azure, their infrastructure takes care of all of this?


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    Try "Cost". MY Virtual machines would cost an arm and a leg on azure 24/7 - it is EXPENSIVE for base load. So expensive you can rent the hardware 2-3 times. – TomTom Jun 24 '12 at 17:15
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    They take care of everything including time zone and leap year management ;-) – Andrew Smith Jun 24 '12 at 17:15
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    I think this question would better be addressed to people of Microsoft itself. – Lucas Kauffman Jun 24 '12 at 17:50

Virtual machine vhd's are backed by Windows Azure Storage. To be more precise, page blobs. A blob is triple-replicated within the data center itself, then georeplicated to a neighboring data center (unless you opt out of georeplication). In the US, those pairs are East<-->West and North Central <--> South Central. In Europe, it's Dublin <--> Amsterdam, and in Asia it's Hong Kong <-->Singapore.

Storage SLA (detailed here): 99.9% availability.

Regarding backups: It's trivial to create a copy of a blob. Up until recently, the API only supported blob-copy within the same storage account (ergo same data center). With the Spring 2012 release, the API was updated, and now supports cross-account blob copy (so you can very easily make periodic backups anywhere you want). Announcement and full description here.

I'm not saying you need to back up your vhd's. It's up to you. Given the relatively inexpensive cost of storage (full details here): Assuming a 50GB vhd, that would run you under $3 monthly (or less than $2 if you turned off georeplication). Having a backup somewhere else would just add another few bucks to your monthly storage cost.

Regarding your 2nd question: First let me point out an article that Michael Washam posted, with a detailed overview of Virtual Machines. This article hopefully answers some of your questions. Let me call out a few items.

All hardware and related infrastructure support is taken care of: Front-end load balancer, hardware health, Host OS updates, availability sets (when load-balancing across multiple VMs), etc. Note that, in that article, there's mention of a new Single-instance SLA of 99.9%. So, while you will have a short period of downtime if only running one VM, the Windows Azure fabric will bring your VM back up fairly quickly if, say, the hardware running your Guest VM fails. To avoid single-instance downtime, consider an Availability Set, where you can load-balance an endpoint across multiple Virtual Machines.

  • Thanks. Regarding the blob copy, when I'm "copying" VMs locally I need to use VSS so that it's captured in a consistent state. Will blob copy use VSS behind the scenes? Didn't find a mention of it in the linked article... – Borek Bernard Jun 26 '12 at 8:29
  • Anyone knows the answer? – Borek Bernard Jun 29 '12 at 21:35
  • Not sure what you're asking - I'm not familiar with VSS. – David Makogon Jul 1 '12 at 16:19
  • Virtual machine's VHD is a large file (tens of gigabytes) and there will be a time difference between when you start the copy operation and when it finishes. During this time span, the VM will be running, modifying some bytes of those tens of gigabytes. Volume Shadow Copy ensures that the file, no matter how large, will be copied in a consistent state. So the question is, will Azure make a consistent copy of my VM or not? – Borek Bernard Jul 1 '12 at 16:24
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    @codingoutloud - Glad you're cleaning up behind me! :) I posted that detail back in 2012, when the IaaS team had a blurb on single-instance SLA on their blog. That was eons ago, and you're correct in that there's no single-instance SLA for a VM. – David Makogon May 15 '14 at 20:53

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