December 2017 - Technology short and sweet

Creating Unmanaged Disk Snapshots

To create a snapshot using PowerShell you retrieve the storage account, reference the blob and create a snapshot. To ensure consistency shutdown your virtual machine beforehand.

$ctx = New-AzureStorageContext -StorageAccountName $StorageAccountName ` -StorageAccountKey $StorageAccountKey $blob = Get-AzureStorageBlob -Context $ctx -Container $ContainerName ` -Blob $BlobName $snapshot = $blob.ICloudBlob.CreateSnapshot()

You can also create incremental snapshots but it is not available in the PowerShell cmdlets only the Azure Rest API, more info here.

Restore Unmanaged Disk Snapshots

Like I pointed out earlier in the case of unmanaged disks the underlying VHD file is manipulated like any other blob. You can copy the snapshot over its base blob to restore the blob to the snapshot state. This means you can replace the OS disk for a VM without rebuilding it. You can also copy it to another blob to clone your disk.

Since snapshots have the same name as their base blob you can’t retrieve a specific snapshot by its name. The easiest way seems to be retrieving all the snapshots for a blob using the Get-AzureStorageBlob cmdlet and filtering the results to get the specific snapshot you need. For example to retrieve a snapshot by SnapShotTime

$snap = Get-AzureStorageBlob –Context $ctx -Prefix $blobName ` -Container $containerName | Where-Object {$_.ICloudBlob.IsSnapshot ` -and $_.SnapshotTime -eq $snapshottime }

Or to retrieve the latest snapshot.

$lastsnap = (Get-AzureStorageBlob –Context $ctx -Prefix $BlobName ` -Container $ContainerName | Where-Object {$_.ICloudBlob.IsSnapshot ` -and $_.SnapshotTime -ne $null} | ` Sort -Property SnapshotTime -Descending )[0]

To restore a snapshot you copy it to a new blob object or you can overwrite an existing blob. This makes it very easy to rollback a VHD to a snapshot without recreating the VM.

$lastsnap = (Get-AzureStorageBlob –Context $ctx -Prefix $BlobName ` -Container $ContainerName | Where-Object ` {$_.ICloudBlob.IsSnapshot -and $_.SnapshotTime -ne $null} | Sort-Object -Property SnapshotTime -Descending)[0] $snapshotblob = [Microsoft.WindowsAzure.Storage.Blob.CloudBlob] ` $lastsnap.ICloudBlob $blob.ICloudBlob.BreakLease() Start-AzureStorageBlobCopy –Context $ctx -ICloudBlob $snapshotblob ` -DestBlob $blobName -DestContainer $ContainerName Get-AzureStorageBlobCopyState -Blob $blobName -Container $ContainerName ` -Context $ctx -WaitForComplete

Take note if you try to overwrite a VHD that is attached to a VM you will receive an error message indicating there is an existing lease on the VHD. The BreakLease call will remove the lease and the lease will be created again when you start the VM.

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Francois Delport

I am a cloud and devops consultant, technology fan and previously a professional C# developer with a keen interest in system design and architecture. Currently I am involved in projects using Azure, the Microsoft stack and DevOps. I am based in Melbourne, Australia. Email: [email protected]

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In post I’m going to take a look at creating and restoring Azure Virtual Machine snapshots for managed disks. These instructions are for Azure Resource Manager (ARM) virtual machines, for Azure Service Manager (ASM) virtual machines read this post. If you are looking for instructions to create and restore snapshots for unmanaged disks read this post.

Why use snapshots?

Snapshots can be used to duplicate a VM relatively quickly since it is faster than copying a blob. You still have to shutdown the source VM to ensure consistency but creating the snapshot only takes a few seconds. Once you have the snapshot you can use it to create new VMs or copies of the VHD blob while the source VM can be powered on again.

Snapshots can also be used as a backup strategy for VMs although Azure Backups provides better functionality with a lot less effort albeit a bit more expensive since you pay for the recovery vault.

The one advantage of snapshots is the ability to overwrite the OS disk without recreating the VM but only for unmanaged disks at the time of writing.

Managed Disk Snapshots

You can create snapshots in the Azure portal by selecting your managed disk from the disks main menu or the disks section on the VM blade.

Take note of the export button, you can use it to export the managed disk to a VHD blob which can be used to create unmanaged disks.

To create a snapshot using PowerShell call the New-AzureRmSnapshot command along with New-AzureRmSnapshotConfig to configure the snapshot options.

$mandisk = Get-AzureRmDisk -ResourceGroupName $rsgName -DiskName $diskname
$snapshot = New-AzureRmSnapshotConfig -SourceUri $mandisk.Id `
  -CreateOption Copy -Location $azureregion 
New-AzureRmSnapshot -Snapshot $snapshot -SnapshotName $Name `
  -ResourceGroupName $rsgName

Restoring Managed Disk Snapshots

~~At the time of writing you couldn’t overwrite or change the managed OS disk on an existing VM but you can create a new managed disk from a snapshot and then create a new VM from the managed disk.~~

Update: You can now swap a Managed Disk on a VM by replacing the disk Id with another one. This still involves creating another disk from your snapshot and swapping it but at least you don’t have to recreate the VM anymore. Thanks to Ralph Herold for pointing it out to me.

$vm = Get-AzureRmVM -ResourceGroupName osrg -Name vm2 
$disk = Get-AzureRmDisk -ResourceGroupName osrg -Name osbackup 
Set-AzureRmVMOSDisk -VM $vm -ManagedDiskId $disk.Id -Name $disk.Name 
Update-AzureRmVM -ResourceGroupName osrg -VM $vm

I assume you can do the same for data disks using Set-AzureRmVMDataDisk but I didn’t try it yet. Full documentation here.

You can create a new Managed Disk from a snapshot in the Azure Portal or PowerShell. In the Azure Portal select Create New Resource from the main portal menu and search for Managed Disks to create a new Managed Disk. There will be a Source Type dropdown where you can select Snapshot and you will see a list of your snapshots to use as the source for the new Managed Disk.

When the create disk operation is completed select the newly created managed disk and create a new VM from the disk.

If you want to script it in PowerShell the steps are basically:

Retrieve the snapshot.
Create a new disk configuration specifying the snapshot as the source.
Create a new managed disk from the disk configuration and attach it to a new virtual machine.

If this is a data disk you can attach it to an existing VM instead of creating a new VM but OS disks can only be attached to new VMs.

...

$snapshot = Get-AzureRmSnapshot -ResourceGroupName $rsg `
  -SnapshotName $yoursnapshotname 
 
$diskconf = New-AzureRmDiskConfig -AccountType $storagetype `
  -Location   $snapshot.Location -SourceResourceId $snapshot.Id `
  -CreateOption Copy

$disk = New-AzureRmDisk -Disk $diskconf -ResourceGroupName $rsg `
  -DiskName $osDiskName
$vm = Get-AzureRmVM ...
$vm = Set-AzureRmVMOSDisk -VM $vm -ManagedDiskId $disk.Id `
  -CreateOption Attach -Windows

...

Full script can be found here.