Azure VM Storage limitation – using Azure blob storage as backup target


One of the frustrating limitations I’ve come across when using Azure Virtual Machines is the limited disk space amount you can have. This limitation is particularly a hurtle when considering Azure storage as a backup target. The maximum amount of disk space you can have on an Azure VM is 16TB. This limitation stems from 3 issues:

  1. Maximum 1,023 GB per disk. Azure VM disks are stored as Page Blobs in Azure storage accounts. A Page Blob has a maximum of 1TB.
  2. Maximum 16 disks per VM. This is in spite of the fact that the VM OS may support 256 disks per SCSI controller, such as Server 2012 R2.
  3. Maximum 2,040 GB per disk. Although this limitation is superseded by the 1TB page blob limitation, it’s worth noting. This limitation stems from the fact that Azure VMs must use VHD disk format which has a maximum of 2TB/disk, in spite of the fact that the VM operating system may support disks as large as 64TB each, such as Server 2012 R2.

Although we can store large files in Azure as Block Blobs, many backup applications require a VM in the cloud to do WAN acceleration, multi-site deduplication, and similar functions.

AzureMVs

Another annoying feature is the artificial coupling of VM CPU, memory, and disk space resources in Azure VMs. For example, to have a VM with with the maximum allowed 16TB disk space, one must use one of the 5 large VM sizes:

AZStor05

This is the output from this Powershell command:

Get-AzureRoleSize | where { $_.MaxDataDiskCount -gt 15 } | Select InstanceSize,Cores,MemoryInMB,MaxDataDiskCount | Out-GridView

For example, the Basic A4 VM annual cost is about $5,500 compared to about $700 for a Small A1 VM which is all is needed for backup.

A VM with 16TB disk space can be setup from the Azure web interface or via Powershell. I’ve used this script to create and attach 16 disks to an A4 sized VM:

$SubscriptionName = “Visual Studio Premium with MSDN”
$StorageAccountName = “portalvhds1brcsk975g4fq”
Set-AzureSubscription -SubscriptionName $SubscriptionName -CurrentStorageAccountName $StorageAccountName

for ($i=0; $i -lt 16; $i++) {
    $i
    Get-AzureVM -ServiceName “v2012r2-az1” -Name “v2012r2-az1” |
        Add-AzureDataDisk -CreateNew -DiskSizeInGB 1023 -DiskLabel “Disk$i” -LUN $i |
        Update-AzureVM
}

The output looked like:

AZStor06

Back to the Azure VM, I created a disk pool, 1 vDisk, and 1 volume using all the 16 TB space, formatted with 64KB blocks:

AZStor04

The 16 disks before and after setting them up as a single vDisk using Storage Spaces

There may be changes coming down the pipeline that would allow up to 30TB/VM or more. For example, the StorSimple 1100 virtual appliance, which is an Azure VM associated with a StorSimple series 8000 storage array has a maximum capacity of 30TB.

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2 responses

  1. Pingback: Setting up Veeam 8 Cloud Connect on Azure | Sam's Corner

  2. Pingback: Using CloudBerry Drive to make Azure Block Blob Storage available to Azure Virtual Machines | Sam's Corner

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