Measuring egress traffic from Azure VMs to Azure Disk Storage

Question

Hi, everyone!

I am looking to conduct some measurements on Azure. Specifically, I wish to measure the egress network traffic from Azure Virtual Machines to Azure Disk Storage volumes. To achieve this, I have used network monitoring tools like tcpdump and bmon, but they showed no traces of network traffic from my VMs to the attached Azure Disk Storage volumes. Additionally, I tried to use perf (perf record -e probe:inet_sendmsg -aR sleep 60) to trace inet_sendmsg calls, but that didn't yield anything as well.

This is very surprising to me -- I was expecting to see increased network utilization since my understanding says that Azure Disk Storage volumes would be attached to the virtual machines as network block devices (https://en.wikipedia.org/wiki/Network_block_device).

I was wondering if the kind members of the community could demystify some of this for me. I would be very grateful for your input on this. 😄

Answer 1

Hi Chaitanya,

Azure managed disks appear to the VMs as physical disks, which is why you don't see the network traffic. The traffic occurs between the physical host the VM is running on and the storage cluster and is not visible to the VM.

Excerpt from: https://learn.microsoft.com/en-us/azure/virtual-machines/managed-disks-overview

Azure managed disks are block-level storage volumes that are managed by Azure and used with Azure Virtual Machines. Managed disks are like a physical disk in an on-premises server but, virtualized. With managed disks, all you have to do is specify the disk size, the disk type, and provision the disk. Once you provision the disk, Azure handles the rest.

Below are list of platform metrics for VMs, some of which relate to managed disks:

Supported metrics for Microsoft.Compute/virtualMachines

https://learn.microsoft.com/en-us/azure/azure-monitor/reference/supported-metrics/microsoft-compute-virtualmachines-metrics

Monitor Azure virtual machines

https://learn.microsoft.com/en-us/azure/virtual-machines/monitor-vm

Please click Accept Answer if the above was useful.

Thanks.

-TP