VM OS disk is "sdb" sometimes ?!

Question

Hi,

When I create a Virtual Machine Linux and attach a data disk, using cli, OS disk might be "sdb" sometimes (and data disk is "sda").
Why is that happened ? I didn't fin any documentation on that. And I haven't seen this with another cloud provider.

I tried to add a sleep of 30 seconds before adding data disk, but result is the same, sometimes OS disk is "sdb".

Here are commands used:

`
az vm create -g testrg -n test --nics test-nic \
 --public-ip-sku Standard --image almalinux:almalinux:8_4-gen2:8.4.20210729 --size Standard_D2s_v5 \ 
--storage-sku Standard_LRS --os-disk-size-gb 64 --os-disk-name test --availability-set test
az vm disk attach -g testrg -n data --vm-name test --sku Standard_LRS --size-gb 128 --new

Thank you for your help.

Have a good day

Answer 1

The term sd stands for SCSI disk. In most cases the OS disk is /dev/sda and in some odd cases it can be /dev/sdb also. This is not controlled by any Azure specific thing. This is a default Linux behavior. The ordering in which Linux enumerates SCSI devices is nondeterministic. It scans SCSI devices in the order they appear on the PCI Bus. Therefore, whichever disk is enumerated first is /dev/sda

In a nut shell, The designation /dev/sda and /dev/sdb can be arbitrary as you can see in this documentation: Persistent_block_device_naming

If the Answer is helpful, please click "Accept Answer" and upvote it

Answer 2

I understand your concern. To address the issue, Microsoft has published an article here: https://learn.microsoft.com/en-us/troubleshoot/azure/virtual-machines/troubleshoot-device-names-problems
Hope, this can help you !

Answer 3

I'm not expert in this part, but I'm still very surprised about this behavior in Azure Cloud. It never happened for me in another other clouds with linux VM.

Any ideas what I could do to avoid this ? Is there specific type of disk or VM ?

It not accept

Answer 4

Thank you for the answer.
However, It not acceptable since deployment of tools are based on the fact that OS disk will be sda.

I'm not expert in this part, but I'm still very surprised about this behavior in Azure Cloud. It never happened for me in another other clouds with linux VM.

Any ideas what I could do to avoid this ? Is there specific type of disk or VM ?