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I am trying to determine the maximum size I can use for an ephemeral OS disk for a NC4as T4 v3, but it doesn't appear that the information is listed in the specs like it is for other VM sizes.
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Hello, @kkcc !
This is the documentation I would reference:
https://learn.microsoft.com/en-us/azure/virtual-machines/nct4-v3-series
Yes that's where I would expect the documentation to show what I'm looking for... but it doesn't have the same column titled "cache size in GB" that say, the M-series has
@kkcc , ah, I see what you mean. Let me do some more investigation on my end.
Sounds good thank you!
Hello again, @kkcc !
It looks like the Max cached and temp storage throughput IOPS/MBps (cache size in GiB) refers to the Write Accelerator, which is unique to the M SKU VMs:
https://learn.microsoft.com/en-us/azure/virtual-machines/how-to-enable-write-accelerator
Write Accelerator is a disk capability for M-Series Virtual Machines (VMs) on Premium Storage with Azure Managed Disks exclusively. As the name states, the purpose of the functionality is to improve the I/O latency of writes against Azure Premium Storage. Write Accelerator is ideally suited where log file updates are required to persist to disk in a highly performant manner for modern databases.
Increasing OS storage
There are 3 main disk roles in Azure: OS, temp, and data disks:
https://learn.microsoft.com/en-us/azure/virtual-machines/managed-disks-overview#disk-roles
If we want to expand the OS disk (typically 127 GB or smaller for Windows and 30 GB on Linux) for a legacy installation, this can be done in the portal or CLI/PowerShell:
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/expand-os-disk
When you create a new virtual machine (VM) in a resource group by deploying an image from Azure Marketplace, the default OS drive is often 127 GB (some images have smaller OS disk sizes by default). Even though it's possible to add data disks to the VM (the number depends on the SKU you chose) and we recommend installing applications and CPU-intensive workloads on these addendum disks, often, customers need to expand the OS drive to support specific scenarios:
• To support legacy applications that install components on the OS drive.
• To migrate a physical PC or VM from on-premises with a larger OS drive.
Using the portal as an example:
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/expand-os-disk#resize-a-managed-disk-in-the-azure-portal
At this point you'll need to expand the volume within the OS. RDP to your VM and use diskpart.
I appreciate the response but I don't think that answers my question. Let me rephrase it. According to this page, the NCasT4_v3 series supports the ephemeral OS disk: https://learn.microsoft.com/en-us/azure/virtual-machines/nct4-v3-series
Then on the ephemeral OS disk documentation that the previous link links to, it says that the size limit for the OS disk when using the ephemeral OS disk is the "Cache size for the VM size" and has links to various VM sizes.
https://learn.microsoft.com/en-us/azure/virtual-machines/ephemeral-os-disks
However, the NCasT4_v3 series is not one of those links, so I don't know how to find the max size I can use for an ephemeral OS disk for the NC4asT4_v3.
@kkcc , thank you for the clarification (and for being patient). Towards the bottom of the ephemeral OS disk documentation, there is an FAQ with a script that uses Get-AzComputeResourceSku to produce a list of VMs that support ephemeral OS disks (note that I've changed location to southcentralus):
$vmSizes=Get-AzComputeResourceSku | where{$_.ResourceType -eq 'virtualMachines' -and $_.Locations.Contains('southcentralus')}
foreach($vmSize in $vmSizes)
{
foreach($capability in $vmSize.capabilities)
{
if($capability.Name -eq 'EphemeralOSDiskSupported' -and $capability.Value -eq 'true')
{
$vmSize
}
}
}
The resulting list gives a value for capability in MaxResourceVolumeMB that matches the Temp storage (SSD) GiB on the NCasT4-v3-series documentation page. In theory that should be related to the ephemeral OS disk max storage but I'm going to reach out internally to confirm.
Awesome thank you!
Although I do want to clarify one thing... I'm referring to OS cache placement which should still give me a temp disk.
@kkcc Sounds good. Thank you again for being patient while I sort this out!
@kkcc , I've got a quick update. It sounds like there are going to be some changes to VM cache and there should be an official announcement at some point soon. Some documentation on VM cache has been pulled until the announcement goes out. Until then I'm still waiting to get an official answer.
I'm affected by the same issue, and it looks like there's still no source for this information (cache size in GiB) for the various VM Sizes. Any word yet?
Solved it: you can use the script above and grab 'CachedDiskBytes' from capabilities. Divide by (1024^3) to get GB.
Here's a list:
