which drive has persistent files

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which drive has persistent files

Justin Mierzejewski 20

I see a C: (Temporary Storage) and a D: (Windows) drive on my Cloud Services VM. I need a place to put persistent files for use with my Web App. Which of these drives should I use for that purpose? I have recently seen files deleted from D: (Windows). However, I am concerned about putting anything on C: (Temporary Storage) as typically that data cannot be guaranteed to persist. Thank you!

Justin Mierzejewski 20 Reputation points

2025-05-12T14:24:55.28+00:00

Hello -

Thank you for the helpful response. I use a Startup Task in my Cloud Service to load some drivers required by my app.

https://learn.microsoft.com/en-us/previous-versions/azure/cloud-services/cloud-services-startup-tasks

Would the Managed Disk be available when the Startup Task is run?

Justin

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Justin Mierzejewski 20 Reputation points

2025-05-12T14:24:55.28+00:00

Hello -

Thank you for the helpful response. I use a Startup Task in my Cloud Service to load some drivers required by my app.

https://learn.microsoft.com/en-us/previous-versions/azure/cloud-services/cloud-services-startup-tasks

Would the Managed Disk be available when the Startup Task is run?

Justin

Answer 1

Michael Taylor 60,326

You didn't clarify what specific Azure service you're using. It sounds like you're using Azure App Services. In that case your app is running in a VM/container that is managed by Azure. As such the D: drive is going to get reset whenever the service starts up. The C: is for you to store data while the service is running. Neither can be used to persist data across service restarts.

Static content, like images and whatnot, should be part of your app's deployment. That structure is restored when your service is started. If you need to dynamically save files and have them persist across service runs then you're going to need to use storage. Depends on what you are running and what you need to persist to determine that. At the high end is setting up a storage account. You can then store whatever you need in a blob and your app can read/write as needed. This is the closest to a regular file system as you'll get with AAS. You can mount the storage as a local drive so your app is none the wiser. Refer to this article on how to do that.

If you're running an actual VM then you can add additional drives as data drives to your VM like you could anything else.

Justin Mierzejewski 20 Reputation points

2025-05-12T14:57:17.89+00:00

Hi Mike -

I'm using "Cloud services (extended support)". I need to install a .NET Framework runtime (SAP Crystal Reports) required by the app, and my understanding is that this is the only Azure app service available to me which can achieve that. Can you confirm?

I need a copy of the SAP Crystal Reports runtime available to the Startup Task.

Would a Managed Disk be available at the time the Startup Task is run?

Thank you for the help!

Justin
Michael Taylor 60,326 Reputation points

2025-05-12T15:15:33.11+00:00

Hmm. I've never used the ACS Extended Support platform so I'm not sure about whether you'd have access at startup. I assume, at startup, you'd have access to other azure services so storage would be available as would managed disks.

I don't know how you package your files for ACS but ideally the binaries needed would be part of that so they are available directly. But installing a component every time your instance starts up seems very expensive to me, in terms of resources. Since you'd need to reinstall each time then you could install to the temp disk. As soon as the service starts up next time you'd just reinstall to the temp disk anyway. This would eliminate the need for another resource.

I'm sure you're aware but just to clarify, ACS is deprecated as of March of 2025 and will be unavailable after March of 2027. You will want to start looking into alternatives.
Mounika Reddy Anumandla 6,850 Reputation points Microsoft External Staff Moderator

2025-05-12T15:37:05.1733333+00:00

Hi Justin Mierzejewski,

AFAIK, the Managed Disk will be available during the Startup Task phase, so you can use it to store and install the SAP Crystal Reports runtime, but the disk must be initialized and mounted properly, and you can use a wait loop to ensure that the disk is available before you proceed with your installation. If this is your first time using the disk, you may need to format it or ensure it’s ready for use in the Startup Task.

As mentioned by Michael Taylor, reinstalling large components during startup could be expensive in terms of time and resources, especially if they don't need to be updated frequently.
Also, as of March 31, 2025, cloud Services (extended support) is deprecated and will be fully retired on March 31, 2027. https://learn.microsoft.com/en-us/azure/cloud-services-extended-support/extensionsHope it clarifies!

Let me know if you have any further queries!
Justin Mierzejewski 20 Reputation points

2025-05-12T17:51:43.23+00:00

Thanks Mike - this is all helpful information. I think I'm good for now. I appreciate your help!

Answer 2

Justin Mierzejewski

Your understanding is correct. In your Azure Cloud Services VM environment, neither the C: (Temporary Storage) nor the D: (Windows) drive is persistent:

C: (Temporary Storage): Backed by the physical host's local disk; data is lost during VM reboots, migrations, or updates.
D: (Windows): Used for the OS and role deployments; data is overwritten during reimaging or re-deployment of your Cloud Service.

Option 1: Attach a Persistent Data Disk
Managed disks simplify disk management by handling the storage for you. Managed disks don't require a storage account. You specify the size and type of disk and it's deployed as a highly available resource. Managed disks also offer cost optimization by providing desired performance without the need for over-provisioning, accounting for fluctuating workload patterns, and minimizing unused provisioned capacity.

The OS disk is a VHD stored in Azure Storage, so it persists even when the host machine is down. We also recommend creating one or more data disks, which are persistent VHDs used for application data.
https://learn.microsoft.com/en-us/azure/architecture/reference-architectures/n-tier/linux-vm#disks
https://learn.microsoft.com/en-us/azure/architecture/reference-architectures/n-tier/windows-vm#disks

Option 2: Use Azure Storage Services

Azure Blob Storage: For storing unstructured data like files and logs.

Azure Files: Provides SMB file shares you can mount to the VM.

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which drive has persistent files

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