Why does "FileSystemUsage" metric for "Microsoft.Web/sites" always return zero?

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While answering Retrieve quota for Microsoft Azure App Service Storage, I stumbled upon the FileSystemUsage metric for Microsoft.Web/sites resource type. As per the documentation, this metric should return Percentage of filesystem quota consumed by the app..

However when I execute Metrics - List REST API operation (and also in the Metrics blade in Azure Portal) for my web app, the value is always returned as zero. I checked it against a number of web apps in my Azure Subscriptions and for all of them the result was zero. I am curious to know the reason for that.

In contrast, if I execute App Service Plans - List Usages REST API operation, it returns me the correct value. For example, if my App Service Plan is S2, I get following response back:

{
  "unit": "Bytes",
  "nextResetTime": "9999-12-31T23:59:59.9999999Z",
  "currentValue": 815899648,
  "limit": 536870912000,//500 GB (50 GB/instance x max 10 instances)
  "name": {
    "value": "FileSystemStorage",
    "localizedValue": "File System Storage"
  }
},

Did I misunderstand FileSystemUsage for Web Apps? Would appreciate if someone can explain the purpose of this metric? If it is indeed what is documented, then why the API is returning zero value?

azure-monitoring azure-web-app-service metrics azure
1 Answers

This should be the default behavior, please check this doc Understand metrics:

Note

File System Usage is a new metric being rolled out globally, no data is expected unless your app is hosted in an App Service Environment.

So currently this metric File System Usage should only be working on ASE.

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