Disk storage considerations

Azure Function Apps have two basic options for local storage used for downloading and expanding NARs.

  1. Temporary storage, under C:\local\Temp - for Consumption plans, the total local disk size is limited to 500 MB, including this Temp directory, making the actual limit somewhat less than 500 MB. This limit can be increased significantly with App Service and Functions Premium plans (for example, the Basic plan starts at 11 GB). This storage is attached directly to the Function App container, and is therefore much faster than the File Share option, discussed next. This storage also requires no additional configuration, but caution must be taken not to fill up the disk. Mitigations for this are discussed below.

  2. File Share storage, which is mapped to C:\home. When setting up the initial function, you already attached a File share from your Storage account, so this should already be available to you. The File share has a default limit of 5 TB, but is not locally attached to your Function App container, and so can be significantly slower than writing to the Temp directory.

Although Temporary storage is faster, DataFlow Functions has a storage optimization that reuses downloaded and unpacked extension NARs. Therefore, the recommended approach is to use the File share for both the WORKING_DIR and EXTENSIONS_DOWNLOAD_DIR. This will incur a significant time penalty on the very first cold start, but afterwards the downloading and unpacking of NARs can be skipped, greatly reducing the function execution time.

You can use the File share for storage by doing the following:

  1. Navigate to your Storage account and click the File shares blade on the left. Select your file share name on the right.

  2. Click the data directory (or create it if it does not yet exist).

  3. Click the + Add directory button, name it extensions, and click OK. Repeat this for a directory called working.

  4. In your Function App, click the Configuration blade on the left, and add (or set) an Application Setting named EXTENSIONS_DOWNLOAD_DIR with a value of C:\home\data\extensions.

  5. In your Function App, click the Configuration blade on the left, and add (or set) an Application Setting named WORKING_DIR with a value of C:\home\data\working.

  6. Save the configuration.

  7. Optionally (but recommended), perform a single "warming" invocation of your Function App.