Download speeds of uncached items when using Front Door in front of Blob Storage very slow even from Azure VMs in same region

Jonathan Lydall 0

I have the following set up:

Azure Blob Storage in South Africa North
Azure Front Door with CDN back by the Blob Storage
Azure managed Windows 11 Virtual Machine in South Africa North

When I download a 120MB (megabyte) file which happens to be uncached by the CDN, it downloads at less than 2MB/s (even from the Windows 11 Azure provisioned VM), when I retry the same download immediately afterwards (i.e. the cache is now hot) it downloads at 10s of MB/s.

The same behaviour happens on my home fibre connection which is also within 100km range of datacenters.

This <2MB/s performance on cold cache when I'm in the same geographical region seems terrible to me, what is the expected speed?

ChaitanyaNaykodi-MSFT 26,101 Reputation points Microsoft Employee

2024-09-25T20:34:04.5366667+00:00

@Jonathan Lydall

Thank you for reaching out.

Can you please confirm if you have enabled caching on you Azure Front Door?

Here is the document on how Azure Front Door handles Delivery of large files If caching is enabled, Front Door uses a technique called object chunking. When a large file is requested, Front Door retrieves smaller pieces of the file from the origin. After Front Door receives a full file request or byte-range file request.From Azure Storage perspective. Can you try downloading the same file using Azcopy and lets us know if you observed the dame latency.

You can also follow the guideline here to help troubleshoot any latency issue.

Thank you!
Jonathan Lydall 0 Reputation points

2024-09-26T08:58:01.6266667+00:00

Can you please confirm if you have enabled caching on you Azure Front Door?

I covered this in my initial post:

When I download a 120MB (megabyte) file which happens to be uncached by the CDN, it downloads at less than 2MB/s (even from the Windows 11 Azure provisioned VM), when I retry the same download immediately afterwards (i.e. the cache is now hot) it downloads at 10s of MB/s.

Also to add, if I download the file directly from Blob storage (just as a normal HTTP(S) download, it downloads at 10s of MB/s.
Jonathan Lydall 0 Reputation points

2024-09-26T10:46:34.1966667+00:00
In answer to your other question, downloading direct from the blob storage HTTP(s) address is super fast, I would expect AzCopy to be the same.

Also worth noting that anyone from Microsoft can test this themselves:

Create a Blob Storage account in South Africa North

Create Front Door link in front of it with caching disabled.

Create a VM using Azure in South Africa North.

Connect to the Azure VM in South Africa North and have it download from the front-door link.

Observe that the download speed is less than 2MB/s.

Change the front-door to have caching enabled.

Re-download from the front-door link.

Observe that the first download is slow.

Re-download from the front-door link.

Observe that the download is now fast.

The problem is that the uncached downloads are slow.
ChaitanyaNaykodi-MSFT 26,101 Reputation points Microsoft Employee

2024-10-01T23:44:20.38+00:00

@Jonathan Lydall

Thank you for getting back and sharing additional details. I think it will be helpful if you could create a support request in this case as access to backend logs will be required. If you do not have a support plan please refer to my private message here. Thank you!

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Download speeds of uncached items when using Front Door in front of Blob Storage very slow even from Azure VMs in same region

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