When creating a managed disk from another disk, does it have the same background copy performance penalty as when creating from a snapshot?

Noah S 121

For creating a disk from a snapshot, Microsoft docs say:

When you create a managed disk from a snapshot, it starts a background copy process. You can attach a disk to a VM while this process is running but you'll experience performance impact.

Is the same true if creating a managed disk using another managed disk as a source? Is it a fully complete clone or is there still a background copy involved?

Luis Arias 7,376 Reputation points

2024-09-17T19:50:48.49+00:00

Hi Noah,

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If the information helped address your question, please Accept the answer.

Luis
deherman-MSFT 37,496 Reputation points Microsoft Employee

2024-09-17T20:36:24.1866667+00:00

@Luis Arias Can you clarify the source for the text that is in italics? I was looking into this but could not find it. Thanks!
Noah S 121 Reputation points

2024-09-18T16:14:02.09+00:00
Hi, Luis. Thanks for the response! However, I'd like to follow up because I'm not seeing the performance I expected from a cloned managed disk in my tests. I performed 2 tests:

Test 1 - Detaching and Attaching the Original Data Disk

Ran a known workload (a build) to get a baseline. On a fresh restart of the VM, it takes about 1 minute.

Detached the data disk

Reattached the data disk

Ran the workload again. It completed in about 1 minute.

Consistently through this test, I saw read speeds of 40 - 90 MBps and Latency of 2 ms reported by the OS. Through Azure metrics, I saw latency of 2.7 ms and a Queue Depth of 3 to 4.

Test 2 - Detaching Original Data Disk, Cloning, and Attaching the Clone

Detached the same data disk

Create a new disk from the data disk. I used the Azure portal to create a new disk in the same region, specifying the original data disk as the source. I kept all properties the same.

Attached the cloned data disk (same LUN)

Ran the workload again. It completed in about 6 minutes.

Metrics also indicated lower performance. Through the OS, I saw read speeds mostly under 5 MBps but up to 30 MBps and latency of 6 - 7 ms. Azure metrics reported latency of 12 ms and Queue Depth of around 11.

If creating a managed disk from an existing disk does not incur a background copy penalty, I would have expected the same performance after attaching such a clone as after attaching the original disk.

Do you have any insight regarding why I'm seeing this behavior? In the FAQs you linked to, I don't see any specific indication of whether disks created from disks use a background copy process, so I'm wondering if that's part of this. Or maybe there is another factor I'm not consider.

Thank you in advance!
deherman-MSFT 37,496 Reputation points Microsoft Employee

2024-09-18T21:06:02.73+00:00

@Noah S I am checking this with our service team to see if we have more information that can be shared. It appears that based on your testing that cloning has a similar performance as creating from snapshot.
Noah S 121 Reputation points

2024-09-18T21:48:52.65+00:00

@deherman-MSFT , if you could get confirmation from the service team, that would be great! I would love to have confirmation from Microsoft of what I'm seeing from tests.

Accepted answer

deherman-MSFT 37,496 Reputation points Microsoft Employee

2024-09-19T15:23:41.1333333+00:00

@Noah S

Thanks for your patience while I confirmed this with our service team.
Any time a disk is created from another source (including being copied from another disk) there is a background copy operation from the source disk to the destination (new) disk. Performance of the target (new) disk will be degraded until the copy is complete.

Hope this helps. Let me know if you have any more questions and I will do my best to assist.

If you still have questions, please let us know in the "comments" and we would be happy to help you. Comment is the fastest way of notifying the experts.

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Noah S 121 Reputation points

2024-09-19T16:58:13.66+00:00

@deherman-MSFT , thank you for confirming! We need to support a process where one VM generates 100 GB+ files and then multiple downstream VMs need to work on their own copies of that data. Speed is key and we have found the performance penalty of snapshots or cloning disks to be too much. Is there an alternative approach you can suggest to quickly making multiple copies of large amounts of data that would not incur the background-copy penalties? If I should start a separate thread for this, please let me know. Thank you.
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When creating a managed disk from another disk, does it have the same background copy performance penalty as when creating from a snapshot?

Test 1 - Detaching and Attaching the Original Data Disk

Test 2 - Detaching Original Data Disk, Cloning, and Attaching the Clone

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