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

Hi Noah,

When you create a managed disk from another managed disk, it does not involve the same background copy process as creating a disk from a snapshot. Instead, It's a fully complete clone of the source disk, meaning there is no performance penalty due to a background copy process.(https://video2.skills-academy.com/en-us/azure/virtual-machines/faq-for-disks?tabs=azure-portal)

This makes the process of creating a managed disk from another managed disk more efficient and less impactful on performance compared to creating a disk from a snapshot.

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

Luis

@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!

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

1. Ran a known workload (a build) to get a baseline. On a fresh restart of the VM, it takes about 1 minute.
2. Detached the data disk
3. Reattached the data disk
4. Ran the workload again. It completed in about 1 minute.
5. 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

1. Detached the same data disk
2. 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.
3. Attached the cloned data disk (same LUN)
4. Ran the workload again. It completed in about 6 minutes.
5. 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!

@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.