unclear why you are allowed to chose a P50 performance tier for a P40 disk when the published specs for a P40 are identical to a P50

@vipullag-MSFT Thanks for raising this question! You can correct that as P40 and P50 performance are identical, there are no current benefits for customers to up tier today. However, we don’t want to block the tier upgrade functionality as we can potentially increase our performance target on larger disk sizes.
Let me provide more information on the performance between P40 and P50.

To provide flexibility for customers to provision an appropriate disk size which matches their workloads, we introduce two disk sizes in P40 (2TB) and P50 (4TB) for both Managed and unmanaged Premium Disks; S40 (2TB) and S50 (4TB) for Standard Managed Disks. Customers can also provision the maximum disk size of 4,095 GB for Standard unmanaged disks.

Larger Premium Disks P40 and P50 will support your IO intensive workload, consequently, offers higher provisioned disk performance. The maximum Premium Disk IOPS and bandwidth is increased to 7,500 IOPS and 250 MBps respectively. Standard Disks, of all sizes, will offer up to 500 IOPS and 60 MBps.

The maximum IOPS limits per VM and per disk are different and independent of each other. Make sure that the application is driving IOPS within the limits of the VM as well as the premium disks attached to it. In other words, VM throughput limit supersedes the aggregated Disks throughput limit. Otherwise, the application performance will experience throttling. For example, a P50 disk provisions 250 MB per second disk throughput. Each high scale VM size also has a specific throughput limit that it can sustain. For example, Standard GS5 VM has a maximum throughput of 2,000 MB per second. If you attach 10 x P50 disk to GS5 VM, you will get throttled at 2,000 MB sec.

“When you attach a premium storage disk to your high scale VM, Azure provisions for you a guaranteed number of IOPS as per the disk specification. For example, a P50 disk provisions 7500 IOPS. Each high scale VM size also has a specific IOPS limit that it can sustain. For example, a Standard GS5 VM has 80,000 IOPS limit.”

Metrics that help diagnose disk IO capping:

• Data Disk IOPS Consumed Percentage - the percentage calculated by the data disk IOPS completed over the provisioned data disk IOPS. If this amount is at 100%, your application running will be IO capped from your data disk's IOPS limit.
• Data Disk Bandwidth Consumed Percentage - the percentage calculated by the data disk throughput completed over the provisioned data disk throughput. If this amount is at 100%, your application running will be IO capped from your data disk's bandwidth limit.
• OS Disk IOPS Consumed Percentage - the percentage calculated by the OS disk IOPS completed over the provisioned OS disk IOPS. If this amount is at 100%, you'll your application running will be IO capped from your OS disk's IOPS limit.
• OS Disk Bandwidth Consumed Percentage - the percentage calculated by the OS disk throughput completed over the provisioned OS disk throughput. If this amount is at 100%, your application running will be IO capped from your OS disk's bandwidth limit.

Metrics that help diagnose VM IO capping:

• VM Cached IOPS Consumed Percentage - the percentage calculated by the total IOPS completed over the max cached virtual machine IOPS limit. If this amount is at 100%, your application running will be IO capped from your VM's cached IOPS limit.
• VM Cached Bandwidth Consumed Percentage - the percentage calculated by the total disk throughput completed over the max cached virtual machine throughput. If this amount is at 100%, your application running will be IO capped from your VM's cached bandwidth limit.
• VM uncached IOPS Consumed Percentage - the percentage calculated by the total IOPS on a virtual machine completed over the max uncached virtual machine IOPS limit. If this amount is at 100%, your application running will be IO capped from your VM's uncached IOPS limit.
• VM Uncached Bandwidth Consumed Percentage - the percentage calculated by the total disk throughput on a virtual machine completed over the max provisioned virtual machine throughput. If this amount is at 100%, your application running will be IO capped from your VM's uncached bandwidth limit.

You can also find new documentation explaining how VM + Disk +Caching can affect applications performance:

https://video2.skills-academy.com/en-us/azure/virtual-machines/linux/disk-performance-linux

Hope this helps!

Kindly let us know if the above helps or you need further assistance on this issue.

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