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P50 disk and host cache

LHocker 21 Reputation points
2021-05-20T18:49:19.767+00:00

Azure docs obscurely state that host cache is not supported on premium disk 4 TiB or larger. Ref: https://learn.microsoft.com/en-us/azure/virtual-machines/premium-storage-performance#disk-caching & https://learn.microsoft.com/en-us/azure/azure-sql/virtual-machines/windows/performance-guidelines-best-practices-storage#caching. If you have P50 disks provisioned, Azure portal still offers the host cache selector and allows you to pick Read-only or Read/write. Are the docs incorrect or does this somehow silently fail?

Assuming the docs are correct, what is required to workaround this? Can you create a custom sized disk of 4095GiB instead of the default 4096GiB with P50? Or, do you need to back all the way down to a P40?

Also, if the doc is correct the UI should be updated to not give you the option of host cache if it's not actually available.

Azure Disk Storage
Azure Disk Storage
A high-performance, durable block storage designed to be used with Azure Virtual Machines and Azure VMware Solution.
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  1. Pranathi Panyam_MSFT 1,026 Reputation points Microsoft Employee
    2021-05-21T14:19:48.31+00:00

    @LHocker Thanks for posting your query here!

    Yes, host cache is not supported on premium disk 4TB or larger. It does ask you to select the cache, but when you save it, within no time it fails with the error "only disk caching type "none" is supported for disks above 4095GB.

    You can go ahead and create a disk with 4095GB, however, P40 & P50 both support the same IOPS & throughput.

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

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  1. LHocker 21 Reputation points
    2021-05-21T14:37:18.823+00:00

    Thanks @Pranathi Panyam_MSFT ! That definitely helps to clarify.

    Any idea why Azure doesn't just default P50 disks to 4095 instead of 4096? It seems under many workloads, this would be significantly more performant? For context, a customer of mine may need to migrate ~ 1.7PB to get the increased performance with caching because they didn't know about this gotcha.