@sakuraime Welcome to Microsoft Q&A, Thank you for posting your here!!
For existing SQL Server VMs, you can modify some storage settings in the Azure portal by following the steps here
- As per SQL Server VM performance best practices . Choose premium storage sets the caching to Read Only for the data drive, and None for the log drive
- Maximum data disk size is up to P80 disk type with 32TB. To know more about size/cache recommendation please refer the document here
- For better performance please create the disk stripe by following this document
- Yes, you are correct if you create 3 disks in a storage pool the performance(IOPS/Throughput) would be combined.
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue
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