Azure SQL Managed Instance Gen4 and Gen5 hardware choices

Question

Azure SQL Database Managed Instance can be created on two different hardware generations Gen5 and Gen4 with the following differences: https://docs.microsoft.com/en-us/azure/sql-database/sql-database-managed-instance-resource-limits#hardware-generation-characteristics

Are there any guidelines in what scenarios should be choose Gen4 or Gen5?

Answer 1

Gen 5 is better for some workloads while Gen 4 is better for the others. However, in the most cases, the primary choice should be Gen5, unless if bigger core/memory ratio or the difference between physical/logical cores make big difference.

• Gen 5 has network acceleration, so in most cases it should provide better IO bandwidth to remote storage on General Purpose than Gen 4, which might be the most biggest bottleneck in your workload.
• Gen 5 is a newer hardware configuration than Gen 4, hence the Gen5 processors are Intel Haswell instead of Intel Broadwell. However, Gen5 uses hyperthreading and a vCore on Gen 5 is a logical processor - this might make some difference, but you would need to try and test. The vCore is the same price on both HW gens.
• Gen 5 uses faster local SSD disks (fast NVMe SSD) than Gen 4, so in Business Critical case there should be an advantage for Gen 5. In both cases tempdb is placed on local SSD both in General Purpose and Business Critical, so workload that are dependent on tempdb would run faster.
• Gen 4 has bigger memory/core ratio than Gen5 - 7 on Gen4 vs 5.1 on Gen5
• Gen4 has only 8-24 cores range with proportional memory 56-178GB, while Gen5 can go up to 80 cores. Also, new configurations such as SKU will less than 8 cores will probably be available only on Gen5 hardware.

Answer 2

Gen 4 is no longer available for new purchases.

Note that with GEN 5 General Purpose you have to buy 2 cores at a minimum, Gen 4 you could buy 1 core. Price per core has not changed, so your total pricing has doubled.

The same with Business Critical. With Gen 4 the minimum cores are 2, while Gen 5 the minimum cores are 4. Again, this is a doubling of costs. This is particularly shocking if you wanted to go from General Purpose to Business Critical because the core costs are already about double.

The other killer now in Business Critical in Gen 5 hardware is that the max # of databases has STAYED at 50. They double your costs and keep you at 50 databases! There is no reason that Business Critical can't start at 2 cores like it does for Gen 4....