Azure SQL Managed Instance Next-gen: Severe Slowdowns During Backups

We recently upgraded our Azure SQL Managed Instance from legacy General Purpose to Next-gen General Purpose. Afterward, automatic full backups caused severe application slowdowns. I’m sharing our experience so others can evaluate this before upgrading.

We had used legacy General Purpose for more than three years without SQL performance problems. We never paid attention to when the automatic backups ran because they had no noticeable impact. After upgrading, we experienced three consecutive Thursday afternoon slowdowns during full backups, severe enough that multiple customers asked whether we were having an outage.

The important difference is how storage performance is allocated. On legacy General Purpose, each database file receives IOPS and throughput based on its allocated size. On Next-gen, IOPS are configured at the instance level, and the allocated IOPS also determine throughput.

Our configuration is 4 vCores with 832 GB of reserved storage, primarily supporting one large production database. Its data file is approximately 703 GiB. Under legacy General Purpose, that file falls in the published tier providing 5,000 IOPS and 210 MB/s.

After upgrading, the default Next-gen allocation was 2,496 IOPS, calculated as three IOPS per GB of reserved storage. Using Microsoft’s throughput formula of IOPS divided by 30, that provides approximately 83.2 MB/s shared across the instance.

In other words, we went from an allowance of approximately 210 MB/s for our main data file alone to approximately 83 MB/s for the instance. Our backups typically run near 100 MB/s, so the reduced allowance left no room for application traffic and caused severe slowdowns. Reverting to legacy restored normal application performance.

This will not necessarily affect every workload the same way. Instances with smaller databases may benefit from the different allocation model. Next-gen also allows additional IOPS to be purchased. However, Microsoft’s positioning of the upgrade as having no additional cost needs an important qualification: the upgrade itself may leave the bill unchanged, but maintaining comparable storage performance can require additional spending.

Moreover, purchasing additional Next-gen IOPS alone would not restore our previous throughput allowance. Our 4-vCore Next-gen configuration has a maximum throughput of 145 MB/s, regardless of how many additional IOPS we purchase, compared with approximately 210 MB/s for our main data file on legacy. Reaching comparable throughput would therefore require increasing the vCore count as well. In our case, reverting restored acceptable performance, so we plan to remain on legacy for now.

Before upgrading, compare your existing per-file IOPS and throughput allowances with the proposed Next-gen instance-wide allocation. Include automatic full backups in that assessment, even if ordinary application traffic is light and CPU utilization is low.

The IOPS allocation rules are documented, but we did not find a clear warning that upgrading with the default settings could substantially reduce available throughput for an existing workload and turn previously unobtrusive backups into a major performance event.

If anyone from Microsoft is reading, please make this tradeoff explicit in the upgrade guidance. An upgrade assessment that compares existing file-level allowances with the proposed instance-level limits would help customers avoid discovering this in production.