Understanding the Standard HDD I/O unit size update and what it means for your workloads

As Azure continues to evolve to support modern workload patterns, we are refining how transactions are measured for Standard HDD managed disks. This update helps align billing more closely with actual I/O behavior while maintaining predictable cost controls for customers. 

In this post, we’ll walk through what’s changing, how transaction billing is calculated, and what this may mean for your workloads. 

With this update, the number of billable transactions for select Standard HDD disk sizes (S4, S6, S70, and S80) will be measured using 16 KiB I/O units. 

• I/O operations ≤ 16 KiB count as one billable transaction
• I/O operations > 16 KiB are counted as multiple billable transactions 

Example 

• A 64 KiB read/write on an S4 disk = 4 billable transactions
• A 4 KiB read/write on an S4 disk = 1 billable transaction 

This update provides more consistent alignment between workload behavior and transaction billing. For latest information on regional availability, please refer to the Standard HDD Managed Disks documentation. 

Built-in transaction caps to help manage costs 

To maintain predictable cost boundaries, Azure will implement hourly caps on billable transactions for the Standard HDD disk sizes with an I/O unit size. 

If a disk exceeds its hourly cap, additional transactions in that hour are not billed. The transaction caps are listed below and on the Azure Disks Pricing Page. 

These caps help provide cost predictability, particularly for bursty workloads. 

Additional protection for large-disk high-throughput workloads 

For larger disk sizes (S70 and S80), Azure is also introducing a per-I/O transaction cap. 

Each I/O is still split into 16 KiB units. With the per-I/O transaction cap, the I/O is split into a maximum of 16 billable transactions no matter how large the I/O is. 

Example 

• A 256 KiB read on S70 → billed as 16 transactions
• A 1 MiB write on S70 → also billed as 16 transactions (not 64) 

This safeguard helps ensure that high-throughput sequential workloads avoid disproportionate transaction billing increases. 

Understanding potential cost impact 

Workloads with larger I/O sizes and sustained IOPS may see changes in the number of billed transactions on their invoice. Workloads with small or infrequent I/O operations may see little to no impact. 

Determining your workload’s cost  

The following table lists three different examples to illustrate how transaction billing* compares for different IO load intensities on an S4 Standard HDD disk vs. an E4 Standard SSD disk. Assume the disks are in Central US region. 

*Any operation against the storage is counted as a transaction, including reads, writes, and deletes. Host caching being enabled might affect the number of transactions against the storage that are counted. 

Choosing the right disk for your workload 

Running workloads on SSD-based disk types may also unlock higher availability, higher reliability, more consistent performance, and lower latency.  

• For OS disks running on Standard HDD, consider migrating to Standard SSD or Premium SSD Managed Disks. Standard HDD OS disks will be retired in September 2028. 
• For non-OS disk workloads, consider high performance and flexible provisioning benefits of Premium SSD v2 Managed Disks, or benefits of Standard SSD or Premium SSD Managed Disks. Learn more here about the different disk types and their benefits.
• To learn more about switching your disk types, view our documentation page. 

To learn more about the billing changes, view the Pricing Page and documentation for Standard HDD Managed Disks.