vSAN support for physical sector size greater than 4K with 512 logical block size

Products

• VMware vSAN
• VMware vSphere ESXi

Issue/Introduction

This article provides information on support regarding physical sector size that are greater than 4K.

Overview

As device vendors continue to innovate, to achieve larger capacity support within traditional 2.5 inch and 3.5 inch form factors, physical block sizes have evolved from 512 bytes to 4K and now 8K.  

Devices vendors are traditionally ahead of application vendors. To bridge this transition gap, emulations likes 512 emulation (512e) are used. 
This means 512 logical block size layered over a larger physical sector size. The device vendor firmware, will handle the translation from 512 bytes to 8K using the read-modify-write (R-M-W) model. 
This translation is proprietary to the device vendors. 

Any 512e devices with non-4K physical block size for ESXi can have performance issues as we do not optimize specifically for these devices in any way.

Symptoms:
In the /var/log/vmkernel.log file of the ESXi host, you see entries similar to:

2018-07-25T20:44:37.588Z cpu5:66850)WARNING: ScsiPath: 4395: The Physical block size "8192" reported by the path vmhba3:C0:T1:L0 is not supported.

Note: The preceding log excerpts are only examples. Date, time, block size and path name may vary depending on your environment.

Environment

VMware vSAN 6.x
VMware vSphere ESXi 6.7
VMware vSphere ESXi 6.5

Resolution

This is a known issue affecting VMware ESXi 6.5.x and 6.7.x.

Currently, there is no resolution.

Workaround:
vSAN interacts with devices based on logical block sizes; and leaves the responsibility of the R-M-W translation to the device firmware.

At VMware, we will continuously keep evaluating way of optimizing. Hence, even though, we support 8K physical sectors sizes with 512 logical block sizes, we will always review ways to optimize where possible.

This message is benign and can be safely ignored if the physical sector size is greater than 4K.

Note: Performance experience of such devices may vary as VMware is not optimized specifically for these devices in any way. These potential performance issues could be associated to the read-modify-write algorithm with the device controller; hence VMware recommends you to contact your storage vendor when you encounter device specific performance issues.

Additional Information

Impact/Risks:
Performance impacts:

The R-M-W that happens during translation, may potentially add latency, depending on the efficiency of the device vendor’s firmware algorithm.