Atomic Test And Set Of Disk Block Returned False For Equality Jun 2026
: ESXi uses ATS (part of the VAAI primitive set) to maintain "liveness" on shared storage. Every few seconds, the host checks its heartbeat slot on the disk and updates it. The Failure
In the world of distributed systems, high-availability clusters, and storage area networks (SANs), data integrity is the highest priority. One of the most cryptic yet significant errors a systems administrator or storage engineer might encounter is:
Check vendor release notes for terms like "COMPARE AND WRITE," "atomic," or "reservation."
The operation failed to "set" the new value because the "test" didn't pass. In short: Someone else already has the keys to that block. : ESXi uses ATS (part of the VAAI
Check if multiple hosts are logging the error for the exact same volume ID (UUID) simultaneously, which points to a contention or fabric issue. Step 2: Correlate with Infrastructure Events
This change must be applied to all hosts sharing the datastore. Also, do not disable ATS heartbeat on hosts that are part of a vSAN cluster, as it can negatively impact the cluster.
Verify that your entire data path is aligned with your hardware vendor's Matrix. One of the most cryptic yet significant errors
Get-AdvancedSetting -Entity VMHost-Name -Name VMFS3.UseATSForHBOnVMFS5 | Set-AdvancedSetting -Value 0 -Confirm:$false
If you are seeing this error frequently, follow this triage path: ESXi host HBAs offline - Broadcom support portal
ATS (Atomic Test and Set) locking mechanisms. It typically indicates that the ESXi host failed to acquire a lock on a datastore because the "test" portion of the atomic operation—which compares the current disk block data to an expected value—returned a mismatch. Broadcom support portal Core Breakdown of the Issue ATS Mechanism Step 2: Correlate with Infrastructure Events This change
If storage saturation is causing the timeouts, throttle the execution queues on your hosts. Lowering the maximum number of concurrent commands allowed per LUN prevents the array from becoming overwhelmed during high-load events. Fall Back to Standard Locking (If Necessary)
If the error is driven by workload density, optimize your storage layout:
I'll search for more details on VAAI ATS. search for "SCSI opcode 0x89 compare and write".'ll open result 0.'ll also search for "test_and_set_bit_lock". have reached the maximum number of steps. I need to formulate an answer based on the information gathered so far. The user wants a long article about the keyword "atomic test and set of disk block returned false for equality". I have collected relevant information, particularly regarding VMware ESXi and ATS miscompare errors. I will structure the article to first explain the foundational concepts of atomic operations and test-and-set, then delve into the specifics of the error message in the context of VMware's VAAI ATS, discuss the root causes, provide troubleshooting steps, and conclude with best practices. I will cite the sources I have found, such as the VMware community threads, KB articles, and Cormac Hogan's blog posts. Now I will write the article. article provides an in-depth look at the "atomic test and set of disk block returned false for equality" error. We will explore what an atomic test and set (ATS) operation is, how it applies to storage systems, and why this specific error occurs, particularly in VMware vSphere environments. By the end, you will understand the technical background and have practical guidance for diagnosing and resolving the issue.
