Instead of locking an entire LUN, ATS locks a specific disk block or sector. The process follows a strict mathematical logic:
In modern block storage, this is heavily utilized via commands like SCSI (ATS - Atomic Test and Set). The workflow follows a strict conditional pattern:
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When the system reports that the "atomic test and set of disk block returned false for equality," it means: Instead of locking an entire LUN, ATS locks
Here is a deep dive into the mechanics of this error, its root causes, and how to resolve it. The Architecture: SCSI ATS and Clustered File Systems
Drop a comment below with your hardware specs. I’ll bet it was an NVMe drive from 2018.
The error Atomic test and set of disk block returned false for equality indicates an between ESXi hosts and a storage array. While the immediate symptom is a "miscompare" of a disk block's content, the root cause is often a deeper issue within the storage stack, such as buggy drivers, firmware flaws, or transient hardware faults. By methodically working through the diagnostic steps outlined here, system administrators can isolate the failing component and apply the necessary resolution to restore stability to their virtual infrastructure. This link or copies made by others cannot be deleted
: In extreme cases, the host may temporarily lose access to the datastore (APD - All Paths Down or PDL - Permanent Device Loss) if the storage queue becomes completely saturated with failed lock retries. Troubleshooting and Resolution Steps
This offloads the test-and-set logic to the storage array itself, which is far more efficient for cluster-wide locking than traditional SCSI reservations that lock entire LUNs.
A test-and-set operation is an atomic instruction used to protect shared resources. It does two things in a single, uninterrupted step: the current value of a variable or storage block. Writes a new value to it if it matches an expected state. Try again later
Momentary issues on the SAN fabric or network. 4. Troubleshooting and Resolution Steps (2026)
: Virtual machines may briefly freeze or experience "gray screen" states while waiting for disk metadata locks to clear.
Depending on your root cause analysis, apply the following technical remediations to stabilize the storage environment. Implement Storage Queue Regulation
(vStorage APIs for Array Integration) is a set of storage primitives that offload certain storage operations from the ESXi host to the storage array. One of the most critical VAAI primitives is Atomic Test and Set (ATS) . On the wire, an ATS operation is implemented as a SCSI Compare and Write (CAW) command :
A momentary loss of connectivity (fiber channel or iSCSI) to the shared storage.