Atomic Test And Set Of Disk Block Returned False For Equality Jun 2026

Understanding this error requires diving into how shared storage manages file locking, the mechanics of atomic operations, and the exact steps needed to resolve the underlying infrastructure instability. 1. The Anatomy of the Error

Resolving an ATS equality error requires a systematic analysis across the compute, fabric, and storage layers. Step 1: Identify the Affected LUN and Host

To temporarily mitigate a critical array failure, you can disable ATS locking globally on a host (forcing standard SCSI locks), though this should be treated as a temporary diagnostics step:

(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 :

The most definitive fix for persistent ATS errors is ensuring compatibility across the stack. Check your hardware vendor’s compatibility matrix. Update the host’s Host Bus Adapter (HBA) drivers, the multipathing software, and the storage array’s controller firmware to the latest stabilized releases. Vendors frequently patch edge-case race conditions in lock handling. Adjust Tuning Parameters (Timeout Values) Understanding this error requires diving into how shared

To solve this, the T10 standard committee introduced , which VMware implemented as part of vSphere Storage APIs for Array Integration (VAAI) . How ATS Works

In vSphere, this can be done by changing the advanced setting: UserVars.VMFS3HardwareAcceleratedLocking to 0 (Disabled).

What and version are you running (e.g., VMware ESXi 7.0/8.0)?

The message is a critical diagnostic error typically associated with VMware ESXi and storage systems using VAAI (vSphere Storage APIs – Array Integration) . Step 1: Identify the Affected LUN and Host

The "atomic test and set of disk block returned false for equality" log entry is a protective mechanism, not a sign of data corruption. It proves that your storage architecture's locking mechanism is successfully preventing two hosts from overwriting the exact same block simultaneously.

Are you seeing any associated or datastore disconnections ? Share public link

Why would the equality test fail? Usually, it's one of three scenarios: 1. "Split Brain" or Multi-Host Contention

In the world of high-performance virtualization, there are few errors as cryptic—and as disruptive—as the infamous: Check your hardware vendor’s compatibility matrix

Storage controllers handle ATS commands at the hardware level. Bugs in the array's microcode or firmware can cause the controller to misreport block states, drop ATS commands, or incorrectly process the compare phase. 3. Multipathing and Path Failovers

Before ATS was introduced, if a host wanted to modify metadata on a shared storage volume (such as creating a virtual disk or powering on a VM), it had to lock the entire logical unit number (LUN) using SCSI reservation locks. This caused massive performance bottlenecks. ATS introduces a granular approach:

If the LUN is thin-provisioned, verify that there is sufficient free space on the array to accommodate its growth. The "could not find an empty HB slot" error is a strong indicator of this issue. If this is the case, the solution is to add more physical space to the thin-provisioned LUN.