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

The error message is a critical alert generated within enterprise storage environments—most notably inside VMware ESXi ( vmkernel.log ) kernels. It signals a failure in Hardware Assisted Locking (VAAI ATS) , meaning that an ESXi host attempted to lock or update a specific disk block on a shared datastore, but the underlying storage array rejected the operation because the expected disk state did not match reality .

This error commonly causes cluster-wide degradation, failing virtual machine operations (such as powering on/off, provisioning, or vMotion), and localized storage lockouts. Understanding why this happens requires looking at how modern hypervisors interact with Storage Area Networks (SANs). 1. The Core Architecture: Understanding VAAI and ATS

If you are seeing this in a log file, I can help you for your storage vendor if you provide the brand of your storage array .

Because it is atomic, no other processor or host can modify the data mid-operation. The Compare-and-Swap (CAS) Logic

: Simultaneous reboots of multiple hosts in a cluster can sometimes trigger this error during datastore mounting. Broadcom support portal Standard Resolutions Performance issues with VM operations The error message is a critical alert generated

If the values match ( True for equality), the host atomically writes its new metadata or lock status to that block.

Imagine a cluster with two nodes both trying to claim ownership of a shared LUN (Logical Unit Number). Each node issues:

Operations like powering a VM on or off, creating a snapshot, vMotion migrations, or cloning vDisks hang indefinitely or fail outright.

Note: This should be used as a temporary troubleshooting step, as disabling ATS can reduce overall metadata performance on highly consolidated clusters. Upgrade Storage Controller Firmware Understanding why this happens requires looking at how

A process issues test-and-set without holding a prior persistent reservation. Result: The storage target rejects the command. Solution: Ensure the initiator has an active, registered reservation key before issuing atomic updates.

If the error is driven by raw contention, reduce the number of virtual machines or active workloads residing on that specific LUN. Distributing high-I/O virtual machines across separate datastores minimizes the frequency of simultaneous metadata modifications. Fallback to Standard SCSI Reservations (As a Last Resort)

Look at the exact timestamp of the error and cross-reference it across all hosts in the cluster.

(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 : Because it is atomic, no other processor or

: If another host has modified that same block, or if extreme latency caused a previous update to be delayed/retried, the "test" part of the command fails because the current disk image differs from the host's in-memory image.

The Hidden Ghost in Your SAN: Understanding "Atomic Test and Set Returned False"

The host reads the current state of a specific disk block.