Atomic Test And Set Of Disk Block Returned False For Equality !!exclusive!! May 2026

At its core, this message indicates a failure in a fundamental synchronization primitive used to prevent data corruption. When this fails, it usually means the system’s "source of truth" regarding who owns a piece of data has been compromised or contested. What is Atomic Test-and-Set (ATS)?

To understand the error, we first have to understand the mechanism. is a hardware-offloaded locking mechanism (often part of the VAAI—vSphere Storage APIs for Array Integration—feature set in VMware environments).

When the system reports that this operation "returned false for equality," it means the phase failed. At its core, this message indicates a failure

The host may mark the storage as "All Paths Down" (APD) or "Permanent Device Loss" (PDL) to protect data integrity.

The host checks the current metadata of a disk block to see if it matches what it expects. To understand the error, we first have to

The "atomic test and set of disk block returned false for equality" error is a protective measure. While it causes disruptive downtime, it exists to prevent the "silent killer" of enterprise computing: By failing the operation when the state doesn't match, the system ensures that two hosts never write to the same block simultaneously, preserving the integrity of your databases and virtual machines.

The VMkernel logs will fill with ATS Miscompare or Status: Op: 0x89 messages. How to Troubleshoot and Fix The host may mark the storage as "All

In clustered environments (like VMware VMFS datastores), hosts use ATS as a "heartbeat" to tell other hosts they are still alive. If the network between the host and the storage has high latency or dropped packets, the update might arrive late or out of sync, causing the "equality" check to fail because the host is working with stale metadata. Impact on Operations When this error occurs, you will typically notice:

The most common cause is that two different hosts are trying to access the same metadata at the exact same time. If Host A updates a block while Host B is still holding onto "old" information about that block, Host B’s next ATS command will fail because the block's state changed behind its back. 2. Storage Array Firmware Incompatibilities