If it matches (equality), the host updates the block with its own signature to claim ownership.
If the host cannot "set" the lock, it cannot write to the disk.
The host sent a command saying: "I want to lock this block. I expect the current owner ID to be 'X'." The storage array looked at the block, saw that the ID was actually 'Y', and replied: "False. The data is not what you expected." Common Causes If it matches (equality), the host updates the
Why would the equality test fail? Usually, it's one of three scenarios: 1. "Split Brain" or Multi-Host Contention
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. I expect the current owner ID to be 'X'
When the system reports that this operation "returned false for equality," it means the phase failed.
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: "Split Brain" or Multi-Host Contention The "atomic test
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)?
The host checks the current metadata of a disk block to see if it matches what it expects.
Look for spikes in command latency. ATS is very sensitive to timing; if the storage is overloaded, ATS failures will increase.
