When an ATS equality test returns false, the host typically retries the operation automatically. In a healthy environment, the second or third attempt succeeds within milliseconds. However, if the underlying issue persists, you will observe several symptoms:
If there is intermittent packet loss, high latency, or link flapping on the Fibre Channel (FC) or iSCSI network, the ATS commands may get delayed in transit. A delayed command is highly likely to fail the equality test because the cluster state has moved on by the time the packet arrives at the storage controller. Impact on Infrastructure
Storage controllers run complex microcode. Bugs in the array's implementation of the SCSI COMPARE AND WRITE command can cause the controller to falsely report miscompares or fail to process the atomic lock quickly enough under heavy load. 3. Network Latency and Packet Loss
If you can tell me or application (e.g., MySQL, Kafka, a custom distributed app) is giving this error, I can provide more specific debugging commands. When an ATS equality test returns false, the
| Aspect | Evaluation | |--------|------------| | | Atomic CAS on disk block failed because block ≠ expected value. | | Typical severity | Moderate — part of normal concurrency, but could indicate bug if unexpected. | | Likely fix if unexpected | Re-read block, ensure correct expected value, implement retries. | | Architectural note | True disk-block atomic CAS is rare; many systems emulate via logging or PERSIST barriers. |
If they match ( equality ), the host immediately writes new data to the block in one atomic operation .
The lights in the room didn't just turn off; they ceased to have ever existed. technical breakdown A delayed command is highly likely to fail
: ESXi uses ATS (part of the VAAI primitive set) to maintain "liveness" on shared storage. Every few seconds, the host checks its heartbeat slot on the disk and updates it. The Failure
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:
A node caches disk block values but fails to invalidate the cache after a write from another node. Result: The node issues a test-and-set based on stale data, causing an unexpected failure. Solution: Disable aggressive caching for shared block devices; use O_DIRECT or O_SYNC where appropriate. use O_DIRECT or O_SYNC where appropriate.
An ATS operation can return false for equality due to several underlying physical and logical issues: 1. High I/O Latency and Storage Congestion
| Issue | Description | Review Recommendation | | :--- | :--- | :--- | | | The thread holding the lock is taking too long (e.g., slow I/O, page fault). | Implement exponential backoff in the spin-loop or switch to a blocking semaphore if wait time exceeds a threshold. | | Deadlock | Thread A holds Lock X and waits for Lock Y; Thread B holds Y and returns false on X. | Review lock ordering policies. The false return is a symptom of a cyclical dependency. | | Forgotten Release | A thread acquired the lock but crashed or returned without releasing it. | The TS will return false indefinitely. Implement watchdog timers or recovery mechanisms to reset "stuck" locks. | | Priority Inversion | A high-priority thread spins on false returns, while the low-priority thread holding the lock is preempted and never runs. | Use priority inheritance protocols. |