The error message says: "returned false for equality."
This means the storage engine performed the atomic operation, but the validation step failed. Specifically:
Current).Current equals Expected (e.g., all zeros).False. They are not equal.So why is this a crisis? Because the system expected to be the only writer, but something else changed the block.
The error “atomic test and set of disk block returned false for equality” is a concurrency control signal, not a disk failure. It tells you that your optimistic lock attempt failed because the disk block’s current value did not match your expected value. By methodically comparing expected vs. actual values, validating cache coherence, and implementing proper retry logic, you can resolve this issue in distributed file systems, lock managers, and custom storage engines.
Remember: atomic operations do not fail silently—they give you clues. Decode them, respect the state on disk, and your system will achieve the consistency it was designed for.
Keywords: atomic test and set, disk block, returned false for equality, compare and swap, distributed lock manager, concurrency control, optimistic locking, split-brain, storage consistency, clustered file system debugging.
If you want, I can produce a short implementation sketch (pseudo-code) for retry + read-after-write verification, or a logging schema for the detailed logs. Which would you prefer?
The "Atomic test and set of disk block returned false for equality" error in VMware vSphere indicates a failure in Hardware Assisted Locking (ATS) due to outdated storage metadata, usually caused by concurrency conflicts or high latency. This failure occurs when an ESXi host attempts to update a storage block that has already been modified by another host, requiring investigation into firmware compatibility or disabling ATS heartbeats. For a detailed technical breakdown of this specific issue, review the discussion at Reddit.
"Atomic test and set of disk block returned false for equality" is a low-level status message typically found in VMware ESXi VMkernel logs It indicates a failure in the Atomic Test and Set (ATS) , which is part of the vStorage APIs for Array Integration (VAAI) Core Concept: What is ATS?
Atomic Test and Set (ATS) is a hardware-assisted locking method used by ESXi to manage metadata updates on shared storage (VMFS datastores). WordPress.com Traditional Method
: Used SCSI reservations to lock an entire LUN (Logical Unit Number), preventing other hosts from accessing it entirely during updates. ATS Method
: Locks only specific disk blocks (sectors) rather than the whole LUN. This allows multiple hosts to perform metadata operations simultaneously on the same LUN, significantly improving performance and scalability. Hitachi Vantara Community Meaning of the "False for Equality" Error
The error occurs when the ESXi host attempts to update a block but finds that the existing data on that block does not match what it expected (the "test" part of "test and set" failed). This typically signifies a lock contention mismatch in state between the host and the storage array. Broadcom support portal Common Causes Performance issues with VM operations
T##:##Z cpu2:#######)ScsiDeviceIO: 4167: Cmd(0x45d90f0d4e48) 0x89, CmdSN 0x2163b3 from world 2101333 to dev "naa..################ Broadcom support portal
vSphere connection to datastore error Atomic test : r/vmware
The message "Atomic test and set of disk block returned false for equality" is a critical diagnostic error typically associated with VMware ESXi and storage systems using VAAI (vSphere Storage APIs – Array Integration).
It indicates a failure in the Atomic Test and Set (ATS) locking mechanism, which is a hardware-assisted method used to lock specific disk sectors (rather than the entire LUN) during metadata updates. Meaning of the Error
The "Equality" Failure: ATS works by comparing the current state of a disk block to an "expected" value. If the values match, the operation proceeds (equality is true). This error means the comparison failed because the disk block's actual data did not match what the host expected, suggesting another host modified it first or there is a communication desync.
Locking Conflict: It often occurs in clustered environments where multiple hosts share the same datastore. A "false for equality" result means the host could not acquire a lock on the metadata because another entity had already updated or locked it.
Storage Latency: High I/O latency or intermittent connectivity issues can cause these "heartbeat" failures, leading to the host losing access to the volume. Common Symptoms
Datastore Disconnects: Hosts may lose access to shared storage or report it as "offline".
VM Freezes: Virtual machines may become unresponsive or report "Invalid" status if the .vmx file lock is lost.
Log Events: Frequent LUN reset or ATS failure messages appearing in the vmkernel.log. Potential Resolutions
Check Firmware: Ensure storage array firmware and ESXi drivers are up to date and compatible.
Address Latency: Investigate network congestion or storage controller overutilization that might cause ATS timeouts. Debugging "Atomic Test and Set of Disk Block
Disable ATS Heartbeat (Workaround): In some cases, vendors (like NetApp or Pure Storage) recommend disabling ATS for heartbeating if the storage array does not support it correctly under specific conditions.
If you are seeing this in a log file, I can help you find the specific VMware KB article for your storage vendor if you provide the brand of your storage array.
The error message "Atomic test and set of disk block returned false for equality" typically indicates a locking failure within VMware ESXi environments using VMFS (Virtual Machine File System).
This occurs during an Atomic Test and Set (ATS) operation, a hardware-accelerated locking primitive where a host attempts to claim or update metadata on a shared storage array. When the "test" (checking if the block's current value matches what the host expects) fails—returning false for equality—it means another host likely changed that block since it was last read, causing a miscompare. Feature Overview: VAAI Atomic Test and Set (ATS)
ATS is part of the vStorage APIs for Array Integration (VAAI), designed to replace traditional, inefficient SCSI reservations.
Primary Function: It provides Hardware-Assisted Locking, allowing a host to lock only specific disk sectors/metadata blocks rather than the entire LUN. Mechanism:
Test: The host reads a block and prepares a "compare" value.
Set: It issues a command to the storage array to update the block only if the current value still matches the "compare" value.
Atomic Nature: The array performs this check and write as a single, indivisible operation.
Benefit: Greatly improves performance in clusters by allowing parallel metadata access, which is critical during "boot storms" or simultaneous VM provisioning. Why the Feature Fails ("False for Equality") The failure usually stems from one of three areas:
Concurrency Contention: Too many hosts are trying to update the same metadata simultaneously (e.g., heavy VM power-on/off cycles), leading to frequent retries and miscompares.
Storage Latency: High I/O latency or "deteriorated performance" on the storage array can cause the ATS heartbeat to time out or mismatch.
Configuration Mismatch: Attempting to extend an "ATS-only" datastore with a non-ATS LUN, or issues with ATS Heartbeats on certain storage firmware. Troubleshooting & Resolution
If you are seeing this error in your logs, consider these steps from industry guides:
Verify Storage Compatibility: Ensure your storage array fully supports VAAI ATS.
Check Performance Logs: Look for ScsiDeviceIO warnings in the VMkernel log that indicate high latency (e.g., jumps from 3ms to 300ms).
Adjust Heartbeat Settings: In some cases, disabling ATS heartbeats (while keeping ATS for metadata) can resolve connectivity drops caused by array timeouts.
Re-mount Datastore: For persistent mount failures, some admins found success by removing and re-adding the datastore via the esxcli command line.
Are you experiencing this error during a specific operation like a VM power-on, or is it happening randomly across the cluster? Performance issues with VM operations
This error message typically appears in VMware ESXi logs (such as vmkernel.log) and indicates a failure in the Atomic Test and Set (ATS) locking mechanism, which is part of the vSphere Storage APIs for Array Integration (VAAI). What it Means
When a host wants to lock a metadata block on a shared datastore, it sends an ATS command (specifically the SCSI COMPARE AND WRITE command) to the storage array.
The "Test": The host provides the data it expects to find in that disk block.
The "Equality": The storage array compares the actual data on the disk with the host's provided data.
The "False" Result: If the data on the disk does not match what the host expected, the equality check returns false (a "miscompare"). Read: The system read the current value of
Because the comparison failed, the storage array refuses to perform the "Set" (write) operation. This is a safety mechanism to prevent data corruption when multiple hosts are competing for the same resource. Common Causes
High Latency: Extreme I/O latency can cause a host to receive outdated information about a block before it tries to lock it, leading to a mismatch when the actual ATS command arrives.
Concurrency Conflicts: If another host successfully updated the block metadata just milliseconds before, the original host's "expected" data is now stale, triggering the miscompare.
Storage Array Issues: Firmware bugs or lack of proper VAAI support on the storage array can cause it to handle ATS commands incorrectly.
Multipathing/Driver Errors: Issues with the HBA (Host Bus Adapter) or the multipathing driver can disrupt the "handshake" between the host and the storage. Troubleshooting Steps
Check Latency: Review your storage performance metrics for spikes in latency that coincide with these log entries.
Verify Compatibility: Ensure your storage array firmware and ESXi drivers are on the VMware Compatibility Guide.
Disable ATS Heartbeat: If you are seeing "Lost access to datastore" messages alongside this error, VMware often recommends disabling ATS for heartbeating (switching back to legacy SCSI reservations) as a workaround on affected arrays.
Update Firmware: Check for known ATS-related bugs in your storage array's firmware version, as some vendors have specific patches for "false ATS miscompares". ESXi host HBAs offline - Broadcom support portal
In the neon-soaked subterranean level of the Sector 7 Data Farm, Elias was the "Janitor"—a title that belied his role as the last line of defense against bit-rot and data corruption. He spent his nights watching the heartbeat of the world’s financial ledger, a rhythmic pulse of green lights. Then, the pulse skipped.
On Terminal 42, a single line of crimson text bled across the screen:
CRITICAL: ATOMIC TEST AND SET OF DISK BLOCK RETURNED FALSE FOR EQUALITY.
Elias froze. An "Atomic Test and Set" was the digital equivalent of a handshake in a dark room. The system checks the data (the Test) and, if it’s what it expects, locks it down and changes it (the Set). It has to happen in one breath, one "atom" of time, so nothing else can sneak in.
"False for equality" meant the handshake had failed. Elias had reached out to grab a specific hand, but found a claw instead.
He bypassed the software layers, diving straight into the raw hex code of the disk block. He expected to see a stray bit flipped by a cosmic ray or a failing magnetic platter. Instead, he saw something impossible. The data in Block 0x4F3 was changing while he looked at it.
It wasn't a hardware failure; it was a ghost. Every time the system checked the value to verify it, the value morphed into something else—a sequence of prime numbers, then a string of coordinates, then a snippet of a nursery rhyme in a language that hadn't been spoken for a thousand years.
The hardware was fine. The "Equality" check failed because the data was alive, and it didn't want to be set.
Elias reached for the physical kill-switch, but the terminal flickered one last message before the screen went black:
TEST FAILED. SUBJECT ELIAS DETECTED. SETTING EQUALITY TO ZERO.
The lights in the room didn't just turn off; they ceased to have ever existed. technical breakdown
of how this error happens in real systems, or should we continue this sci-fi horror
Understanding the "Atomic Test-and-Set of Disk Block Returned False for Equality" Error
In the world of distributed systems, high-availability clusters, and storage area networks (SANs), data integrity is the highest priority. One of the most cryptic yet significant errors a systems administrator or storage engineer might encounter is: "atomic test and set of disk block returned false for equality."
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)? So why is this a crisis
To understand the error, we first have to understand the mechanism. Atomic Test-and-Set is a hardware-offloaded locking mechanism (often part of the VAAI—vSphere Storage APIs for Array Integration—feature set in VMware environments).
In traditional storage, locking a file required "SCSI Reservations," which locked an entire LUN (Logical Unit Number). This was inefficient. ATS allows for discrete locking. Instead of locking the whole "parking lot," the system only locks a "single parking space" (a specific disk block). The process works like this:
Test: The host checks the current metadata of a disk block to see if it matches what it expects.
Set: If it matches (equality), the host updates the block with its own signature to claim ownership.
Atomic: This happens in a single, uninterruptible operation. Decoding the Error: "Returned False for Equality"
When the system reports that this operation "returned false for equality," it means the Test phase failed.
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
Why would the equality test fail? Usually, it's one of three scenarios: 1. "Split Brain" or Multi-Host Contention
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
Not all storage arrays implement VAAI/ATS the same way. If there is a bug in the array's microcode or if the host's driver is sending a malformed request, the array might reject the ATS heartbeat, leading to "false for equality" errors even if no real contention exists. 3. Network Latency and Heartbeating Issues
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:
Virtual Machines freezing: If the host cannot "set" the lock, it cannot write to the disk.
Datastore disconnects: The host may mark the storage as "All Paths Down" (APD) or "Permanent Device Loss" (PDL) to protect data integrity.
Log Spam: The VMkernel logs will fill with ATS Miscompare or Status: Op: 0x89 messages. How to Troubleshoot and Fix
Check Firmware and Drivers: Ensure your HBA (Host Bus Adapter) drivers and the storage array firmware are on the vendor's "Compatibility Matrix."
Review Storage Latency: Look for spikes in command latency. ATS is very sensitive to timing; if the storage is overloaded, ATS failures will increase.
Disable ATS Heartbeating (Last Resort): In some specific storage environments (notably certain older NAS or SAN setups), the ATS heartbeating mechanism is too aggressive. VMware allows you to revert to traditional SCSI reservations for heartbeating while keeping ATS for other tasks, though this should only be done under the guidance of support.
Verify VAAI Support: Use command-line tools (like esxcli storage core device vaai status get) to ensure the array is actually reporting ATS as "supported." Conclusion
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: data corruption. 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.
Title: The Silent Witness: On the Philosophy of Atomic Test-and-Set and the Refutation of Sameness
In the intricate architecture of modern computing, few instructions carry as much weight—both literal and metaphorical—as the atomic test-and-set. It is the gatekeeper of concurrency, the arbiter of resources, and the sentinel that ensures the chaotic potential of parallel execution resolves into orderly sequence. Yet, our attention is often fixated on the "success" of this operation—the moment the lock is acquired, and the critical section is entered. We rarely pause to consider the deeper implications of its failure: the moment the test-and-set returns false for equality.
When the disk block reports that the atomic test-and-set has returned false, it is not merely a technical error or a transient state. It is a profound philosophical statement about the nature of reality, time, and the impossibility of true sameness in a dynamic system.
Some advanced storage controllers support atomic operations directly on hardware sectors.
false.Scenario: 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.
The Test-and-Set instruction is defined by the following atomic (indivisible) sequence:
0 or UNLOCKED).1 or LOCKED) and return True (Success).False (Failure).