Many "false for equality" errors stem from firmware bugs in:
Check vendor release notes for terms like "COMPARE AND WRITE," "atomic," or "reservation."
This is the nightmare scenario. Your drive says it wrote the data, but it didn't.
When the OS asks, "Is this zero?" the drive lies and says "Yes" (because it forgot it wrote something else). Then the atomic compare fails.
List all nodes connected to the same LUN:
sg_inq /dev/sdX
sg_persist -i /dev/sdX # Show all registered initiators
Check kernel logs (dmesg), system logs (/var/log/messages), and application logs:
grep -i "atomic test and set" /var/log/messages
dmesg | grep -i "compare.*write\|reservation"
journalctl -xe | grep "false for equality"
Here is where the debugging gets tricky. This error rarely means "a bug in the code." It usually means a hardware or configuration mismatch.
When we look deeply at an atomic test-and-set returning false for equality on a disk block, we are seeing a mechanism of humility. It is a safeguard against arrogance. Without this failure, systems would overwrite one another, data would corrupt, and the "truth" of the disk would be a palimpsest of conflicting intentions.
The "false" is a notification that the universe does not exist in the state we imagined it to be. It forces the software to pause, to re-evaluate, and to try again. It teaches the machine that reality is a shared resource, that time flows differently for different observers, and that access is not ownership.
In the end, the "false" returned is not a denial of service, but a promise of integrity. It ensures that when a change finally does occur—when the test returns "true"—it is valid, it is exclusive, and it is real. The false for equality is the price we pay for a consistent world, a digital sentinel standing guard against the entropy of simultaneous desire.
The Hidden Ghost in Your SAN: Understanding "Atomic Test and Set Returned False"
In the world of high-performance virtualization, there are few errors as cryptic—and as disruptive—as the infamous:
Atomic test and set of disk block returned false for equality.
If you've spotted this in your VMkernel logs, you aren't just looking at a minor storage hiccup. You've encountered an ATS Miscompare
, a technical standoff between your ESXi host and your storage array that can lead to datastore disconnects, VM crashes, and major performance degradations. What is Atomic Test and Set (ATS)?
Before we dive into the failure, we have to understand the success. Atomic Test and Set (ATS)
is a VAAI (vSphere Storage APIs for Array Integration) primitive.
Think of it as a "surgical lock." In older systems, if a host wanted to update a piece of metadata on a shared datastore, it had to lock the entire LUN (SCSI Reservation), preventing every other host from talking to that storage. ATS changed this by allowing a host to lock only a specific disk block
The "Atomic" part means the operation happens in one indivisible step:
: The host checks if the block on the disk still matches what it has in memory. : If they match, it immediately writes the new data. Why Does it Return "False for Equality"?
The error occurs when the "Test" phase fails. The host says, "I expect this block to look like ," but the storage array replies, "Actually, it looks like . I'm not letting you write". This is an ATS Miscompare There are three main culprits behind this mismatch: The "Slow Motion" Race Condition
: Under extreme latency, an ATS command might time out. The host assumes the write failed and tries again using the old "test" image. However, if the first write actually made it to the disk just before being aborted, the second attempt will fail because the disk has already changed. High Concurrency Overload
: Performing too many metadata-heavy operations at once—like powering on 50 VMs simultaneously or deploying a massive template—can overwhelm the storage array’s ability to track these surgical locks. Multipathing & Firmware Bugs
: Incompatibilities between specific storage array firmwares and ESXi (particularly ESXi 6.5 and VMFS6) have historically triggered these errors. The Impact: Why You Should Care
This isn't just a log entry you can ignore. When an ATS miscompare happens, the ESXi host often loses trust in its connection to the storage. This can trigger: SCSI Resets
: The host may issue a full reset on the LUN to "clear the air," which aborts all active I/O for every VM on that datastore. Degraded Path Redundancy
: You may see vCenter alarms stating "Path redundancy to storage degraded". Host Hangs : In severe cases, the
service can enter a degraded state, making the host unresponsive to management commands. How to Fix It
If you are seeing this error frequently, follow this triage path: ESXi host HBAs offline - Broadcom support portal
The error message "atomic test and set of disk block returned false for equality" is a critical VMware ESXi kernel error related to VAAI (vSphere Storage APIs Array Integration)
ATS (Atomic Test and Set) locking mechanisms. It typically indicates that the ESXi host failed to acquire a lock on a datastore because the "test" portion of the atomic operation—which compares the current disk block data to an expected value—returned a mismatch. Broadcom support portal Core Breakdown of the Issue ATS Mechanism
: ATS is used as a hardware-accelerated locking mechanism to replace traditional SCSI reservations. It allows a host to lock a single block rather than an entire LUN. The Error Meaning
: The host sent a request to write a new lock value if the current value matched what it expected. The storage array responded that the values did
match, effectively denying the host access to that disk block. Resulting Symptoms Datastores becoming inaccessible or "grayed out" in vCenter.
Virtual machine operations (powering on, snapshots, migrations) failing or hanging.
Hosts losing "scratch" partition configurations or taking an unusually long time to boot. Broadcom support portal Common Causes Communication & Latency
: High I/O latency or intermittent path failures can cause the "test" value to become stale before the "set" command is completed. Inconsistent Metadata
: VMFS inode inconsistency or corruption in the catalog directory, specifically related to file mechanisms, can prevent successful ATS locks. Driver/Firmware Bugs : Outdated or buggy HBA (Host Bus Adapter)
drivers, such as those for Emulex or Fibre Channel cards, often trigger these synchronization failures. Configuration Mismatches
: Misconfigured LUN numbers (e.g., using LUN 0 when the host group expects a different ID) or storage arrays not fully supporting VAAI specifications. Broadcom support portal Recommended Resolutions Reboot the Affected Host Many "false for equality" errors stem from firmware bugs in:
: This is often the first step to clear stale locks and restore temporary connectivity to the datastore. Validate Data Path Compatibility
: Ensure that HBAs, firmware, and storage array versions are all on the VMware Compatibility Guide Update HBA Drivers
: Verify if there is a known buggy driver for your hardware (common in versions like ESXi 6.7u3) and apply recommended patches. Consult Storage Vendor
: If the error persists, the storage array may be misreporting its state or requiring a specific ATS configuration. Engage Broadcom Support : For severe cases involving
or VMFS corruption, official support may be needed to run remediation procedures like vclock file resets storage array compatibility for your current hardware setup? ESXi host HBAs offline - Broadcom support portal
The phrase "atomic test and set of disk block returned false for equality" typically points to a low-level synchronization failure within a filesystem or a storage area network (SAN). This error indicates that a system attempted to update a specific block of data but found that the block’s current state did not match the expected "baseline" state.
In modern computing, ensuring data integrity across distributed systems or multi-core processors requires these "atomic" operations to prevent race conditions and data corruption. 🛠️ Understanding the Atomic Operation
At the heart of this issue is the Compare-and-Swap (CAS) or Test-and-Set logic.
The Goal: Change the value of a disk block from "State A" to "State B."
The Check: Before writing "State B," the system verifies that the block is still actually in "State A."
The Failure: If the system finds "State C" instead, the equality test fails. The operation returns false, and the write is aborted to prevent overwriting someone else's data. 🔍 Common Causes for the Equality Failure
When this error appears in logs (common in environments like VMware ESXi, Linux LVM, or clustered filesystems), it usually stems from one of the following: 1. Multi-Host Contention (Split Brain)
In clustered environments, two different servers (hosts) might believe they own the same disk block. If Host 1 updates the block while Host 2 is still processing, Host 2’s next atomic command will fail because the block "fingerprint" has changed unexpectedly. 2. VAAI (vStorage APIs for Array Integration) Issues
In VMware environments, the Hardware Accelerated Locking feature uses atomic test-and-set commands (ATS). If the underlying storage array has a firmware bug or a momentary timeout, the ATS primitive may return a false equality, leading to VM freezes or "Lost access to volume" messages. 3. Latency and Connectivity Spikes
High "noise" on a Fiber Channel or iSCSI network can cause delayed packets. If a test command is delayed and the data changes in the intervening milliseconds, the eventual set command will fail the equality check. 4. Hardware Degradation
A failing drive controller or a "bit-rot" scenario can cause the data read during the "test" phase to be inconsistent. If the checksums don't align perfectly, the atomic operation triggers a safety shutdown of that specific task. 🛠️ Troubleshooting and Resolution
If you are seeing this error in your system logs, follow these steps to isolate the cause: Check Storage Logs Look for SCSI Sense Codes (e.g., H:0x0 D:0x2 P:0x0 Valid).
Identify if the error is isolated to a single LUN (Logical Unit Number) or spans the entire array. Review Locking Mechanisms
For VMware: Check if "ATS+SCSI2" locking is enabled. Sometimes reverting to standard SCSI reservations can bypass a buggy ATS implementation on older storage firmware.
For Linux: Use multipath -ll to ensure that paths are healthy and not flapping, which causes synchronization mismatches. Firmware Updates
Storage providers (Dell, HPE, Pure Storage, etc.) frequently release patches for VAAI and ATS logic. Ensure your Host Bus Adapter (HBA) and Storage Array firmware are in sync. Analyze Resource Contention
Reduce the number of VMs or processes accessing a single volume. Excessive metadata updates (like taking many snapshots simultaneously) can overwhelm the atomic locking capacity of the disk. 💡 Summary Table Description Operation Type Atomic Compare-and-Swap (CAS) Context Filesystem metadata updates / Distributed locking The "False" Result Means the block was modified by another process first Risk Level High (Potential for data inconsistency if ignored) Primary Fix Firmware updates or reducing I/O contention
To help me give you more specific advice, could you tell me:
What Operating System or Hypervisor (e.g., ESXi, Ubuntu, Windows Server) are you using? What is the brand of the storage hardware?
Did this occur during a specific task, like a backup or a VM migration?
Here’s a good, clear review for that scenario, depending on who your audience is:
For a developer / code review context:
“The atomic test-and-set operation on the disk block returned
falsewhen checking for equality, indicating that the current value in the block did not match the expected value. This suggests a concurrent modification or a stale expected value — the operation failed as designed, preventing a potential race condition or lost update.”
For a bug report or log comment:
“Atomic compare-and-swap on disk block failed: equality check returned false. Expected value did not match actual block content. Possible causes: concurrent write by another process, or cached expected value outdated.”
For a performance / correctness review (e.g., database or filesystem):
“Correct behavior observed: atomic test-and-set returned false on equality check, meaning the block had been modified since the expected value was read. The operation correctly aborted without updating, preserving consistency.”
Report: Atomic Test and Set of Disk Block Returned False for Equality
Introduction
The following report documents an issue encountered during a recent testing phase, where an atomic test and set operation on a disk block returned an unexpected result, indicating that the block's contents were not equal as anticipated.
Test Environment
Test Description
The test in question involved performing an atomic test and set operation on a disk block. This operation typically checks the current value of a disk block and, if it matches a specified expected value, atomically sets it to a new value. The goal was to verify the integrity and consistency of disk operations under various conditions.
Observed Issue
During the execution of the test:
Analysis
The return of false for equality during an atomic test and set operation on a disk block suggests that:
Steps to Reproduce
true, indicating the block's value matched the expected value before being updated.Recommendations
Conclusion
The observation that an atomic test and set operation on a disk block returned false for equality highlights a potential issue with data consistency or concurrent access. Further investigation and debugging are necessary to resolve the root cause and ensure the reliability of disk operations.
Action Plan
Responsibilities
Timeline
Status Update
This report will be updated with findings from the investigation and any corrective actions taken.
Title: The Ghost in the Machine: Debugging "Atomic Test-and-Set of Disk Block Returned False for Equality"
Tagline: When the storage layer lies about a simple comparison, distributed systems start to hallucinate.
If you work with distributed databases (like Cassandra, ScyllaDB, or FoundationDB), Ceph, or any system that uses complex consensus algorithms (Raft/Paxos), you might eventually stumble upon a terrifying log message:
atomic test and set of disk block returned false for equality
This error is cryptic. It sounds like a C++ template metaprogramming error or a cosmic ray hit your RAM. But in reality, it is the storage engine’s way of screaming, "Reality is broken."
Let’s dissect what this means, why it happens, and why your database cluster might refuse to talk to itself because of it.
Engineers building high-performance LSM-trees or B-tree storage engines sometimes use block-level TAS to avoid finer-grained locks. This error indicates that a concurrent write or a partial block update corrupted the expected state.
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.
In a storage context, the error "atomic test and set of disk block returned false for equality" typically indicates a locking failure in VMware ESXi environments using VAAI (vSphere Storage APIs for Array Integration) .
It occurs when a host attempts to update a disk block (such as a VMFS metadata heart-beat) but finds that the data currently on the disk does not match what it expected to see before making the change . Core Mechanism: Atomic Test and Set (ATS)
Traditional storage uses "SCSI Reservations" to lock an entire LUN (volume), which can cause performance bottlenecks. Modern systems use ATS (also known as Hardware Assisted Locking) to lock only specific disk blocks .
The "Test": The host reads a block and compares it to a "test-image" (expected data) .
The "Set": If they match (equality), the host immediately writes new data to the block in one atomic operation .
The Failure: If the block on the disk has changed since the host last checked it, the equality test returns false. The array then returns an "ATS Miscompare" error . Common Causes of This Error
Race Conditions: Multiple ESXi hosts are trying to access or update the same metadata block at the same time .
Delayed I/O (Timeouts): An earlier ATS "set" command actually reached the disk even though the host thought it timed out. When the host retries with the original "test" data, it no longer matches the already-updated disk content .
Storage Array Issues: Firmware bugs or misconfigurations on the storage array can lead to incorrect reporting of block states.
Network/Fabric Instability: Dropped packets or high latency in the SAN can cause the host and storage to become out of sync regarding the lock state . Troubleshooting Steps
Check VMkernel Logs: Look for "ATS Miscompare" or SCSI sense key MISCOMPARE (0xE or 14) in your ESXi logs .
Verify VAAI Support: Ensure your storage array's firmware is compatible with the version of ESXi you are running .
Monitor Path Latency: High latency often triggers the "timeout and retry" loop that leads to miscompares .
Consider Disabling ATS: As a last resort for stability, you can temporarily disable ATS heartbeat to revert to traditional SCSI reservations, though this may impact performance .
Are you seeing this error in a VMware VMkernel log, or is it appearing during a specific operation like mounting a datastore?
The error message "atomic test and set of disk block returned false for equality"
a critical indicator of a metadata mismatch or locking failure in VMware vSphere environments . It typically occurs during an Atomic Test and Set (ATS) Check vendor release notes for terms like "COMPARE
operation, where an ESXi host attempts to update a datastore's heartbeat or lock a file but finds that the data on the disk does not match what it expected. Core Cause: ATS Miscompare Heartbeat Mechanism
: 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
: 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 host often loses access to the datastore, causing virtual machines to hang, crash, or enter a "grayed out" state. Common Triggers Storage Latency
: High I/O latency can lead to ATS heartbeats failing, causing the service to degrade. Firmware/Driver Incompatibility
: Outdated HBA drivers or storage array firmware frequently cause these synchronization issues. Heavy Load
: 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
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?
Subject: Atomic Test and Set of Disk Block Returned False for Equality
Incident Report
Date: [Insert Date] Time: [Insert Time] System/Component: [Insert System/Component Name] Error Description:
An atomic test and set operation on a disk block returned false for equality, indicating a potential issue with data consistency or synchronization. This error was encountered during [insert operation or process].
Error Details:
Impact:
The returned false value for equality may lead to:
Root Cause Analysis:
Preliminary analysis suggests that the issue might be related to:
Recommendations:
To resolve this issue, we recommend:
Action Plan:
The following steps will be taken to address this issue:
Responsibilities:
Timeline:
This report will be updated as more information becomes available. If you have any questions or concerns, please do not hesitate to reach out.
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").
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
This phrase seems to describe a low-level concurrency or transactional issue, likely in the context of database systems, file systems, or persistent memory. Here’s a technical review of what this could mean and the implications. When the OS asks, "Is this zero