Atomic Test And Set Of Disk Block Returned False For Equality Jun 2026
This error typically appears in vmkernel.log files and indicates a failure in a locking mechanism designed to prevent data corruption. This article explains what this error means, why it happens, and how to troubleshoot it in 2026. 1. What is an "Atomic Test and Set" Operation?
Operations like powering a VM on or off, creating a snapshot, vMotion migrations, or cloning vDisks hang indefinitely or fail outright.
Check if "ATS+SCSI2" locking is enabled. Sometimes reverting to standard SCSI reservations can bypass a buggy ATS implementation on older storage firmware.
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 This error typically appears in vmkernel
If it matches (equality), the host updates the block with its own signature to claim ownership.
Historically, hosts used SCSI reservations ( SCSI-2 Reservation ), which locked the entire LUN. This caused massive performance degradation because other hosts had to wait for the entire volume to unlock just to modify a single file.
: The host provides the data it expects to find in that disk block. What is an "Atomic Test and Set" Operation
If the data does not match, the array returns an status, which the hypervisor surfaces as "Atomic test and set of disk block returned false for equality" . 2. Root Causes of the Error
At the heart of this issue is the or Test-and-Set logic.
: 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. Sometimes reverting to standard SCSI reservations can bypass
If rebooting is not possible, try unmounting and remounting the datastore to refresh the locks. B. Long-Term Fixes (Prevent Future Occurrences)
In vSphere, this can be done by changing the advanced setting: UserVars.VMFS3HardwareAcceleratedLocking to 0 (Disabled).
If storage saturation is causing the timeouts, throttle the execution queues on your hosts. Lowering the maximum number of concurrent commands allowed per LUN prevents the array from becoming overwhelmed during high-load events. Fall Back to Standard Locking (If Necessary)
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