: Tasks like snapshot creation, VM provisioning, or migrations fail with timeout errors.
: 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.
In the world of low-level systems programming and distributed databases, few error messages are as cryptic—and as critical—as If you have encountered this error while working with a clustered file system, a distributed lock manager, or a custom storage engine, you know the frustration it brings. The operation failed unexpectedly, leaving your application in an inconsistent state.
Under extreme storage load, the time it takes for an ATS command to travel from the host to the array increases. If the host takes too long to execute the operation, another host may swoop in, modify the metadata first, and complete its write. When the first host's delayed command finally reaches the disk, the on-disk state has changed, causing an instant miscompare. B. Out-of-Sync Firmware and Multipathing Glitches : Tasks like snapshot creation, VM provisioning, or
During an ATS operation, the ESXi host issues a COMPARE AND WRITE command. It sends two images to the storage array: a (what it believes is currently written on the disk) and a "Set" image (the modification it wants to write). The storage array evaluates this atomically:
If they match (), the array updates the block with the new data.
: 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 When the first host's delayed command finally reaches
Understanding the "Atomic Test-and-Set of Disk Block Returned False for Equality" Error
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returned false for equality" error in VMware vSphere indicates a failure in Hardware Assisted Locking (ATS) due to outdated storage metadata, usually caused by concurrency conflicts or high latency.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 .
To understand the error, you must first understand the technology behind it. SCSI Comparison Same (Atomic Test-and-Set) : It provides Hardware-Assisted Locking
In many APIs, a failed TAS simply returns false (0). However, the message tells you the test condition failed—not that the set was impossible due to hardware error. This distinction is crucial:
Hosts losing "scratch" partition configurations or taking an unusually long time to boot. Broadcom support portal Common Causes Communication & Latency
A 4-node GlusterFS cluster began throwing “atomic test and set of disk block returned false for equality” errors after a power outage. Metadata operations hung, and thick provisioning failed.
: It provides Hardware-Assisted Locking , allowing a host to lock only specific disk sectors/metadata blocks rather than the entire LUN. Mechanism :
Too many hosts attempting to lock the same metadata block at the same time. 3. Causes of "Atomic Test and Set" Failures