Atomic Test And Set Of Disk Block Returned False For Equality |link| -

Traditionally, shared storage environments used SCSI reservations (SCSI-2) to lock an entire storage LUN (Logical Unit Number) when a host needed to update metadata. This metadata update happens during routine tasks like creating a virtual machine, powering it on, or expanding a virtual disk. However, locking the entire LUN created a massive performance bottleneck because all other hosts connected to that LUN had to wait.

This failure is often referred to in VMware documentation as an . When this happens, the ESXi host cannot update the cluster metadata or maintain its "heartbeat" on the datastore. As a result, the host may lose access to the volume, VMs might become unresponsive, and tasks like cloning or powering on machines will fail. Root Causes of ATS Miscompare Errors

Raft requires strict persistence. To become the leader, a node must write a "no-op" entry to disk using a test-and-set to ensure no split-brain occurs.

If you are using older file systems like VMFS5, migrate to VMFS6, which features improved automatic space reclamation and more efficient metadata handling. Step 5: Adjust ATS Miscompare Thresholds (Advanced) This failure is often referred to in VMware

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.

But why is the equality false? In the context of disk blocks, we must consider the content. If the block is a counter, a flag, or a pointer, the failure to match implies that the value has evolved. The equality is false because time has moved forward.

This means the storage engine performed the atomic operation, but the validation step failed. Specifically: Root Causes of ATS Miscompare Errors Raft requires

When an ATS operation returns "false for equality," it means a mismatch occurred. This mismatch is rarely caused by a failing hard drive; it is almost always an orchestration or communication fault. 1. Multi-Host Contention and Race Conditions

ATS leverages the standard SCSI command COMPARE AND WRITE (Opcode 0x89 ). The operation relies on strict mathematical equality:

This failure acts as a boundary condition for the selfhood of a process. In concurrent programming, a process defines itself by its resources. "I am the process that owns Block X." When the test-and-set returns false, the process is stripped of that potential identity. It is told, "You are not the one. You do not own this. You are equal to the task, but the world does not match your view of it." detects the already updated block

A known architectural race condition occurs when an ESXi host aborts a timed-out heartbeat I/O. In many cases, the "Set" image actually makes it to the physical disk right before the abort command finishes processing. When the ESXi host automatically retries the operation using its original "Test" image, the storage array looks at the disk, detects the already updated block, and correctly flags a mismatch. 3. Fabric and Path Connectivity Dropping

Hosts losing "scratch" partition configurations or taking an unusually long time to boot. Broadcom support portal Common Causes Communication & Latency

: For persistent mount failures, some admins found success by removing and re-adding the datastore via the esxcli command line.

[ ESXi Host 1 ] -----\ (ATS Lock Request: "Is block X empty? If yes, write my ID") +---> [ Shared Storage LUN ] ---> Lock Verified & Applied [ ESXi Host 2 ] -----/ (ATS Lock Request: "Is block X empty?" -> RETURNS FALSE) The Evolution of VMFS Locking

Modern drives use 4096-byte (4K) sectors. Legacy software sometimes assumes 512-byte sectors. If you try to perform an atomic test-and-set on a 512-byte chunk that straddles two physical 4K blocks, you aren't testing one atomic unit. You are testing half of block A and half of block B. The disk firmware will return a "false" because the comparison wasn't aligned to its native boundary.