In VMware vSphere , Virtual Machine File System (VMFS) uses this method to manage locking on LUNs (Logical Unit Numbers). If multiple ESXi hosts try to update the same metadata block on a shared LUN simultaneously, the "test and set" operation ensures only one host succeeds.
Two processes attempt to acquire the same disk-based lock. Result: One succeeds; the other receives the "false for equality" error and should retry or fail gracefully. Solution: Implement exponential backoff and retry logic.
The mechanism is often implemented via commands or similar primitives (e.g., NVMe Compare and Write, or Linux’s BLKZEROOUT with verification).
ATS is a storage hardware acceleration feature (part of VAAI) used by ESXi to lock individual disk blocks rather than an entire LUN.
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 In VMware vSphere , Virtual Machine File System
Check physical switch ports for CRC errors or frame drops that could indicate a degrading physical path. Step 4: Reduce Metadata Contention
To understand why this error occurs, you must understand .
Modern storage systems rely on at the block level to achieve:
At the heart of this issue is the or Test-and-Set logic. Result: One succeeds; the other receives the "false
Check logs for specific NAA disk string, verify storage array load, restart host management daemons.
: 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.
If a storage array is performing background operations—such as replication, automated tiering, deduplication, or taking hardware snapshots—it may manipulate block allocations without notifying the hypervisor immediately. Similarly, if another host in the cluster loses network connectivity but remains active (a split-brain scenario), both hosts might attempt to claim the same metadata heartbeats simultaneously. 3. Symptoms and Operational Impact
Hosts losing "scratch" partition configurations or taking an unusually long time to boot. Broadcom support portal Common Causes Communication & Latency ATS is a storage hardware acceleration feature (part
: This is often the first step to clear stale locks and restore temporary connectivity to the datastore. Validate Data Path Compatibility
VMware ESXi hosts may temporarily lose access to the VMFS datastore, marking it as completely unavailable or "All Paths Down" (APD).
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
Check with your storage vendor (e.g., Dell EMC, HPE, NetApp, Pure Storage). Ensure your storage controllers are running the latest validated firmware. Vendors frequently release patches that optimize VAAI/ATS lock handling and fix race conditions within the controller cache. Step 3: Verify Multipathing Configurations
| Step | Action | Process | |:----:|:-------|:--------| | 1 | | The ESXi host constructs a SCSI COMPARE_AND_WRITE (opcode 0x89) command. It specifies the LBA (Logical Block Address) of the disk block to operate on, the block of data to compare against (the "expected" data), and the new block of data to write if the comparison succeeds. | | 2 | Storage Array Comparison | The storage array receives the command. It reads the current block from its media, fetches the "compare" data from the command, and performs a bitwise equality check between the two. | | 3 | Equality Check & Write | If the current block data matches the expected data, the array proceeds to atomically write the new data to the block. It then returns a status of "success". | | 4 | Atomic Lock Acquisition | This successful operation serves as an atomic lock acquisition, proving to the VMFS filesystem that it has exclusive rights to modify the metadata for that filesystem extent. | | 5 | Status Return & Completion | The array returns a SCSI status code. On success, it's GOOD . On a compare failure, it returns COMPARE AND WRITE MISCOMPARE , which the VMware storage stack interprets and logs as the error in question. |
If the values do not match ( False for equality), the storage array rejects the write operation entirely.