Look for spikes in command latency. ATS is very sensitive to timing; if the storage is overloaded, ATS failures will increase.
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 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
Mixing drives with different sector sizes (such as native 4Kn drives and 512-byte emulation/512e drives) in the same storage pool can cause misalignment. When the host expects a specific block boundary for an ATS operation and encounters a mismatch, it returns an equality failure. Symptoms and Impact on the Environment
VAAI is a set of storage primitives that offload specific tasks from the ESXi host to the storage array, improving performance and scalability. One of these primitives is ATS , also known as "Hardware Assisted Locking". Before ATS , VMFS used SCSI-2 reservations, which locked an entire LUN (Logical Unit Number), preventing any other host from accessing it, even for unrelated operations. ATS solves this by allowing fine-grained, block-level locking. Instead of locking the whole LUN, a host can lock only the specific disk block it needs to update. This significantly reduces lock contention in shared-storage environments. Look for spikes in command latency
The host expected the disk block to look like Value A . However, right before the host could write its update, a different host changed that exact disk block to Value B . Because the block no longer matches the expected state, the storage array rejects the command and returns false . This prevents data corruption but halts the pending storage operation. 2. Common Root Causes
The most frequent cause is excessive metadata activity on a single volume. If dozens of virtual machines on different hosts are booting up, taking snapshots, or migrating via vMotion simultaneously on the same datastore, multiple hosts will compete for the same metadata blocks. One host will win the lock, causing the others to trigger an ATS equality failure. 2. Storage Array Firmware Bugs
To maximize storage efficiency, modern datacenters connect multiple physical servers (hosts) to a centralized storage array. When these hosts write data to the same shared volume simultaneously, they must use a mechanism to prevent overwriting each other's data. The Role of Atomic Test-and-Set (ATS) Symptoms and Impact on the Environment VAAI is
Check your performance monitors for spikes in device latency and commands aborted. If you see latency climbing above 20–30 milliseconds during the error windows, your storage fabric or array spindles are likely oversaturated. Step 3: Verify Firmware and Driver Interoperability
In the world of high-performance virtualization, there are few errors as cryptic—and as disruptive—as the infamous:
In distributed systems, a node that was partitioned may have written to a block while another node believed it held the lock. After healing, the TAS fails. Examine cluster membership logs and fencing mechanisms. the TAS fails.
Verify that Jumbo Frames (MTU 9000) are configured uniformly across the entire path if using iSCSI.
: Introduced via VMware vStorage APIs for Array Integration (VAAI), ATS allows the host to lock only a specific disk block instead of the whole LUN.