A node caches disk block values but fails to invalidate the cache after a write from another node. Result: The node issues a test-and-set based on stale data, causing an unexpected failure. Solution: Disable aggressive caching for shared block devices; use O_DIRECT or O_SYNC where appropriate.
Before we dive into the failure, we have to understand the success. Atomic Test and Set (ATS)
Understanding "Atomic Test-and-Set of Disk Block Returned False for Equality" A node caches disk block values but fails
If transient network spikes are causing the issue, modifying how long a host waits before timing out an ATS operation can stabilize the environment. For instance, in vSphere environments experiencing heavy load, administrators sometimes increase the VMFS3.HardwareAcceleratedLocking heartbeat timeout configuration parameters to allow the system more breathing room during I/O spikes. Optimize VM Distribution to Reduce Contention
An unexpected failure in a storage area network (SAN) or virtualized file system can halt infrastructure operations instantly. One of the most cryptic errors system administrators encounter in cloud and enterprise virtualization environments is the failure of a storage locking primitive, often logged as: Before we dive into the failure, we have
The error message "atomic test and set of disk block returned false for equality" directly corresponds to the MISCOMPARE sense code (0x1D, qualifier 0x00).
Resolving an ATS equality failure requires a systematic approach to isolate whether the issue lies in the host layer, the fabric layer, or the storage array layer. Step 1: Analyze the System Logs Optimize VM Distribution to Reduce Contention An unexpected
| Aspect | Evaluation | |--------|------------| | | Atomic CAS on disk block failed because block ≠ expected value. | | Typical severity | Moderate — part of normal concurrency, but could indicate bug if unexpected. | | Likely fix if unexpected | Re-read block, ensure correct expected value, implement retries. | | Architectural note | True disk-block atomic CAS is rare; many systems emulate via logging or PERSIST barriers. |
Ensure your storage array's firmware is compatible with the version of ESXi you are running .
In a typical distributed system, TAS is used to implement optimistic concurrency control. For example, a metadata server might attempt to claim ownership of a disk block by atomically testing whether its "owner ID" field is 0 (unowned) and setting it to its own ID. If the operation returns true , the block was successfully claimed. If it returns false (the error condition), someone else has already claimed it—or the block state became corrupted.
esxcli system settings advance parameter set -o /VMFS3/HeartbeatTimeout -i 20 Use code with caution.