Atomic Test And Set Of Disk Block Returned False For Equality Jun 2026

: Ensure your storage array fully supports VAAI ATS.

This implies that or another mechanism changed the block between when Process A last checked it and when the atomic operation executed. 3. Common Causes and Scenarios

The most definitive fix for persistent ATS errors is ensuring compatibility across the stack. Check your hardware vendor’s compatibility matrix. Update the host’s Host Bus Adapter (HBA) drivers, the multipathing software, and the storage array’s controller firmware to the latest stabilized releases. Vendors frequently patch edge-case race conditions in lock handling. Adjust Tuning Parameters (Timeout Values)

a critical indicator of a metadata mismatch or locking failure in VMware vSphere environments . It typically occurs during an Atomic Test and Set (ATS) : Ensure your storage array fully supports VAAI ATS

When you see the error "atomic test and set of disk block returned false for equality" , the array has executed step 3. The host’s cached map of the storage metadata is out of sync with the actual physical reality on the disk array. Common Root Causes

If your host experiences flapping network paths to the storage array (due to a bad fiber channel cable, faulty SFP module, or misconfigured switch), an ATS command may be sent down one path, but the acknowledgment gets lost during a failover. When the host retries over the new path, the state has changed, triggering the error. 4. Non-Standard Sector Formats (Advanced Format Disks)

: High I/O latency or intermittent connectivity issues can cause these "heartbeat" failures, leading to the host losing access to the volume. Common Symptoms Common Causes and Scenarios The most definitive fix

grep -i "atomic test and set" /var/log/messages dmesg | grep -i "compare.*write\|reservation" journalctl -xe | grep "false for equality"

Keywords: atomic test and set, disk block, returned false for equality, compare and swap, distributed lock manager, concurrency control, optimistic locking, split-brain, storage consistency, clustered file system debugging.

| API/Command | Purpose | |-------------|---------| | sync_file_range(2) + fdatasync(2) | Control write ordering | | io_uring_ops with IORING_OP_COMPARE_AND_WRITE | Linux native TAS on block devices | | fcntl(F_OFD_SETLK) | POSIX file locking (not block-level) | | nvme compare and nvme write | NVMe’s compare-and-write primitives | | rados cas (Ceph) | Object-level atomic compare-and-swap | Vendors frequently patch edge-case race conditions in lock

The host checks the current metadata of a disk block to see if it matches what it expects.

Severe delays in storage communication can cause the host to time out or see outdated metadata.

The error is a concurrency control signal, not a disk failure. It tells you that your optimistic lock attempt failed because the disk block’s current value did not match your expected value. By methodically comparing expected vs. actual values, validating cache coherence, and implementing proper retry logic, you can resolve this issue in distributed file systems, lock managers, and custom storage engines.

Look for spikes in command latency. ATS is very sensitive to timing; if the storage is overloaded, ATS failures will increase.

When a host wants to lock a metadata block on a shared datastore, it sends an ATS command (specifically the SCSI COMPARE AND WRITE command) to the storage array.