Atomic Test And Set Of Disk Block Returned False For Equality (2024)

The message "Atomic test and set of disk block returned false for equality" is a critical diagnostic error typically associated with VMware ESXi and storage systems using VAAI (vSphere Storage APIs – Array Integration).

: Misconfigured LUN numbers (e.g., using LUN 0 when the host group expects a different ID) or storage arrays not fully supporting VAAI specifications. Broadcom support portal Recommended Resolutions Reboot the Affected Host The message "Atomic test and set of disk

If false is returned consistently, the system enters a spin-loop.

CPU Waste: On single-core systems, spinning while receiving false wastes CPU cycles that could be used by the lock holder to release the lock.

Cache Coherency Traffic: In multi-core systems, repeated TS instructions returning false generate high traffic on the memory bus (invalidating and re-reading the "lock" variable cache line), known as "cache thrashing."

Verdict: The mechanism is correct.
Action Item: Code handling this return value must be optimized to avoid performance degradation. It is recommended to use an adaptive mutex: spin for a short duration (expecting the lock holder to release soon), and if false persists, yield the CPU (sleep) to allow the lock holder to complete its work.

Steps to Reproduce

Update Firmware: Check for known ATS-related bugs in your storage array's firmware version, as some vendors have specific patches for "false ATS miscompares". ESXi host HBAs offline - Broadcom support portal If false is returned consistently, the system enters

Responsibilities

High Latency: Extreme I/O latency can cause a host to receive outdated information about a block before it tries to lock it, leading to a mismatch when the actual ATS command arrives. Verdict: The mechanism is correct