![]() With VAAI ATS, the lock granularity is reduced to a much smaller level of control (specific metadata segments, not an entire volume) for the VMFS that a given host needs to access. This behavior not only caused metadata lock queues but also prevented standard I/O to a volume from VMs on other ESXi hosts which were not currently holding the lock. In a cluster with multiple nodes, all metadata operations were serialized and hosts had to wait until whichever host, currently holding a lock, released that lock. Prior to the introduction of VAAI ATS, VMFS used LUN-level locking via full SCSI-2 reservations to acquire exclusive metadata control for a VMFS volume. VMware resolved the first issue with the introduction of Atomic Test and Set (ATS), also called Hardware Assisted Locking.
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