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ERROR_SVHDX_UNIT_ATTENTION_AVAILABLE
Meaning in the SCSI command path
ERROR_SVHDX_UNIT_ATTENTION_AVAILABLE tells the initiator that the virtual disk has Unit Attention information to report. Unit Attention is a SCSI changed-state condition: it is not the same as an ordinary permission error, media error, or a malformed command. It commonly appears after a state transition that requires the initiator to refresh what it knows about the device or shared-storage ownership.
The Shared VHDX layer can carry this condition across the remote virtual-disk path. A generic Unit Attention result does not identify the change by itself. The associated sense data determines whether the event concerns capacity, persistent reservations, registrations, an operating-definition change, or another unit-attention reason. This is why treating every instance as an automatic retry is unreliable.
How to handle the result
- Collect the associated sense data and identify the precise Unit Attention reason before resuming a sequence of dependent I/O.
- Re-read relevant device information and the current shared-disk state. For cluster-aware software, include persistent-registration and reservation status in that check.
- Use the more specific Shared VHDX results when present. For example, preempted reservations, released reservations, and changed operating definitions require different recovery decisions.
- Preserve the time order: a Unit Attention may be the first visible sign of a failover, registration change, device reconfiguration, or storage-format transition.
What not to conclude
This condition alone does not prove data corruption and does not say that a guest can safely reclaim shared-disk ownership. It says that an initiator must account for a state change. In a guest cluster, letting the cluster stack perform its normal fencing and ownership recovery is safer than issuing ad hoc SCSI Persistent Reserve Out commands from a troubleshooting host.
References
- Microsoft Open Specifications: Remote Shared Virtual Disk Protocol
- Linux kernel documentation: SCSI persistent reservations
- T10: SCSI reservation and Unit Attention semantics
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