Reading and resetting the reservation state

6.10 LPAR mode z/VM guest KVM guest

How the DASD device driver handles I/O requests depends on the last_known_reservation_state sysfs attribute of the DASD.

About this task

The last_known_reservation_state attribute reflects the reservation state as held by the DASD device driver and can differ from the actual reservation state. Use the tunedasd -Q command to find out the actual reservation state. The last_known_reservation_state sysfs attribute can have the following values:

none
The DASD device driver has no information about the device reservation state. I/O requests are processed as usual. If the DASD is reserved by another system, the I/O requests remain in the queue until they time out, or until the reservation is released.
reserved
The DASD device driver holds a valid reservation for the DASD and I/O requests are processed as usual. The DASD device driver changes this state if notified that the DASD is no longer reserved to this system. The new state depends on the reservation policy.
ignore
The state is changed to none.
fail
The state is changed to lost.
lost
The DASD device driver had reserved the DASD, but subsequently another system has unconditionally reserved the DASD. The device driver processes only requests that query the actual device reservation state. All other I/O requests for the device are returned as failed.
When the error that led another system to unconditionally reserve the DASD is resolved and the DASD has been released by this other system, there are two methods for resuming operations.
  • Setting the DASD offline and back online.
  • Resetting the reservation state of the DASD.
Attention: Do not resume operations by resetting the reservation state unless your system setup maintains data integrity on the DASD despite:
  • The I/O errors that are caused by the unconditional reservation
  • Any changes to the DASD through the other system

You reset the reservation state by writing reset to the last_known_reservation_state sysfs attribute of the DASD. Resetting is possible only for the fail reservation policy (see Handling lost device reservations) and only while the value of the last_known_reservation_state attribute is lost.

To find out the reservation state of a DASD issue a command of this form: 
# cat /sys/bus/ccw/devices/<device_bus_id>/last_known_reservation_state
where <device_bus_id> specifies the DASD.

Example

The command in this example queries the reservation state of a DASD with bus ID 0.0.7009. 
# cat /sys/bus/ccw/devices/0.0.7009/last_known_reservation_state
reserved