Quiescence

Quiescence: during a DR unconfigure/disconnect operation on a system board with non-pageable Open Boot PROM (OBP) or kernel memory, the operating system is briefly paused, which is known as operating system quiescence. All operating system and device activity on the backplane must cease for a few seconds during a critical phase of the operation.

Before it can achieve quiescence, the operating system must temporarily suspend all processes, processors, and device activities. If the operating system cannot achieve quiescence, it displays the reasons, which may include the following:

• A user thread did not suspend

• Real-time processes are running

• A device exists that cannot be paused by the operating system

The conditions that cause processes to fail to suspend are generally temporary. Examine the reasons for the failure. If the operating system encountered a transient condition--a failure to suspend a process--you can try the operation again.

Suspend-Safe and Suspend-Unsafe Devices

suspend-safe: a suspend-safe device is one that does not access memory or interrupt the system while the operating system is in quiescence. A driver is suspend-safe if it supports operating system quiescence (suspend/resume). It also guarantees that when a suspend request is successfully completed, the device that the driver manages will not attempt to access memory, even if the device is open when the suspend request is made.

suspend-unsafe: a suspend-unsafe device is one that allows a memory access or a system interruption while the operating system is in quiescence.

Suspend-safe drivers provide the ability to:

• Stop user threads

• Execute the DDI_SUSPEND call in each device driver

• Stop the clock

• Stop the CPUs

The operating system refuses a quiescence request if a suspend-unsafe device is open. To manually suspend the device, you may have to close the device by killing the processes that have it open, asking users not to use the device, or disconnecting the cables. For example, if a device that allows asynchronous unsolicited input is open, you can disconnect its cables prior to activating operating system quiescence and reconnect them after the operating system resumes. This action prevents traffic from arriving at the device and, thus, the device has no reason to access the backplane.

Testing for Suspend-Safe Drivers

The quiesce-test option tests for suspendable drivers.

# cfgadm -x quiesce-test sysctrl#:slot#

Tape Devices

The sequential nature of tape devices prevents them from being reliably suspended in the middle of an operation, and then resumed. Therefore, all tape drivers are suspend-unsafe. Before executing a DR operation that activates operating system quiescence, make sure all tape devices are closed or not in use.