Ensuring correct channel path status

Red Hat Enterprise Linux 8.6 LPAR mode z/VM guest KVM guest

Ensure that you have varied the path offline before you perform a planned task on it.

Before you begin

Do not vary channel paths offline that provide access to vital resources, such as the root file system.

KVM: A KVM guest might have only one channel path through which all CCW devices are accessed.

Tasks that require the channel path to be offline include:

Pulling out or plugging in a cable on a path.
Configuring a path off or on at the SE.

To vary the path offline, issue a command of the form:

# chchp -v 0 <chpid>

where <chpid> is the channel path ID.

After the operation completed and the path is available again, vary the path online by using a command of the form:

# chchp -v 1 <chpid>

Alternatively, you can write on or off to the channel path status attribute in sysfs to vary the path online or offline.

# echo on|off > /sys/devices/css0/chp0.<chpid>/status

An unplanned change in path availability can occur due to, for example, unplanned cable pulls or a temporary path malfunction. Then, the PIM/PAM/POM values (as obtained through lscss) might not be as expected. To update the PIM/PAM/POM values, vary one of the paths that lead to the affected devices.

Example:

# chchp -v 0 0.12
# chchp -v 1 0.12

Rationale: Linux® does not always receive a notification (machine check) when the status of a path changes (especially for a path that comes online again). To make sure Linux has up-to-date information about the usable paths, path verification is triggered through the Linux vary operation.