Enabling and disabling immediate failure of I/O requests

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

Prevent devices in mirror setups from being blocked while paths are unavailable by making I/O requests fail immediately.

About this task

By default, if all path have been lost for a DASD, the corresponding device in Linux® waits for one of the paths to recover. I/O requests are blocked while the device is waiting.

If the DASD is part of a mirror setup, this blocking might cause the entire virtual device to be blocked. You can use the failfast attribute to immediately return I/O requests as failed while no path to the device is available.

Attention: Use this attribute with caution and only in setups where a failed I/O request can be recovered outside the scope of a single DASD.

Procedure

Use one of these methods:

You can enable immediate failure of I/O requests when you load the base module of the DASD device driver.
Example:
To define a device range (0.0.7000-0.0.7005) and enable immediate failure of I/O requests specify:
```
dasd=0.0.7000-0.0.7005(failfast)
```
You can use the sysfs attribute failfast of a DASD to enable or disable immediate failure of I/O requests on or off.
To enable immediate failure of I/O requests, write 1 to the failfast attribute.
Example:
```
echo 1 > /sys/bus/ccw/devices/<device_bus_id>/failfast
```
To disable immediate failure of I/O requests, write 0 to the failfast attribute.
Example:
```
echo 0 > /sys/bus/ccw/devices/<device_bus_id>/failfast
```