Setting the access mode

Red Hat Enterprise Linux 9.2 z/VM guest

You might want to access the DCSS device with write access to change the content of the DCSS or set of DCSSs that map to the device.

About this task

There are two possible write access modes to the DCSS device:

shared

In the shared mode, changes to DCSSs are immediately visible to all z/VM guests that access them. Shared is the default.

Note: Writing to a shared DCSS device bears the same risks as writing to a shared disk.

exclusive-writable

In the exclusive-writable mode you write to private copies of DCSSs. A private copy is writable, even if the original DCSS is read-only. Changes that you make to a private copy are invisible to other guests until you save the changes.

After saving the changes to a DCSS, all guests that open the DCSS access the changed copy. z/VM retains a copy of the original DCSS for those guests that continue accessing it, until the last guest stops using it.

To access a DCSS in the exclusive-writable mode the maximum definable storage size of your z/VM virtual machine must be above the upper limit of the DCSS. Alternatively, suitable authorizations must be in place.

For either access mode the changes are volatile until they are saved.

Procedure

Set the access mode before you open the DCSS device. To set the access mode to exclusive-writable, set the DCSS device's shared attribute to 0. To reset the access mode to shared set the DCSS device's shared attribute to 1.

Issue a command of this form:

# echo <flag> > /sys/devices/dcssblk/<dcss-name>/shared

where <dcss-name> is the DCSS name that represents the DCSS device.

You can read the shared attribute to find out the current access mode.

Example

To find out the current access mode of a DCSS device represented by the DCSS name MYDCSS:

# cat /sys/devices/dcssblk/MYDCSS/shared
1

1 means that the current access mode is shared. To set the access mode to exclusive-writable issue:

# echo 0 > /sys/devices/dcssblk/MYDCSS/shared