What you should know about DCSSs

6.10 z/VM guest

The DCSS device names and nodes adhere to a naming scheme. There are different modes and options for mounting a DCSS.

Important: DCSSs occupy spool space. Be sure that you have enough spool space available (multiple times the DCSS size).

DCSS naming scheme

The standard device names are of the form dcssblk<n>, where <n> is the corresponding minor number.

The first DCSS device that is added is assigned the name dcssblk0, the second dcssblk1, and so on. When a DCSS device is removed, its device name and corresponding minor number are free and can be reassigned. A DCSS device that is added always receives the lowest free minor number.

Creating device nodes

User space programs access DCSS devices by device nodes. Your distribution might create these device nodes for you.

If no device nodes are created for you, you must create them yourself, for example, with the mknod command. See the mknod man page for further details.

Tip: Use the device names to construct your nodes.

To create standard DCSS device nodes of the form /dev/<device_name> issue commands of this form:

# mknod /dev/dcssblk0 b <major> 0
# mknod /dev/dcssblk1 b <major> 1
# mknod /dev/dcssblk2 b <major> 2
...

When the DCSS device driver is loaded, it dynamically allocates a major number to DCSS devices. A different major number might be used when the device driver is reloaded, for example when Linux® is rebooted. Check the entry for dcssblk in /proc/devices to find out which major number is used for your DCSSs.

Accessing a DCSS in exclusive-writable mode

You must access a DCSS in exclusive-writable mode, for example, to create or update the DCSS.

To access a DCSS in exclusive-writable mode at least one of the following conditions must apply:
  • The DCSS fits below the maximum definable address space size of the z/VM® guest virtual machine.

    For large read-only DCSS, you can use suitable guest sizes to restrict exclusive-writable access to a specific z/VM guest virtual machine with a sufficient maximum definable address space size.

  • The z/VM user directory entry for the z/VM guest virtual machine includes a NAMESAVE statement for the DCSS. See z/VM: CP Planning and Administration, SC24-6271 for more information about the NAMESAVE statement.
  • The DCSS was defined with the LOADNSHR operand.

    See z/VM: CP Commands and Utilities Reference, SC24-6268 for information about the LOADNSHR operand.

    See DCSS options about saving DCSSs with the LOADNSHR operand or with other optional properties.

DCSS options

The z/VM DCSS device driver always saves DCSSs with default properties. Any previously defined options are removed.

For example, a DCSS that was defined with the LOADNSHR operand loses this property when it is saved with the z/VM DCSS device driver.

To save a DCSS with optional properties, you must unmount the DCSS device, then use the CP DEFSEG and SAVESEG commands to save the DCSS.

See z/VM: CP Commands and Utilities Reference, SC24-6268 for information about DCSS options.