Sharing DASD without Using Virtual Reserve/Release

If the guest operating systems do not support reserve/release CCWs, you can still share DASD between virtual machines. However, no write protection is provided using this method. To do this, define a minidisk for one virtual machine and have other virtual machines link to that minidisk. The process is similar to that described for virtual reserve/release with the following exceptions:

  1. You do not have to specify a V as part of the access mode on the MDISK directory statement.
  2. You do not have to be concerned with the SHARED setting.

The major difference between this method of sharing DASD and virtual reserve/release is best illustrated by the case where you have included an access mode of MW (multiple write) on the MDISK definition. With virtual reserve/release, although many virtual machines can obtain write access to the minidisk, only one virtual machine can write to it at any one time if it uses virtual reserve release. If you do not use virtual reserve/release, two virtual machines can both obtain write access to the minidisk at the same time. To ensure data integrity, you should use virtual reserve/release when you need to give multiple virtual machines write access to a minidisk.

Note: Sharing DASD without using virtual reserve/release is not recommended. You could read incomplete data or have an abend.

The SET WRKALLEG command supports the MVS™ Sysplex Environment. SET WRKALLEG allows MVS guests running in a single virtual machine to share minidisks without using reserve and release CCWs. For more information, see the SET command in z/VM: CP Commands and Utilities Reference.