IBM DS6000 Series manual 327

However, there is no forced error indicator in the SCSI architecture, and the revector operation is nonatomic. As a substitute, the OpenVMS shadow driver exploits the SCSI commands READ LONG (READL) and WRITE LONG (WRITEL), optionally supported by some SCSI devices. These I/O functions allow data blocks to be read and written together with their disk device error correction code (ECC). If the SCSI device supports READL/WRITEL, OpenVMS shadowing emulates the DSA forced error with an intentionally incorrect ECC. For details see Scott H. Davis, Design of VMS Volume Shadowing Phase II — Host-based Shadowing, Digital Technical Journal Vol. 3 No. 3, Summer 1991, archived at:

http://research.compaq.com/wrl/DECarchives/DTJ/DTJ301/DTJ301SC.TXT

The DS6000 provides volumes as SCSI-3 devices and thus does not implement a forced error indicator. It also does not support the READL and WRITEL command set for data integrity reasons.

Usually the OpenVMS SCSI Port Driver recognizes if a device supports READL/WRITEL, and the driver sets the NOFE (no forced error) bit in the Unit Control Block. You can verify this setting with the SDA utility: After starting the utility with the analyze/system command, enter the show device command at the SDA prompt. Then the NOFE flag should be shown in the device’s characteristics.

The OpenVMS command for mounting shadow sets provides a qualifier /override=no_forced_error to support non-DSA devices. To avoid possible problems (performance loss, unexpected error counts, or even removal of members from the shadow set), we recommend you apply this qualifier.

Appendix A. Operating systems specifics 327