IBM TotalStorage DS6000 Series User Manual
Page 351
Appendix A. Operating systems specifics
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:
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.