Technical data

System Management Features
4.13 Volume Shadowing for OpenVMS
The device specified must be a member of a shadow set that is mounted on
the node where the command is issued.
• /MVTIMEOUT=yyyyyy DSAnnnn:
Specifies the mount verification timeout value to be used for this shadow set,
specified by its virtual unit name, DSAnnnn.
The value supplied by this qualifier overrides the SYSGEN parameter
MVTIMEOUT for this specific shadow set.
The valid range for yyyyyy is 1 to 16,777,215 seconds.
The device specified must be a shadow set that is mounted on the node where
the command is issued.
•
/READ_COST=zzz ddcu:
The valid range for zzz is 1 to 4,294,967,295 units.
The device specified must be a member of a shadow set that is mounted on
the node where the command is issued.
This qualifier allows you to modify the default ‘‘cost’’ assigned to each member
of a shadow set, so that reads are biased or prioritized toward one member
versus another.
The shadowing driver assigns default READ_COST values to shadow set
members when each member is initially mounted. The default value depends
on the device type, and its configuration relative to the system mounting it.
There are default values for a DECRAM device; a directly connected device in
the same physical location; a directly connected device in a remote location; a
DECram served device; and a default value for other served devices.
The value supplied by this qualifier overrides the default assignment. The
shadowing driver adds the value of the current queue depth of the shadow set
member to the READ_COST value and then reads from the member with the
lowest value.
Different systems in the cluster can assign different costs to each shadow set
member.
If the /SITE command qualifier has been specified, the shadowing driver will
take site values into account when it assigns default READ_COST values.
Note that in order for the shadowing software to determine if a device is in
the category of ‘‘directly connected device in a remote location,’’ the /SITE
command qualifier must have been applied to both the shadow set and to the
individual device.
Reads requested for a shadow set from a system at Site 1 are performed from
a shadow set member that is also at Site 1. Reads requested for the same
shadow set from Site 2 can read from the member located at Site 2.
•
/READ_COST=y DSAnnnn
The valid range for y is any non-zero number. The value supplied has no
meaning in itself. The purpose of this qualifier is to switch the read cost
setting for all shadow set members back to the default read cost settings
established automatically by the shadowing software. DSAnnnn must be a
shadow set that is mounted on the node from which this command is issued.
• /SITE=(nnn, logical_name)(ddcu: DSAnnnn:)
4–22 System Management Features