During the life of a shadow set, the state of any shadow set
member relative to the rest of the members of the shadow set can
vary. The shadow set is considered to be in a steady state when
all of its members are known to contain identical data. Changes in the composition
of the shadow set are inevitable because:
Disk drives occasionally need corrective
maintenance.
New disks are added to replace other disks.
System failures occur, requiring merge operations
to take place within the shadow set.
Controllers fail, requiring maintenance.
System management functions, such as backup, are
required.
For example, suppose an operator dismounts a member of a shadow
set and then remounts the member back into the shadow set. During
the member's absence, the remaining members of the shadow set may
have experienced write operations. Thus, the information on the
member being remounted into the shadow set will differ from the
information on the rest of the shadow set. Therefore, a copy (or
minicopy) operation is required.
As another example, consider a situation where a shadow set
is mounted by several systems in an OpenVMS Cluster configuration.
If one of those systems fails, the data on the members of the shadow
set may differ because of outstanding or incomplete write operations
issued by the failed system. The shadowing software resolves this
situation by performing a merge operation.
In any event, copy and merge operations allow volume shadowing
to preserve the consistency of the data written to the shadow set. A shadow set is considered
to be in a transient state when one or more
of its members are undergoing a copy or a merge operation.
Additionally, volume shadowing maintains shadow set consistency
by:
Maintaining consistent data on shadow
set members by automatically detecting and replacing bad blocks on
one shadow set member and rewriting those bad blocks with good data
from another shadow set member.
Notifying all nodes when a member is added or removed
from a shadow set, and ensuring the shadow set membership is consistent
clusterwide.
Volume shadowing uses two internal mechanisms to coordinate
shadow set consistency:
Storage control blocks (SCBs) Volume shadowing uses a storage control block (SCB) as a primary
method for controlling shadow set membership. Each physical disk
contains an SCB in which the shadowing software records the names
of all the current members of the shadow set. Each time the composition
of the shadow set changes, the SCB on all members is updated. This
feature simplifies clusterwide membership coordination and is also
used by the MOUNT qualifier /INCLUDE to reconstruct a shadow set.
Shadow set generation number Volume shadowing uses a shadow set generation number as a
primary method of determining shadow set member validity and status.
A shadow set generation number is an incrementing value that is
stored on every member of a shadow set. Each time a membership change
occurs to the shadow set (members are mounted, dismounted, or fail),
the generation number on the remaining members is incremented. Thus,
if a shadow set's generation number is 100 and a member is dismounted
from the set, the generation numbers on the remaining members are
incremented to 101. The removed member's generation number remains
at 100. When mounting shadow sets, the shadowing software uses the
generation numbers, found in the SCB on the physical units, to determine
the need for and direction of copy operations.
Table 1 Information in the Storage Control Block (SCB)
SCB Information
Function
Volume label
Identifies a unique name
for the volume. Every member of a shadow set must use the same volume
label.
BACKUP revision
number
A BACKUP/IMAGE restoration
rearranges the location of data on a volume and sets a revision
number to record this change. The Mount utility (MOUNT) checks the
revision number of the proposed shadow set member against the numbers
on current or other proposed shadow set members. If the revision
number differs, the shadowing software determines whether a copy
or merge operation is required to bring the data on the less current
members up to date.
Volume shadowing generation number
When a member joins a shadow
set, it is marked with a volume shadowing generation number. You
can zero the generation number by using the /OVERRIDE=SHADOW_MEMBERSHIP
qualifier with the MOUNT command.
Mount and dismount status
The SCB mount status field is used as
a flag that is set when a volume is mounted and cleared when it
is dismounted. There is also a count of the number of nodes that
have mounted the shadow set write-enabled. The MOUNT command checks this
field when a disk is mounted. If the flag is set, this indicates
that the disk volume was incorrectly dismounted. This will occur
in the event of system failure. When mounting shadow sets that were
incorrectly dismounted, or where the write count field is not correct,
the shadowing software automatically initiates merge operations.
Upon receiving a command to mount a shadow set, the volume
shadowing software immediately determines whether a copy or a merge
operation is required; if either is required, the software automatically
performs the operation to reconcile data differences. If you are
not sure which disks might be targets of copy operations, you can
specify the /CONFIRM or /NOCOPY qualifiers when you use the MOUNT
command. To disable performing any copy operations, use the /NOCOPY
qualifier. If you mount a shadow set interactively, use the /CONFIRM
qualifier to instruct MOUNT to display the targets of copy operations
and request permission before the operations are performed.
When you dismount an individual shadow set member, you produce
a situation similar to a hardware disk failure. Because files remain
open on the virtual unit, the removed physical unit is marked as not being properly
dismounted.
After one of the devices is removed from a shadow set, the
remaining shadow set members have their generation number incremented,
identifying them as being more current than the former shadow set member.
This generation number aids in determining the correct copy operation
if you remount the member into a shadow set.