HP XP7 Storage - Manual (Page 167)

Because most UNIX file systems (excluding JFS and VxFS) have no journal files, the fence level
should be defined as Never. When a takeover by the S-VOL occurs, fsck is executed on the volume
and the file system is cleaned up, even if the S-VOL is undefined at the secondary host. The data
that is lost depends on how much differential data is contained in the P-VOL when the S-VOL is
suspended. During operation, error recovery should be performed when the suspended status
(PSUE or PDUB) is detected (when one error occurs).
Snapshot operations
Snapshot normally creates virtual volumes for copying on write without specifying LUNs as S-VOLs.
However, to use a Snapshot volume via the host, it is necessary to map the Snapshot S-VOL to a
LUN. Therefore, RAID Manager provides a combined command to enable the user or application
to use the same RAID Manager command in order to maintain Business Copy compatibility.
Snapshot uses two techniques, one called "V-VOL mapping" (or virtual volume mapping) and the
other is "Snapshot using copy on write" or "Copy-on-Write Snapshot." Snapshot volumes are also
put into pooling volumes called a "Snapshot pool," and a Snapshot pool is specified as a pool ID
when a Snapshot is made. Snapshot and volume mapping is illustrated in
Figure 56 (page 167) .
Figure 56 Snapshot and volume mapping
Snapshot volumes
The specifications for Snapshot volumes are:
•
Allowable type of paired volume: The supported volume type is OPEN-V only for P VOL, and
OPEN-0V for S VOL.
•
Number of volumes (Snapshot) can be paired: This depends on P VOL capacity, Snapshot
pool capacity, and shared memory capacity on the RAID storage system.
•
Duplicated writing mode: Copying on write.
•
Number of mirror volumes:
Fast Snap: Up to 1,024 secondary volumes can be defined for each P VOL. ◦
◦ Snapshot: Up to 64 secondary volumes can be defined for each P VOL.
For details on Fast Snap or Snapshot specifications such as maximum number of mirrored volumes,
volumes and operations, please see the HP XP7 Fast Snap User Guide or HP XP7 Snapshot User
Guide for your storage system.
Snapshot operations 167