User`s guide

2. The REDUCE command allows multiple disks to be simultaneously removed
from multiple mirrorsets as a single, synchronized operation. This permits
snapshot copies of stripesets and striped mirrorsets in a way that retains the
integrity of the entire stripeset.
3. The NODESTROY option on the INITIALIZE command permits the snapshot
copy to be configured and added as a unit (distinct from the original) without
destroying any of the data.
6.9 Mirrorset Considerations for Snapshot Copies
If you want to use mirroring to create a snapshot copy of data, consider the
following:
1. The only useful snapshot is a snapshot of all the data on a unit. This is
because the host sees the unit as a single integrated storage space, and the
host uses this storage space without regard to any physical boundaries in the
underlying disks.
This means that if the unit is layered on one or more storagesets, all devices
in the storageset must be snapshot at the same time.
Thus, if the unit is layered on a simple disk container, then the snapshot need
only involve one disk. Likewise, if the unit is layered on a simple mirrorset,
the snapshot need only involve one disk (because for mirrorsets, all members
are equivalent). For stripesets and striped mirrorsets, however, multiple
snapshot disks are involved, one for each member of the stripeset.
To preserve the host view of the data, when you configure the snapshot unit
to gain access to the snapshot data, you must carefully configure the snapshot
in a manner identical to the configuration at the time the snapshot member
was removed from the mirrorset. Since all snapshot disks will have been
mirrorset members at the time of removal, they must be defined as mirrorset
members when used in the snapshot unit as well. Therefore:
• A snapshot of a single disk unit must be configured as a single-member
mirrorset
• A snapshot of a mirrorset unit must be configured as a single-member
mirrorset
• A snapshot of a stripeset must be configured as a stripeset of single-
member mirrorsets
• A snapshot of striped mirrorsets must be configured as a stripeset of
single-member mirrorsets
See the CLONE utility discussion in Section 7.3.12 for examples of this
consideration in practice.
2. Mirroring snapshots cannot be used for RAIDset units. They can only be used
to snapshot the data in disk, simple mirrorset, simple stripeset, or striped
mirrorset units.
3. The bulk of the overhead in making a snapshot is consumed during the copy
phase. Once the snapshot member copy is complete (it is in the NORMAL
state), it is kept current with all other members from then on. Thus, the
extraction of the snapshot from the mirrorset can be done with similar results
at any time after the copy is complete. Because the extraction operation is
quick, it can be synchronized manually with host activity in any convenient
fashion.
Working with RAID Arrays 6–19