User`s guide

6.7 Planning Your Mirrorsets
The following items should be considered before creating your mirrorsets:
• Mirrorset size (1 to 6 members)
• Mirrorset replacement policy
• Mirrorset spares
• Mirrorset hardware requirements—write-back cache module
• Mirrorset firmware requirements—HSOF Version 2.5 firmware
6.8 Using Mirrorsets to Obtain Snapshot Copies of Data
A mirrorset is a storageset consisting of multiple devices, each containing
an identical copy of the same data. Because individual devices can be added
and removed from a mirrorset while it is in use (with minimal impact on the
mirrorset), and because removed members contain a complete copy of all the
data on the mirrorset at the time of removal, mirroring is used in some system
management situations to obtain snapshot copies of data.
The general strategy used in these situations contains the following steps:
1. During normal, steady-state operation, run the unit with the number of
drives appropriate for the availability required.
2. When the time comes to take a snapshot of the data, activate mirroring
on the unit in question (if it is not already active), and add a physical disk
into the mirrorset. As with the addition of any new mirrorset member, the
mirroring facility copies the data from the existing members to the new
‘‘snapshot’’ member.
3. When the copy to the snapshot member is complete, extract the snapshot
member from the mirrorset. In some situations, the application is quiesced
so that there is no activity to the mirrorset when the snapshot member is
extracted; other situations are insensitive to when the snapshot member is
extracted.
4. If mirroring is not the normal steady-state mode for the unit, deactivate
mirroring (UNMIRROR) on the unit that was copied.
5. Set the snapshot copy aside for use as a ready backup. Alternatively, the
snapshot copy can be mounted and a conventional backup utility run against
it to backup the contents to tape. This latter approach allows the backup
utility to be run at locations and times more convenient to system operation
than the traditional fixed backup window.
Some file systems and applications recommend against mirror snapshots as a
backup strategy, because the snapshot technique is not well suited to the way
they use data. But for some systems, mirror snapshots are a useful technique.
The HS controllers contain the following mirrorset commands to ease the task of
using mirroring to obtain data snapshots:
1. The MIRROR and UNMIRROR commands allow a specific disk to be
converted from a disk container to a mirrorset and back again to a simple
disk container, all while the disk is in use. This means that units that will be
snapshot do not have to be configured permanently as mirrorsets; mirroring
can be activated and deactivated on the units only for the time it takes to
obtain the snapshot.
6–18 Working with RAID Arrays