HP P6000 Replication Solutions Manager User Guide (T3680-96089, October 2012)

Mirrored write-back is the default mode for all virtual disks, provided that a cache battery or UPS
is present and operational for the preferred controller. Mirrored write-back provides the high
performance of write back under normal operation, but reverts to the safety of write-through in the
event of failure.
You cannot use the replication manager to set the write cache mirror policy.
Read cache
The virtual disk read cache policy specifies how host reads from virtual disks are performed. Values
are:
• On. The storage system satisfies host read requests, when possible, from the storage system's
cache memory. On is the default.
• Off. The storage system always satisfies host read requests from the disk media.
Read caching increases host read performance because data is read from the high speed cache
memory faster than from disk media.
Containers
A container is disk space that is reserved, or preallocated, for later use as a mirrorclone, snapclone
or a snapshot. A container has most of the same properties as a virtual disk, including a name
and size. However, a container:
• Cannot be presented to a host
• Cannot store data
Implementation of container features is controller software dependent. See Controller software
features - local replication.
Containers for preallocated replication
Preallocated replication refers to a snapclone or snapshot that is created by copying data from a
source virtual disk to a container and immediately converting the container into a virtual disk.
Compared to standard snapclones and snapshots, the preallocated methods are faster. In cases
where host I/O must be suspended, the improved speed of preallocated replication reduces the
time that a host application is suspended.
Container planning
A container must be the same size as the virtual disk that you want to copy. For example, if you
have virtual disks of 10-, 20-, and 30-GB to copy, you should create 10-, 20-, and 30-GB containers.
A container of 25-GB would not be usable with any of those disks.
When used for mirrorclones and preallocated snapclones, the container can be in a different disk
group than the source virtual disk. When used for preallocated snapshots, the container must be
in the same disk group as the source virtual disk.
Conversion of virtual disks to containers
A snapclone virtual disk can be converted to a container. This is useful when you repeatedly create
a snapclone of the same virtual disk, for example, when making daily backups of a virtual disk.
See the following job commands: ConvertStorageVolumeIntoContainer,
ConvertStorageVolumesIntoContainers, and ConvertStorageVolumesInManagedSetIntoContainers.
For more information on jobs, see the HP P6000 Replication Solutions Manager Job Command
Reference.
The following virtual disks cannot be converted into containers:
• Virtual disks that are presented to hosts
• Snapshots
Virtual disk concepts 257