• When using the replication manager CLUI, you can enter the names of any resourc e. If the
command is invalid for the resource, the c ommand will fail when it is executed.
Cache policies
Each virtual d isk on a storage system has a read cache a nd a write cache. The following are key c ache
policies (settings): write cache, write cache mirror and read cache.
Write cache
The virtual disk write cache policy specifies how host writes to virtual disks are performed. Values are:
• Write-back. The storage system notifies a host that a write operation is complete after data is
written to the storage system cache memor y (but before the data is written from the cache to disk
media). Write-back is the default.
• Write-through. The storage system notifiesahostthatawriteoperationiscompleteafterthe
data is written from the cache to disk media.
Write-back caching improves host write performance because data is written to the high speed ca che
memor y faster then to disk media. Write-through caching is safer, in terms of fault tolerance.
Storage systems that are fully functional use only write-back. Under certain failure conditions, storage
systems automatically switch to write-through for data safety.
Actual and requested values. Policy values are reported as actual and requested. An actual value is the
policy that is in effect for the virtual disk. A requested value is a value that has been requested but is not
yet in effect. For example, a policy change can be requested when a virtual disk is not p resented to a
hostbutthechangedoesnottakeeffectuntilthediskispresentedtoahost.
Write cache mirror
The virtu
al disk write cache mirror policy specifies whether each controller in a storage system contains a
copy of t
he other controller’s write cache. The following values are reported:
• Enabled. In the storage system, each controller contains a copy of the other controller’s write
cache. Enabled is the default.
• Disabled. In the storage system, the controller does not contain a copy of the companion
contro
ller’s write cache.
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 o p eration, but reverts to the safety of write-through in the event of failure.
You can
not 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 da ta is read from the high speed ca che memory
faster than from disk media.
Containers
A container is disk space that i s reserved, or preallocated, for later use as a mirrorclone, snapclone
or a s
napshot. 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
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Virtual disks