HP 418800-B21 HP StorageWorks Replication Solutions Manager 4.0.1 user guide ( - Page 430

• When using the replication manager CLUI, you can enter the names of any resource. If the command is invalid for the resource, the command will fail when it is executed. Cache policies Each virtual disk on a storage system has a read cache and a write cache. The following are key cache 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 memory (but before the data is written from the cache to disk media). Write-back is the default. • Write-through. The storage system notifies a host that a write operation is complete after the data is written from the cache to disk media. Write-back caching improves host write performance because data is written to the high speed cache memory 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 presented to a host but the change does not take effect until the disk is presented to a host. Write cache mirror The virtual disk write cache mirror policy specifies whether each controller in a storage system contains a copy of the 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 controller'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 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 430 Virtual disks