HP 418800-B21 HP StorageWorks Replication Solutions Manager 4.0.1 user guide ( - Page 440

The snapshot property indicates whether a virtual disk can be locally replicated using the snapshot method. Values are: • Yes. The virtual disk complies with snapshot guidelines. Snapshot replication can be performed. • No. The virtual disk does not comply with snapshot guidelines. Snapshot replication cannot be performed. Preallocated snapshots Preallocated snapshot refers to a fully allocated 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 a standard fully allocated snapshot, creating a preallocated snapshot is faster. In cases where host I/O must be suspended, the improved speed of preallocated snapshots reduces the time that a host application is suspended. When a preallocated snapshot is created, the source virtual disk write cache must be flushed before replication is started. (See cache policies write cache.) This ensures that the source virtual disk and snapclone copy contain identical data. The following table shows how a write cache flush is implemented. IMPORTANT: When using jobs or the CLUI, you must explicitly ensure that write caches are flushed. Method GUI action Job CLUI Flush implementation Write cache setting after replication The replication manager automatically sets the source disk to write-though mode and ensures the flush is completed before starting the replication. You must include job commands to set the source disk to write-through mode and wait for the flush to complete before starting the replication. You must issue CLUI commands to set the source disk to write-through mode and wait for the flush to complete before starting the replication. When replication is completed, the controller software automatically sets the source disk and preallocated copy (converted container) to write-back mode. If you want the source disks or copies to be in write-through mode, you must explicitly set them. This feature is controller software version dependent. See controller software features - local replication. See also virtual disk containers. Snapshot FAQ • How can I tell a snapshot from other types of virtual disks? Because snapshots are not independent virtual disks, they are identified differently than original (active) virtual disks. See virtual disks types. • How long does it take to create a snapshot? A snapshot requires only a matter of seconds, no matter how large the original (active) virtual disk. • If it is virtual, can a host write to a snapshot? Yes. A snapshot is functionally equivalent to a physical disk with both read and write capability. • After I create a snapshot, can I delete the original (active) virtual disk? No. A snapshot always relies, at least in part, on the original (active) virtual disk for data. If the original virtual disk is deleted, its associated snapshot becomes unusable. A snapshot should be thought of as a temporary copy. • Can I make multiple snapshots of an original (active) virtual disk? Yes. However, there is a limit. See virtual disks snapshot guidelines. • What is the maximum number of snapshots on a storage system? There is no limit. However, the greater the number of snapshots, the longer it takes to shut down the storage system during maintenance and upgrade activities. 440 Virtual disks