HP StorageWorks SAN Virtualization Services Platform Best Practices Guide (5697-0935, May 2011)

User-created PiTs
User-created PiTs (through a script or the GUI) have the following benefits:
• Can be created when the application is stopped (typically part of a script)
• Are application-consistent and applications recover more quickly
However, there are other considerations regarding user-created PiTs. For example, user-created
PiTs are not deleted the same as automatic PiTs. See the HP StorageWorks SAN Virtualization
Services Platform Administrator Guide for details.
Alternatively, you can use thin provisioned virtual disks. Thin provisioned virtual disks mirror quickly
because only the used blocks are transferred instead of a full copy including unused blocks.
Detach, split, and merge
Asynchronous mirroring has merge, split, and detach features that enable you to switch the mirroring
direction between SVSP domains. The original destination SVSP domain of a task can become the
source SVSP domain and mirror data to the SVSP domain that was originally the source domain.
Asynchronous mirroring resumes the process from the cutoff point without having to make a full
copy. The procedure is useful for both a planned failover and an unplanned failover resulting from
a primary site failure.
Detach
Detaching an asynchronous mirror task deletes the task but does not delete the source virtual disk,
destination virtual disks, and the PiTs on the source and the destination. Detaching is enabled
regardless of the status of the tasks. However, the source and destination might have different
states and data, so there is a risk of losing the latest data. It is useful if an unplanned failure or
disaster occurs at the source; you can detach the task and use a destination virtual disk for recovery.
Split
Splitting an asynchronous mirror task deletes the task but does not delete the source and destination
virtual disks or their PiTs. Where the detach operation involves the risk of losing the latest data, a
split ensures that no data is lost. A split is usually a step in a planned failover.
You can only split a task if certain conditions apply. These conditions ensure that the split results
in identical data on the source and the destination. Splitting enables you to transfer the production
host from a virtual disk on the source site to a mirror of the virtual disk on the destination site without
losing application data.
Before you split an asynchronous mirror task, complete the following procedure:
1. Stop the applications that are using the asynchronous mirror source virtual disk.
2. Unmount the virtual disk on the host side. All host application writes are now flushed to the
virtual disks.
3. Remove all host permissions from the task source virtual disk.
4. Create a user PiT on the task source virtual disk. This will close the active temporary virtual
disk of the last PiT and allows the tasks to copy this data to the destinations.
5. Wait until the user PiT is fully replicated to the destination virtual disk.
6. Suspend the task.
7. Split the asynchronous mirror.
Merge
Merge is an operation that you can perform on a virtual disk or virtual disk group that re-establishes
an asynchronous mirror task that was split or broken. When you merge a task, you identify an
existing virtual disk as the source virtual disk and the former mirroring partner as the destination
virtual disk. This allows you to reverse the direction of mirroring by using the former destination
50 Asynchronous mirroring