HP StorageWorks SAN Virtualization Services Platform Manager User Guide (5697-0454, May 2010)

18 Using mirroring
The mirror service enables you to mirror virtual disks asynchronously between domains, and
synchronously within a domain.
In asynchronous mirroring, the host application writes data to an original virtual disk and the data is
copied to mirror virtual disks in the background while the application continues to write. Asynchronous
mirroring is useful for maintaining an offsite backup of virtual disks for disaster recovery, enabling
you to install more cost-effective storage equipment on your disaster recovery site and use high-end
storage systems on your main site. Continuous Access is the term that HP uses to denote asynchronous
mirroring to a remote site.
In synchronous mirroring, the host application receives a response to a write command only after
data has been successfully written to all mirror virtual disks. Synchronous mirroring is also provided
for the setup virtual disk on the three to five storage pools used in the local domain. Mirroring of the
setup virtual disk is established when you perform the one-time operation of creating the setup virtual
disk. For information about creating the setup virtual disk, see “Creating the setup virtual
disk” on page 373.
Asynchronous mirroring is supported between two domains, connected by Internet Small Computers
Systems Interface (iSCSI). Synchronous mirroring is supported within the local domain.
The mirror service includes data integrity technology that ensures I/O consistency (also called
write-order or crash consistency) between mirror copies and includes features that enable you to fail
over rapidly to mirrored copies while maximizing data integrity.
In order to maintain I/O and crash consistency for the data of an application that uses multiple virtual
disks, all of those disks must belong to the same virtual disk group, and therefore to the same
asynchronous mirror group.
Asynchronous mirroring supports mirroring of one source virtual disk to multiple destination virtual
disks.
Asynchronous mirroring
Asynchronous mirroring is useful for maintaining an off site backup of virtual disks for disaster recovery.
In asynchronous mirroring, the host application writes data to an original virtual disk, while the VSM
servers copy the data in the background to mirrored virtual disks on a destination site.
The mirror service supports asynchronous mirroring of virtual disks, virtual disk groups (VDGs), and
snapshots between two domains. Adding virtual disks to a VDG enables you to mirror multiple virtual
disks as a group.
Asynchronous mirroring is supported across domains connected over iSCSI, and is suitable for mirroring
data to a disaster recovery site connected over low bandwidth Ethernet connections. The mirror service
provides features to enable you to recover lost data from a mirrored virtual disk or VDG, and transfer
production from an original virtual disk, VDG, or snapshot, to a mirrored virtual disk or VDG.
To perform asynchronous mirroring, you first create an asynchronous mirror group that defines the
source element to be mirrored. The source element can be a virtual disk, a snapshot, or a VDG. When
you create the group, you also create one async mirror task. The task continually mirrors the source
element to a specified destination until you detach or delete the task. After the group and the first task
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