HP SAN Virtualization Services Platform - Manual (Page 268)

are created, you can create up to two additional tasks for additional destination virtual disks. Multiple
tasks can be defined per group, representing multiple destinations for the same source element. Each
source element can only belong to one VDG.
When you create an async mirror task with up to three remote copies for added disaster tolerance,
the mirror service creates a snapshot PiT on the source element. A task begins by copying the source
element as it is at the time of PiT creation to the destination virtual disk in the background. Meanwhile,
the host application can continue to make modifications to the source element, which are redirected
to the PiT temporary virtual disk. After the initial build of the destination virtual disk (called
normalization, or the initial full copy operation), the task updates the source element by creating
subsequent source PiTs, and copying and integrating them into the destination virtual disk.
If you run an async mirror task on a virtual disk that was never presented to any host, the mirror service
assumes that the virtual disk is empty and skips the initial copy of the source virtual disk to the
destination virtual disk. This enables you to save time if you are creating an async mirror group on a
new, empty virtual disk. You can purposely delay assigning host permission to the source virtual disk
until after creating the group, in order to skip the initial copy.
CAUTION:
Failure to present a disk full of data before creating a remote mirror task may result in the loss of data
at the remote site.
By default, the mirror service automatically creates PiTs at regular intervals. These PiTs are called
standard PiTs. The minimum time between standard PiT creations is configurable, and at the specified
time, the mirror service checks that conditions are suitable for creating a new standard PiT.
NOTE:
During the initial product releases, the minimum time between standard PiT creations is limited to 10
minutes
The following conditions must all be satisfied before the mirror service creates a new standard PiT:
• There is no more than one PiT waiting for deletion on the source element. If more data is in the
PiT, the data can be copied within the PiT interval. The automatic creation of subsequent PiTs is
delayed until the previous PiT has been copied to the remote site.
• Creating a new PiT will not exceed the maximum number of permitted PiTs for the source element
(32 PiTs per virtual disk presented to a user-defined host (UDH) and 32 PiTs per virtual disk
presented to a VSM server), with a minimum of 10 minutes between intervals. See the HP Storage-
Works SAN Virtualization Services Platform release notes if these values change.
• The source virtual disk was modified since the last PiT creation.
If these conditions are satisfied, the service creates a new standard PiT. If not, it waits for the specified
interval and checks the conditions again.
You can trigger PiT creation on an async mirror group at any time. PiTs that a user creates are called
user PiTs. They enable you to synchronize the PiT creation with a specific application state that you
want to capture.
You can configure the mirror service not to create standard PiTs on a specific group. In this case, as
soon as a new user PiT is created, the previous user PiT is copied to the mirror destination. The new
user PiT on the source accumulates all data changes until a user triggers the creation of the next user
PiT.
Using mirroring 268