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

A synchronous mirror group can be created only if the virtual disk:
• Has host permissions.
• Status is Normal.
• Does not contain PiTs.
• Is not a member of a virtual disk group (VDG).
NOTE: See the HP StorageWorks SAN Virtualization Service Platform Administrator Guide for
information and examples about test and recovery of various synchronous mirror failure scenarios.
Dirty regions
When the synchronous mirror group receives a write, each task issues that write to its virtual disk.
This is done sequentially in that the completion from one task is necessary before the request is
sent to the second task. If the DPM terminates before all writes have completed, it may leave the
synchronous mirror in an inconsistent state. This can only happen if there are at least two copies
with a Normal status. The recovery for this type of failure can be very lengthy as it requires
synchronizing all the virtual disks in the task to bring them back to a state where they are consistent.
This is done by copying the virtual disk of one of the Normal copies to the other copy's virtual
disks. The lengthy recovery process for this type of failure can be shortened considerably by
enabling the Dirty Regions On mode. In this mode, all recent writes are tracked in a bitmap stored
in the reallocation table's virtual disk of each task. When a write is received, the corresponding
region is marked dirty. After all writes have completed successfully, the region is marked clean
again. The recovery process is reduced from synchronizing all of the content on the virtual disks
to synchronizing the dirty regions identified in the bitmap. Running a task in Dirty Regions On
mode carries a small performance penalty that must be weighed against the possibility of
synchronizing an entire virtual disk to restore a virtual disk after failure. Dirty Regions On mode is
recommended for larger virtual disks.
Segment size
SVSP uses coarse grain segment size for tracking changes to virtual disks that are being replicated.
The current segment size for SVSP is 1 MB. Applications such as databases that exhibit random
access with small I/O request sizes typically write to many different segments between synchronous
mirrors because they have a relatively low locality of reference. Consequently, performance on
synchronous mirrors can be reduced because many segments (larger than the host request) need
to be written.
Synchronous mirror configurations
Figure 17 (page 42) shows the recommended configuration for SVSP, because the DPM can still
use all available processing capabilities of the ASIC during a fabric failure. The two port colors
represent separate fabrics. A standard cabling configuration will lose access to half the processing
capabilities of the DPM.
Figure 17 Current SVSP cabling configuration
42 Synchronous mirroring