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

6 Virtual disks
Virtual disk characteristics
Before creating a virtual disk and presenting to a host, consider the following characteristics:
Name
Choose the virtual disk name carefully. If you change the name after creation, you must take the
virtual disk offline momentarily.
TIP: Name a virtual disk for the data it contains, since it may be moved to a different host or to
different storage. Do not name it after the host or the storage where the virtual disk was first created.
DPM group
Determine the DPM group in which the virtual disk will reside. Moving a virtual disk to another
DPM group requires taking the virtual disk offline momentarily.
Primary and secondary DPMs
Plan ahead to balance workload across DPMs as evenly as possible. The primary or secondary
DPM property is selected during the virtual disk creation process. If this property is not explicitly
selected, the VSM server alternates the virtual disks between the two members of the DPM group.
Pool characteristics
Front-end virtual disks are allocated from capacity on back-end virtual disks using an algorithm
that roughly distributes the front-end virtual disks across the multiple back-end virtual disks. The
algorithm was based on the following:
• A storage pool will contain a minimum of 8–16 back-end LUs.
• When configuring additional virtual disks, a back-end LU is selected, and the algorithm
searches for contiguous free space. This algorithm also applies to temporary virtual disks that
support PiTs, snapshots, thin provisioning, and so on.
Virtual disk path presentations and resource limits
Front-end presentations
It is important when creating virtual disks to consider that SVSP v3.x allows for 16 K disk presentation
paths per DPM pair. Considering that hosts typically have two adapter paths for redundancy, this
results in 8 K virtual disk paths per DPM pair. To calculate the maximum virtual disks per host,
divide 8 K by the number of hosts. This may be a limitation if you want to create many small virtual
disks (that is, more than two per host), have many host paths, or have many hosts. See “Fabric
topology” (page 10) for additional details.
Back-end resource limits
The DPM physical storage container (PSC), the path from a DPM to a back-end LU, has a limit of
64 outstanding I/Os per PSC. Operating systems (like OpenVMS) may generate 256 outstanding
I/Os per LUN; therefore, you may require multiple PSCs for maximum performance.
PiT and snapshots
PiT usage may produce a performance bottleneck when the PiT is first created and has a workload
of high random writes with small request sizes. Online transaction processing (OLTP) databases
Virtual disk characteristics 27