68 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
Strive to distribute volumes evenly across available I/O Groups and nodes within the clustered 
system. Although volume characteristics depend on the storage pool from which it is created, 
any volume can be assigned to any node.
When you create a volume, it is associated with one node of an I/O Group, the preferred 
access node. By default, when you create a volume, it is associated with the I/O Group node 
by using a round-robin algorithm. However, you can manually specify the preferred access 
node if needed.
No matter how many paths are defined between the host and the volume, all I/O traffic is 
serviced by only one node (the preferred access node). 
If you plan to use volume mirroring, put each copy in a different storage pool backed by 
different back-end storage subsystems for maximum availability. However, depending on your 
needs, it might be sufficient to use a different set of physical drives, a different storage 
controller, or a different back-end storage for each volume copy. Strive to place all volume 
copies in storage pools with similar performance characteristics. Otherwise, the volume 
performance as perceived by the host might be limited by the performance of the slowest 
storage pool.
3.11.1  Planning for image mode volumes
Use image mode volumes to present to hosts data that is written to the back-end storage 
before it was virtualized. An image mode volume directly corresponds to the MDisk from 
which it is created. Therefore, volume logical block address (LBA) x = MDisk LBA x. The 
capacity of image mode volumes is equal to the capacity of the MDisk from which it is 
created.
Image mode volumes are extremely useful tool in storage migration and when introducing 
IBM SAN Virtualization Controller to a storage environment.
3.11.2  Planning for thin-provisioned volumes
A thin-provisioned volume has a virtual capacity and a real capacity. Virtual capacity is the 
volume storage capacity that a host sees as available. Real capacity is the actual storage 
capacity that is allocated to a volume copy from a storage pool. Real capacity limits the 
amount of data that can be written to a thin-provisioned volume.
When planning use of thin-provisioned volumes, consider expected usage patterns for the 
volume. In particular, the actual size of the data and the rate of data change.
Thin-provisioned volumes require more I/Os because of directory accesses. For fully random 
access, and a workload with 70% reads and 30% writes, a thin-provisioned volume requires 
approximately one directory I/O for every user I/O. Additionally, thin-provisioned volumes 
require more processor processing, so the performance per I/O Group can also be reduced.
However, the directory is two-way write-back-cached (as with the Storwize V7000 fastwrite 
cache), so certain applications perform better. 
Also, the ability to thin-provision volumes can be a worthwhile tool that allows hosts to see 
storage space significantly larger than what is allocated within the storage pool. Thin 
provisioning can also simplify storage allocation management. You can define virtual capacity 
of a thinly provisioned volume to an application based on the future requirements, but allocate 
real storage based on today’s use.