Chapter 7. Volumes 243
/SM590000Volumes can be compressed.
/SM590000Volumes can be configured as VMware vSphere Virtual Volumes, sometimes referred to 
as VVols, which allows VMware vCenter to manage storage objects. The storage system 
administrator can create these objects and assign ownership to VMware administrators to 
allow them to manage these objects.
Volumes have two major modes: Managed mode and image mode. For managed mode 
volumes, the sequential policy and the striped policy define how the extents of the volume 
are allocated from the storage pool.
/SM590000Volumes can be replicated synchronously or asynchronously. An IBM Spectrum Virtualize 
system can maintain active volume mirrors with a maximum of three other IBM Spectrum 
Virtualize systems, but not from the same source volume.
These volume characteristics are described next.
7.1.3  I/O operations data flow
Volumes can have one or two copies. Although a mirrored volume looks to its users the same 
as a volume with a single copy, some differences exist in how I/O operations are performed 
internally for volumes with single or two copies.
When host requests an I/O operation to any IBM Spectrum Virtualize volume type, the 
multipath driver on the host identifies the preferred node for the volume and by default uses 
only paths to this node to communicate I/O requests. 
Read I/O operations data flow
If the volume is mirrored (that is, two copies of the volume exist), one copy is known as the 
primary copy. If the primary is available and synchronized, reads from the volume are 
directed to that copy. The user selects which copy is primary when the volume is created, but 
can change this setting at any time. In the management GUI, an asterisk indicates the 
primary copy of the mirrored volume. Placing the primary copy on a high-performance 
controller maximizes the read performance of the volume
For non-mirrored volumes, only one volume copy exists; therefore, no choice exists for read 
source, and all reads are directed to the single volume copy.
Write I/O operations data flow
For write I/O operations to a mirrored volume, the host sends the I/O request to the preferred 
node, which is responsible for destaging the data from cache. Figure 7-1 on page 244 shows 
the data flow for this scenario.