GLYPH<SM590000> Can be configured to have single data copy, or mirrored to make them resistant to disk subsystem failures and/or to improve the read performance. 

GLYPH<SM590000> Can be compressed. 

GLYPH<SM590000> Can be configured as VMware vSphere Virtual Volumes. Such volumes are sometimes referred to as VVols. They allow 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. 

These volumes have two major modes: managed mode and image mode. For managed-mode volumes, two policies define how the extents of the volume are allocated from the storage pool: the sequential policy and the striped policy. 

GLYPH<SM590000> 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. 

Subsequent sections of this chapter describe the preceding characteristics of volumes. 

## 7.1.3 I/O operations data flow 

Volumes can have one or two copies. A mirrored volume looks to its users exactly the same as one with a single copy. However, there are some differences 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. By default, the host uses only paths to this node to communicate I/O requests. 

## Read I/O operations data flow 

If the volume is mirrored, that is, there are two copies of the volume, 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 

GLYPH<SM590000> For non-mirrored volumes, there is only one volume copy. Therefore, there is no choice 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 266 shows the data flow for this scenario. 

