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.  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. 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. 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. Subsequent sections of this chapter describe the preceding characteristics of volumes. 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.  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 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. Write I/O operations data flow Read I/O operations data flow 7.1.3 I/O operations data flow Chapter 7. Volumes  265