Tip:  Migrating volumes across I/O groups can be a disruptive action. Therefore, specify  the correct I/O group at the time the volume is created. Also, when a volume is being created, it is possible to define a list of I/O groups in which a  volume can be accessible to hosts. It is recommended that a volume is accessible to hosts  only by the caching I/O group. You can have more than one I/O group in the access list of  a volume in some scenarios with specific requirements, like when a volume is being  migrated to another I/O group. GLYPH<SM590000> By default, the  preferred node , which owns a volume within an I/O group, is selected for a  load balancing  basis. Even though it is not easy to expect the workload when the volume  is created, you should distribute the workload evenly on each node within an I/O group. GLYPH<SM590000> With the exception of a few cases, cache mode of a volume should be set to readwrite.  More details can be found in section 5.12, “Volume cache mode” on page 135. GLYPH<SM590000> The maximum number of volumes per I/O group and system is described in Table 5-1. Table 5-1 Maximum number of volumes in IBM FlashSystem 9100 Volume type Maximum  number Comments Volumes (VDisks) per system 10,000 Each basic volume uses 1 VDisk, each with one  copy. HyperSwap volumes per  system 1,250 Each HyperSwap volume uses 4 VDisks, each with  one copy, 1 active-active remote copy relationship  and 4 FlashCopy mappings. Volumes per I/O group (volumes per caching I/O  group) 10,000 Volumes accessible per I/O  group  10,000 Thin-provisioned  (space-efficient) volume copies  in regular pools per system 8,192 Compressed volume copies in  regular pools per system 2,048 Maximum requires a system containing four  control enclosures; see the Compressed volume  copies in regular pools per I/O group limit below Compressed volume copies in  regular pools per I/O group 512 With 32 GB Cache upgrade and 2nd Compression  Accelerator card installed. Compressed volume copies in  data reduction pools per system - No limit is imposed here beyond the volume copy  limit per data reduction pool Compressed volume copies in  data reduction pools per I/O  group - No limit is imposed here beyond the volume copy  limit per data reduction pool Deduplicated volume copies in  data reduction pools per system - No limit is imposed here beyond the volume copy  limit per data reduction pool Deduplicated volume copies in  data reduction pools per I/O  group - No limit is imposed here beyond the volume copy  limit per data reduction pool Chapter 5. Volumes  105