The sum of all bitmap memory allocation for all functions except FlashCopy must not exceed  552 MiB. Other parameters for a volume that you can adjust include cache characteristics. Typically, a  volume’s read and write data is held in the cache of its preferred node. And a mirrored copy of  write data is held in the partner node of the same I/O Group. However, it is possible to create  a volume with different cache characteristics, if this is required. Cache setting of a volume can have the following values: Having cache-disabled volumes makes it possible to use the native copy services in the  underlying RAID array controller for MDisks (LUNs) that are used as IBM Spectrum Virtualize  Image-mode volumes. However, using IBM Spectrum Virtualize Copy Services rather than  the underlying disk controller copy services gives better results. Note:  Disabling volume cache is a prerequisite for using native copy services on  image-mode volumes that are defined on storage systems that are virtualized by IBM  Spectrum Virtualize. Consult with IBM Support before you turn off the cache for volumes in  your production environment to avoid performance degradation. For each volume there are two parameters that describe its capacity: In a  fully allocated  volume, these two values are the same. In a  thin-provisioned  volume, the  real capacity can be as little as a few percent of virtual capacity. The  real capacity  is used to  store the user data and in the case of thin-provisioned volumes, metadata of the volume. The  real capacity can be specified as an absolute value, or as a percentage of the virtual capacity. Thin-provisioned volumes can be used as volumes that are assigned to the host, by  FlashCopy to implement thin-provisioned FlashCopy targets. When you create a mirrored  volume, you can create a thin-provisioned volume as a second volume copy, whether the  primary copy is a fully or thin-provisioned volume.  When a thin-provisioned volume is initially created, a small amount of the real capacity is  used for initial metadata. This metadata holds a mapping of a logical address in the volume to  a  grain  on a physically allocated extent. When a write request comes from a host, the block  address for which the write is requested is checked against the mapping table. There might  be a previous write to a block on the same grain, as the incoming request. Chapter 7. Volumes  273 GLYPH<SM590000> The real physical capacity that is allocated to the volume from the storage pool;  determines how many MDisk extents are initially allocated to the volume. GLYPH<SM590000> The virtual capacity that is reported to the host and to IBM Spectrum Virtualize  components (for example, FlashCopy, cache, and remote copy).  7.1.8 Fully allocated and thin-provisioned volumes GLYPH<SM590000> readwrite . All read and write I/O operations that are performed by the volume are stored in  cache. This is the default cache mode for all volumes. GLYPH<SM590000> readonly . Only read I/O operations that are performed on the volume are stored in cache. GLYPH<SM590000> disabled . No I/O operations on the volume are stored in cache. I/Os are passed directly to  the back-end storage controller rather than being held in the node’s cache.  7.1.7 Volume cache mode