Chapter 7. Volumes 251
7.1.7  Volume cache mode
Another adjustable volume parameter is its cache characteristics. Under normal conditions, a 
volume’s read and write data is held in the cache of its preferred node, with a mirrored copy of 
write data 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:
/SM590000readwrite: 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.
/SM590000readonly: Only read I/O operations that are performed on the volume are stored in cache.
/SM590000disabled: 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. 
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. The use of IBM Spectrum Virtualize Copy Services rather than the 
underlying disk controller copy services gives better results.
7.1.8  Fully allocated and thin-provisioned volumes
For each volume the following parameters describe its capacity:
/SM590000The real physical capacity that is allocated to the volume from the storage pool; 
determines how many MDisk extents are initially allocated to the volume.
/SM590000The virtual capacity that is reported to the host and to IBM Spectrum Virtualize 
components (for example, FlashCopy, cache, and remote copy). 
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 creating a mirrored 
volume, a thin-provisioned volume can be created as a second volume copy, whether the 
primary copy is a fully or thin-provisioned volume. 
When a thin-provisioned volume is 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. If a previous write to a block on the same grain exists as 
the incoming request, physical storage was allocated for this logical block address and can be 
used to service the request; otherwise, new physical grain is allocated to store the data, and 
the mapping table is updated to record that allocation. 
Note: Disabling volume cache is a prerequisite for the use of native copy services on 
image mode volumes that are defined on storage systems that are virtualized by IBM 
Spectrum Virtualize. Contact IBM Support before turning off the cache for volumes in your 
production environment to avoid performance degradation.