22 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
The lower cache delivers the following additional functions:
/SM590000Ensures that the write cache between two nodes is in sync
/SM590000Caches partitioning to ensure that a slow back end cannot use the entire cache
/SM590000Uses a destage algorithm that adapts to the amount of data and the back-end 
performance
/SM590000Provides read caching and prefetching
Combined, the two levels of cache also deliver the following functions:
/SM590000Pins data when the LUN goes offline
/SM590000Provides enhanced statistics for IBM Tivoli® Storage Productivity Center, and maintains 
compatibility with an earlier version
/SM590000Provides trace for debugging
/SM590000Reports medium errors
/SM590000Resynchronizes cache correctly and provides the atomic write functionality
/SM590000Ensures that other partitions continue operation when one partition becomes 100% full of 
pinned data
/SM590000Supports fast-write (two-way and one-way), flush-through, and write-through
/SM590000Integrates with T3 recovery procedures
/SM590000Supports two-way operation
/SM590000Supports none, read-only, and read/write as user-exposed caching policies
/SM590000Supports flush-when-idle
/SM590000Supports expanding cache as more memory becomes available to the platform
/SM590000Supports credit throttling to avoid I/O skew and offer fairness/balanced I/O between the 
two nodes of the I/O Group
/SM590000Enables switching of the preferred node without needing to move volumes between I/O 
Groups
Depending on the size, age, and technology level of the disk storage system, the total 
available cache in the system can be larger, smaller, or about the same as the cache that is 
associated with the disk storage. Because hits to the cache can occur in either the IBM 
Storwize V7000 or at the disk controller level of the overall system, the system as a whole can 
take advantage of the larger amount of cache wherever the cache is located. Therefore, if the 
storage controller level of the cache has the greater capacity, expect hits to this cache to 
occur, in addition to hits in the IBM Storwize V7000 cache.
Also, regardless of their relative capacities, both levels of cache tend to play an important role 
in enabling sequentially organized data to flow smoothly through the system. The IBM 
Storwize V7000 cannot increase the throughput of the underlying disks in all cases because 
this increase depends on both the underlying storage technology and the degree to which the 
workload exhibits hotspots or sensitivity to cache size or cache algorithms.
IBM Spectrum Virtualize V7.3 introduced a major upgrade to the cache code. In association 
with IBM Storwize V7000 Gen2 hardware, it provided another cache capacity upgrade.
At V8.1, these limits were enhanced with IBM Storwize V7000 Gen2+. Before this release, the 
IBN SAN Volume Controller memory manager (PLMM) only addressed 64 GB of memory. 
In V8.1, the underlying PLMM was rewritten and the structure size increased. The cache size 
can be now upgraded up to 128 GB per canister and the entire memory can now be used.