The SAN Volume Controller uses cache partitioning to limit the potential negative effects that  a poorly performing storage controller can have on the clustered system. The cache partition  allocation size is based on the number of configured storage pools. This design protects  against an individual overloaded back-end storage system filling system write cache and  degrading the performance of the other storage pools. For more information, see Chapter 2,  “System overview” on page 7. Table 3-3 shows the limit of the write-cache data that can be used by a single storage pool. No single partition can occupy more than its upper limit of write cache capacity. When the  maximum cache size is allocated to the pool, the SAN Volume Controller starts to limit  incoming write I/Os for volumes that are created from the storage pool. That is, the host writes  are limited to the destage rate, on a one-out-one-in basis.  Only writes that target the affected storage pool are limited. The read I/O requests for the  throttled pool continue to be serviced normally. However, because the SAN Volume Controller  is destaging data at a maximum rate that the back-end storage can sustain, read response  times are expected to be affected.  All I/O that is destined for other (non-throttled) storage pools continues as normal. Chapter 3. Planning  75 Number of storage pools Upper limit 1 100% 2 066% 3 040% 4 030% 5 or more 025% Table 3-3 Limit of the cache data 3.9.1 The storage pool and SAN Volume Controller cache relationship GLYPH<SM590000> Storage pool reliability, availability, and serviceability (RAS) considerations: GLYPH<SM590000> Storage pool performance considerations: – It might make sense to create multiple storage pools if you are attempting to isolate  workloads to separate disk drives.  – Create storage pools out of MDisks with similar performance. This technique is the only  way to ensure consistent performance characteristics of volumes created from the  pool. – Put image mode volumes in a dedicated storage pool or pools. – The storage pool goes offline if an MDisk is unavailable, even if the MDisk has no data  on it. Do not put MDisks into a storage pool until they are needed. – An alternative approach is to create few la rge storage pools. All MDisks that constitute  each of the pools should have the same performance characteristics. – The number and size of storage pools af fects system availability. Using a larger  number of smaller pools reduces the failure domain in case one of the pools goes  offline. However, an increased number of storage pools introduces management  overhead, impacts storage space use efficiency, and is subject to the configuration  maximum limit.