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GLYPH<SM590000> Storage pool reliability, availability, and serviceability (RAS) considerations: 

– 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. 

– 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 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. 

– Put image mode volumes in a dedicated storage pool or pools. 

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. 

## 3.9.1 The storage pool and SAN Volume Controller cache relationship 

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. 

<img src='content_image/998312.jpg'>

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. 

