Chapter 3. Planning 67
– 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.
This rule does not apply when you intentionally place MDisks from different storage 
tiers in the pool with the intent to use Easy Tier to dynamically manage workload 
placement on drives with appropriate performance characteristics.
3.10.1  The storage pool and Storwize V7000 cache relationship
The Storwize V7000 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 individual overloaded back-end storage system from filling system write 
cache and degrading the performance of the other storage pools. For more information, 
see Chapter 2, “System overview” on page 9.
Table 3-3 lists the limit of the write-cache data that can be used by a single storage pool.
Table 3-3   Limit of the cache data
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 Storwize V7000 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 Storwize V7000 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.
3.11  Volume configuration
When planning a volume, consider the required performance, availability, and cost of storage 
backing that volume. Volume characteristics are defined by the storage pool in which it is 
created.
Every volume is assigned to an I/O Group that defines which pair of Storwize V7000 nodes 
will service I/O requests to the volume.
Number of storage pools Upper limit
11 0 0 %
2 066%
3 040%
4 030%
5 or more 025%
Important: No fixed relationship exists between I/O Groups and storage pools.