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Chapter 12. Performance considerations 267
Figure 12-10 Spreading data across ranks
The stripe size
Each striped logical volume that is created by the host’s logical volume manager has a stripe
size that specifies the fixed amount of data stored on each DS8000 logical volume (LUN) at
one time.
12.4.5 Determining the number of connections between the host and DS8000
When you have determined your workload requirements in terms of throughput, you have to
choose the appropriate number of connections to put between your open systems and the
DS8000 to sustain this throughput.
A Fibre Channel host port can sustain a maximum of 206 MB/s data transfer. As a general
recommendation, you should at least have two FC connections between your hosts and your
DS8000.
Note: The recommendation is to use host striping wherever possible to distribute the
access patterns across the physical resources of the DS8000.
Note: The stripe size has to be large enough to keep sequential data relatively close
together, but not too large so as to keep the data located on a single array.
The recommended stripe sizes that should be defined using your host’s logical volume
manager are in the range of 4 MB to 64 MB.
You should choose a stripe size close to 4 MB if you have a large number of applications
sharing the arrays and a larger size when you have very few servers or applications
sharing the arrays.
Extent Pool
Rank 5
Rank 6
Rank 7
Rank 8
8GB LUN
Extent
1GB
Rank 1
Extent pool 1
Rank 3
Rank 4
2
GB LUN 1
Extent
1GB
Extent Pool 5
Rank 2
Non-balanced implementation: LUNs across ranks
More than 1 rank per extent pool
2
GB LUN 2
2GB LUN 3
2GB LUN 4
Extent pool 2Extent pool 2
Extent pool 3
Extent pool 4
Balanced implementation: LVM striping
1 rank per extent pool
LV stripped across 4 LUNs