IBM DS8000 manual Number of iSeries Fibre Channel adapters, Size and number of LUNs

some benefit in larger cache sizes. However, in general, with large iSeries main memory sizes, OS/400 Expert Cache can reduce the benefit of external cache.

Number of iSeries Fibre Channel adapters

The most important factor to take into consideration when calculating the number of Fibre Channel adapters in the iSeries is the throughput capacity of the adapter and IOP combination.

Since this guideline is based only on iSeries adapters and Access Density (AD) of iSeries workload, it doesn't change when using the DS8000. (The same guidelines are valid for ESS 800.)

Note: Access Density is the capacity of occupied disk space divided by the average I/O /sec. These values can be obtained from the OS/400 System, Component and Resource Interval performance reports.

Table B-2shows the approximate capacity which can be supported with various IOA/IOP combinations.

Table B-2 Capacity per I/O Adapter

I/O Adapter	I/O Processor	Capacity per IOA	Rule of thumb
2787	2844	1022/AD	500GB
2766	2844	798/AD	400GB
2766	2843	644/AD	320GB

For most iSeries workloads, Access Density is usually below 2, so if you do not know it, the Rule of thumb column is a typical value to use.

Size and number of LUNs

As discussed in “Logical volume sizes” on page 374, OS/400 can only use fixed logical volume sizes. As a general rule of thumb, we recommend that you should configure more logical volumes than actual DDMs. As a minimum, we recommend 2:1. For example, with 73 GB DDMs, you should use a maximum size of 35.1 GB LUNs. The reason for this is that OS/400 does not support command tag queuing. Using more, smaller LUNs can reduce I/O queues and wait times by allowing OS/400 to support more parallel I/Os.

From the values in Table B-2,you can calculate the number of iSeries Fibre Channel adapters for your required iSeries disk capacity. As each I/O adapter can support a maximum of 32 LUNs, divide the capacity per adapter by 32 to give the approximate average size of each LUN.

For example, assume you require 2 TB capacity and are using 2787 I/O adapters with 2844 I/O processors. If you know the access density, calculate the capacity per I/O adapter, or use the rule-of-thumb. Let’s assume the rule-of-thumb of 500 GB per adapter. In this case, we would require four I/O adapters to support the workload. If we were able to have variable LUNs sizes, we could support 32 15.6 GB LUNs per I/O adapter. However, since OS/400 only supports fixed volume sizes, we could support 28 17. 5 GB volumes to give us approximately 492 GB per adapter.

398DS8000 Series: Concepts and Architecture