Capacity sizing in IBM i
The IBM i operating system (formerly known as the OS/400) addresses data as 520-byte blocks that include an 8-byte header reserved for metadata. In other words, the usable space for actual
data is 512 bytes per segment, because an IBM i host consists of 520 bytes with 512 bytes user data and 8 bytes metadata.
This implementation means that every 4096 bytes of user data written consumes 4608 bytes of disk space.
In summary, the reported usable IBM i capacity (provisioned capacity as seen by the IBM i operating system) is approximately 89% of the effective capacity as configured in the storage
system.
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Rule of thumb: To calculate the IBM System i capacity for IBM Storage Virtualize based storage systems, simply
multiply by 0.889 (8/9) the capacity that Storage Modeller reports for the desired configuration.
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The 520-to-512-byte mapping is currently supported with all IBM Storage Virtualize systems (all models of San Volume Controller. former Storwize systems, and System 9100 and 9200).
IBM i attachment to a System 900 system (which is a legacy system, no longer available for selection in Storage Modeller) is only supported with 4096-byte sector volumes. In this case, IBM
i compresses a subset of its main memory pages — including the metadata — so they fit into a 4096-byte sector. In this case, when the System 900 storage is not managed by IBM Storage
Virtualize, no capacity adjustment is required.
The DS8000 can present volumes with 520-byte sectors, and this format can be chosen when creating the volume. In this case, no capacity adjustment is required.
Also see the following data import topics, which touch on IBM i issues: