IBM 990 - Manual (Page 82)

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zSeries 990 Technical Guide
Only Linux and z/VM operating systems can run in Linux Only mode:
Linux for zSeries uses 64-bit addressing and operates in the z/Architecture mode.
Linux for S/390 uses 31-bit addressing and operates in the ESA/390 Architecture mode.
z/VM can be configured to use 64-bit addressing and operate in the z/Architecture mode,
or to use 31-bit addressing and operate in the ESA/390 architecture mode.
2.2.9 Reserved storage
Reserved storage can optionally be defined to a logical partition allowing a non-disruptive
image memory upgrade for this partition. Reserved storage can be defined to both central
and expanded storage, and to any image mode except the Coupling Facility mode.
A logical partition must define an amount of central storage and, optionally (if not a Coupling
Facility image), an amount of expanded storage. Both central and expanded storages can
have two storage sizes defined: an initial value and a reserved value:
The initial value is the storage size allocated to the partition when it is activated.
The reserved value is an additional storage capacity beyond its initial storage size that a
logical partition can acquire dynamically. The reserved storage sizes defined to a logical
partition do not have to be available when the partition is activated. They are just
predefined storage sizes to allow a storage increase from the logical partition point of view.
Without the reserved storage definition, a logical partition storage upgrade is disruptive,
requiring:
Partition deactivation
An initial storage size definition change
Partition activation
The additional storage capacity to a logical partition upgrade can come from:
Any unused available storage
Another partition that has released some storage
A concurrent CPC memory upgrade
A concurrent logical partition storage upgrade uses Dynamic Storage Reconfiguration (DSR)
and the operating system must use the Reconfigurable Storage Units (RSUs) definition to be
able to add or remove storage units. Currently, only z/OS and OS/390 operating systems have
this support.
2.2.10 LPAR storage granularity
Storage granularity for Central Storage and Expanded Storage in LPAR mode varies as a
function of the total installed storage, as shown in Table 2-10 on page 71.
This information is required for Logical Partition Image setup and for z/OS and OS/390
Reconfigurable Storage Units definition.