A memory upgrade is considered to be concurrent when it requires no change of the physical memory cards. A memory card change is disruptive when no use is made of Enhanced Drawer Availability (EDA). In a multiple CPC drawer system, a single CPC drawer can be concurrently removed and reinstalled for a repair with EDA. 

For model upgrades involving the addition of a CPC drawer, the minimum usable memory increment (256 GB) is added to the system. During an upgrade, adding a CPC drawer is a concurrent operation, as is adding physical memory in the new drawer. 

## Concurrent memory upgrade 

If physical memory is available, memory can be upgraded concurrently by using Licensed Internal Code Configuration Control (LICCC). 

## Redundant array of independent memory 

RAIM technology makes the memory subsystem (in essence) a fully fault-tolerant N+1 design. The RAIM design automatically detects and recovers from failures of dynamic random access memory (DRAM), sockets, memory channels, or dual inline memory modules (DIMMs). 

The RAIM design is fully integrated in the z15, and was enhanced to include one Memory Controller Unit (MCU) per processor chip, with five memory channels and one DIMM per channel. A fifth channel in each MCU enables memory to be implemented as RAIM. This technology has significant RAS capabilities in the area of error correction. Bit, lane, DRAM, DIMM, socket, and complete memory channel failures (including many types of multiple failures) can be detected and corrected. 

For more information about memory design and configuration options, see IBM z15 (8561) Technical Guide , SG24-8851. 

## 2.3.4 Hardware system area 

The HSA is a fixed-size, reserved area of memory that is separate from the client-purchased memory. The HSA is used for several internal functions, but the bulk of it is used by channel subsystem functions. 

The fixed-size 256 GB HSA for z15 is large enough to accommodate any LPAR definitions or changes, which eliminates most outage situations and the need for extensive planning. 

A fixed, large HSA allows the dynamic I/O capability of the z15 to be enabled by default. It also enables the dynamic addition and removal of the following features: 

GLYPH<SM590000> LPAR to new or existing channel subsystem (CSS) 

GLYPH<SM590000> CSS (up to six can be defined in z15) 

GLYPH<SM590000> Subchannel set (up to four can be defined in z15) 

GLYPH<SM590000> A total of 85 logical partitions 

GLYPH<SM590000> Devices, up to the maximum number permitted, in each subchannel set 

GLYPH<SM590000> Logical processors by type 

GLYPH<SM590000> Cryptographic adapters 

