GLYPH<SM590000> Communication with and control of optional expanded storage 

GLYPH<SM590000> Error checking and correction 

Main storage can be accessed by all processors, but cannot be shared between LPARs. Any system image (LPAR) must include a defined main storage size. This defined main storage is allocated exclusively to the LPAR during partition activation. 

## 3.6.3 Hardware system area 

The HSA is a non-addressable storage area that contains system LIC and configuration-dependent control blocks. On z15 servers, the HSA has a fixed size of 256 GB and is not part of the purchased memory that you order and install. 

The fixed size of the HSA eliminates planning for future expansion of the HSA because the hardware configuration definition (HCD)/input/output configuration program (IOCP) always reserves space for the following items: 

GLYPH<SM590000> Six channel subsystems (CSSs) 

GLYPH<SM590000> A total of 15 LPARs in CSSs 1 through 5, and 10 LPARs for the sixth CSS for a total of 85 LPARs 

GLYPH<SM590000> Subchannel set 0 with 63.75-K devices in each CSS 

GLYPH<SM590000> Subchannel set 1 with 64-K devices in each CSS 

GLYPH<SM590000> Subchannel set 2 with 64-K devices in each CSS 

GLYPH<SM590000> Subchannel set 3 with 64-K devices in each CSS 

The HSA features sufficient reserved space to allow for dynamic I/O reconfiguration changes to the maximum capability of the processor. 

## 3.6.4 Virtual Flash Memory (FC 0643) 

IBM Virtual Flash Memory (VFM, FC 0643) is the replacement for the Flash Express features that were available on the IBM zEC12 and IBM z13. No application changes are required to change from IBM Flash Express to VFM. 

IBM VFM provides up to 6.0 TB of virtual flash memory in 512 GB increments. The minimum is 0, while the maximum is 12 features. The number of VFM features ordered reduces the maximum orderable memory for the z15 server. 

