z15 can be configured with of a maximum of five CPC drawers that are designed as a field replaceable unit (FRU). Connections among the CPC drawers are established through symmetric multiprocessing (SMP) cables. Each CPC drawer consists of two logical PU (processor units) clusters and contains four central processor (CP) single-chip modules (SCMs) and one storage controller (SC) SCM. In addition to SCMs, CPC drawers host memory DIMMs, connectors for I/O, oscillator interface, and manifolds. 

z15 inherits I/O infrastructure reliability improvements from z14, including Forward Error Correction (FEC) technology that enables better recovery of FICON channels. FICON Express16SA and FICON Express16S+ features continue to provide a new standard for transmitting data over 16 Gbps links by using 64b/66b encoding. 

The z15 air-cooled configuration includes a newly designed front-to-rear radiator cooling system. The radiator pumps, blowers, controls, and sensors are N+2 redundant. In normal operation, one active pump supports the system. A second pump is turned on and the original pump is turned off periodically, which improves reliability of the pumps. The replacement of pumps or blowers is concurrent with no affect on performance. 

A water-cooling system is also an option with the z15, with water-cooling unit (WCU) technology. Two redundant WCUs run with two independent chilled water feeds. One WCU and one water feed can support the entire system load. The water-cooled configuration is backed up by the rear door heat exchangers in the rare event of a problem with the chilled water facilities of the customer. 

RAS also includes the following enhancements: 

GLYPH<SM590000> Integrated sparing 

GLYPH<SM590000> Error detection and recovery improvements in caches and memory 

GLYPH<SM590000> 25GbE RoCE Express2 

GLYPH<SM590000> PCIe coupling links 

GLYPH<SM590000> Enhanced channel logging 

GLYPH<SM590000> A fixed HSA with its size increased to 256 GB on the z15 

GLYPH<SM590000> OSA firmware changes to increase the capability of concurrent maintenance change level (MCL) updates 

GLYPH<SM590000> System power cycle 

GLYPH<SM590000> CFCC level 24 

GLYPH<SM590000> IBM RMF reporting 

The z15 continues to support concurrent addition of resources, such as processors or I/O cards, to an LPAR to achieve better serviceability. If another SAP is required on a z15 (for example, as a result of a disaster recovery situation), the SAPs can be concurrently added to the CPC configuration. 

It is possible to concurrently add CP, zIIP, IFL, and ICF processors to an LPAR. This function is supported by z/VM, and (with appropriate PTFs) by z/OS and z/VSE. It is possible to add concurrently memory to an LPAR as well. This feature is supported by z/OS and z/VM. 

The z15 supports adding Crypto-Express features to an LPAR dynamically by changing the cryptographic information in the image profiles. Users can also dynamically delete or move Crypto-Express features. This enhancement is supported by z/OS, z/VM, and Linux on IBM Z. 

