IBM DS8000 manual SAN fabric

use fast-write, in which the data is written to volatile memory on one complex and persistent memory on the other complex. The server then reports the write as complete before it has been written to disk. This provides much faster write performance. Persistent memory is also called NVS or non-volatile storage.

Processor Complex 0

SAN fabric

Host ports

Processor Complex 1

Volatile
memory
Persistent memory
N-way	RIO-G
SMP

Host adapter

in I/O enclosure

First RIO-G loop

Device adapter in I/O enclosure

Host adapter

in I/O enclosure

Device adapter in I/O enclosure

	Volatile
	memory
Persistent memory
G	N-way
RIO-
	SMP

Front storage enclosure with 16 DDMs

Figure 2-3 DS8000 architecture

Fibre channel switch Fibre channel switch

Fibre channel switch Fibre channel switch

Rear storage enclosure with 16 DDMs

When a host performs a read operation, the servers fetch the data from the disk arrays via the high performance switched disk architecture. The data is then cached in volatile memory in case it is required again. The servers attempt to anticipate future reads by an algorithm known as SARC (Sequential prefetching in Adaptive Replacement Cache). Data is held in cache as long as possible using this smart algorithm. If a cache hit occurs where requested data is already in cache, then the host does not have to wait for it to be fetched from the disks.

Both the device and host adapters operate on a high bandwidth fault-tolerant interconnect known as the RIO-G. The RIO-G design allows the sharing of host adapters between servers and offers exceptional performance and reliability.

Chapter 2. Components 23