6. DATA SOURCES › 6.4 Expanded Storage Concepts › 6.4.1 Overview

6.4.1 Overview

Expanded storage is optional, high-speed semiconductor
storage initially made available for the IBM 3090 processor
series.  The storage is installed in the central processor
complex; that is, it is an integrated part of the complex and
not a stand-alone piece of equipment.

In some respects, expanded storage can be thought of as a
slower speed extension of central storage.  The two differ in
that expanded storage is not directly addressable and can be
accessed only by moving 4 KB blocks of data from expanded
storage into central storage.  All data movement to and from
expanded storage must be through central storage.  There is
no way to directly transfer data between expanded storage and
auxiliary or online storage.

In other respects, expanded storage is similar to high-
performance paging devices.  However, the system control
program knows more about expanded storage than it does about
solid state paging devices, and uses this intelligence to
optimize storage access, including paging and swapping
operations as well as special kinds of application and system
control data.

z/OS in z/Architecture mode does not support expanded storage
anymore.  The hiperspace APIs and internal uses of expanded
storage are reimplemented to use real storage.

The primary use of expanded storage is as a substitute for
auxiliary storage (page/swap data sets on DASD) in situations
demanding higher speed.  The time it takes to read a page
from auxiliary to central storage in a well-tuned system may
range from a few milliseconds (with cached DASD controllers
or solid-state channel-attached devices on dedicated paging
channels) to 20 to 30 milliseconds for non-cached 3380 DASD
on non-dedicated channels.  Indirect measurements suggest
that typical times for page transfers from expanded to
central storage take something less than 100 microseconds.
This represents a reduction in page resolution time of
approximately two orders of magnitude.

Expanded storage, therefore, adds a new level to the storage
hierarchy between central and auxiliary storage.  Now, page
movement within the storage hierarchy can be divided into the
following categories:

    From         To           Terminology
    ---------    ---------    ----------------------
    Central      Auxiliary    Page-out or swap-out
    Auxiliary    Central      Page-in or swap-in
    Central      Expanded     Expanded storage write
    Expanded     Central      Expanded storage read
    Expanded     Auxiliary    Page migration

When expanded storage was first introduced, the system
control program used it exclusively for paging and swapping.
New SCP versions have introduced additional uses of expanded
storage, such as virtual I/O (VIO) and hiperspaces (both
conventional and expanded-storage-only).