2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.4 Expanded Storage Concepts › 2.2.4.4.1 Overview of Expanded Storage

2.2.4.4.1 Overview of Expanded Storage

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 4K 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 storage.

In other respects, expanded storage is similar to high
performance paging devices such as the 2305-2 or 4305.
However, the system control program knows more about expanded
storage than it does about solid state paging devices.  It
uses this intelligence to optimize storage access, including
paging and swapping operations and special kinds of
application and system control data in ESA systems.

A primary use of expanded storage is as a substitute for
auxiliary storage (page/swap data sets on DASD) in situations
demanding better performance.  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 or 3390 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.

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



Expanded storage, therefore, added 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