2. PERFORMANCE REPORTING ANALYSIS › 2.3 Auxiliary Storage Management Analysis › 2.3.4 MVS Concepts

2.3.4 MVS Concepts

The Auxiliary Storage Manager (ASM) maintains copies of all
virtual storage pages that are not currently assigned to a
real memory address.  ASM controls two types of data sets:

    o  Paging space data sets
    o  Swapping space data sets

To provide as much parallel operation as possible and to
spread out the I/O overhead associated with paging, the
following classes of page data sets are supported by MVS:

    o  PLPA -- Provides primary storage for paged-out
       portions of pageable link pack area modules.

    o  Common -- Provides primary storage for paged-out
       portions of the common system area.

    o  Duplex -- When duplexing is specified, a secondary
       copy of common system area and PLPA pages is
       maintained in the duplex data set.  This provides for
       additional reliability.  Initially, the PLPA is
       written to both the duplex and PLPA data set.
       Subsequently, all common system area page-outs are
       written to both the duplex and common data set.  When
       an unrecoverable read error occurs in the common or
       PLPA data set, ASM attempts to read that page from the
       duplex data set.

    o  Local -- The local page data sets hold all the paged-
       out private area virtual storage of individual address
       spaces that are in real memory (i.e., swapped-in), as
       well as most of  the pages of swapped-out address
       spaces.  Precisely which private area pages are
       directed to paging space data sets and which are
       directed to swapping space data sets during a swap
       varies with the different levels of  the Auxiliary
       Storage Manager.  Section 2.3.4.3.1., Physical Swap
       Algorithm, discusses the swap process for MVS/SP 1.3
       and MVS/XA.  Virtual I/O (VIO) data sets (special data
       sets accessed by applications that use paging to
       provide I/O functions) are also contained in the local
       data sets.

Swap data sets may be provided to speed up swap processing.
They contain all local system queue area (LSQA) private area
virtual pages and, when the extended swap IUP or MVS/SP 1.3
(or beyond) is used, the swap-in working set of all swapped-
out address spaces.

If swap data sets are not defined, LSQA pages (and any other
pages directed to the swap data sets) are written to the
local page data sets.  This normally interferes with the
demand paging I/O and, depending on the level of the system,
may cause response degradation.  In an MVS/XA system with a
well-configured paging subsystem using 3380s, the amount of
degradation is probably very small, if any, depending on the
amount of swapping.

Section 2.3.2, Usage Guidelines, discusses tuning ASM
configurations for the different paging and swapping
algorithms and explains the use of the Auxiliary Storage
Management reports in the analysis of the ASM configuration.

This concepts section discusses the swapping and paging
algorithms used by ASM.  The following sections explain ASM
operation, the management of page traffic, and the principles
of tuning an ASM configuration:

    1 - Auxiliary Storage Management Overview
    2 - Paging
    3 - Swapping