This article summarizes the key enhancements to VSAM (Virtual Sequential Access Method) from DFSMS 1.3 to DFSMS 1.12. Some major enhancements include empty control area reclamation to improve space reuse, partial release for multi-volume datasets, extended addressing to support larger datasets, and VSAM RLS (Record Level Sharing) support utilizing coupling facility caches to enable data sharing across systems. Other enhancements focused on extending capacity limits and improving functions like system managed buffering.

1. VSAM NEW (NOT SO) FEATURES – FROM DFSMS 1.3 TO 1.12
Journal of Computer Resource Management
Prepared by the CMG Publications Committee
Journal of Computer Resource Management, a publication of CMG
August, 2011 – Alvaro Guimaraes Salla
This article describes briefly the new features of Virtual Sequential Access Method
(VSAM) a component of DFSMS. It encompasses the most important VSAM
enhancements from DFSMS 1.3 to DFSMS 1.12. The sequencing of the items is
from the most important to the least one, in the author´s opinion.
In the early 1970s, Virtual Sequential Access Method
(VSAM) was introduced by IBM as a collection of three ENHANCEMENTS DESCRIPTION
data set organizations — sequential, indexed, and
direct-access, together with the access method Follows the description of the VSAM enhancements.
software and utilities to be used on the mainframe IBM
operating systems. General functions
The word virtual means only that VSAM was
introduced at approximately the same time as the Empty CA reclaim. Orphan data control areas are
initial IBM virtual storage operating systems OS/VS1 control areas with lots of free space (or totally free)
and OS/VS2. Since then VSAM has been continually located below the HURBA mark. They orphans are
improved and enhanced. created through the following scenario: if at load time
an specific data CA was loaded with records
. belonging to a specific key range and later on the
majority of these records are deleted (and the same
VSAM, AN ACCESS METHOD OF z/OS key value is not re-utilized), as the example of a
timestamp key, the CA is under populated, and this
free space is never reclaimed for records with keys
VSAM is a z/OS access method, that is, a set of
out of such specific and original key range. This set of
programs written by IBM to help application code to
keys (like the timestamp ones) are called creeping
access data through I/O operations. keys, the ones that continues to grow in its numeric
The access method runs under the application task value. Traditionally, VSAM was not designed to
that invokes it using a branch instruction (no PSW process well creeping keys. The solution is to add
change status, such as, problem mode, PSW Key)). these empty CAs to a free CA list (that originally
contains the ones above HURBA) when VSAM
VSAM Functionality realizes the last original record is deleted. This
function applies only to KSDS clusters and to ICF
The major access method functions are: catalogs. It is important to know that for an existing
Blocking by clustering logical records in one VSAM data set, by just enabling CA reclaim, does not
physical block reclaim the previous internal CAs totally free. Only the
Buffering by managing the buffer pool ones that will be totally freed in the future, will be
efficiently reclaimed. This function will even decrease the need
An access method understands the way of defrags.
logical records are organized on the 3390 VSAM Partial Release on multiple volumes data set.
tracks. Then, it is in charge of writing virtual Partial release was only available for sequential data
sets. Later was made available only for VSAM, but
channel programs for accessing such records.
for single volume data sets.
Invoking the I/O driver through an SVC
Ignore DEFINE parameters IMBED, REPLICATE,
(usually SVC 00 – also named EXCP) KEYRANGE, ORDERED options.
Synchronize through the Wait function, the Health Check to detect and identify catalogs with
running task with the end of the I/O operation IMBED and REPLICATE.
(if needed). DFSMS Data Set Separation, to allocate data sets in
Execute I/O error recovery. distinct physical DASD controllers.

2. Media Manager the unique VSAM I/O driver, existing data sets. Through data sharing implemented by
exception for Improved Control Interval Processing the use of coupling facility structures VSAM RLS offers a
(ICIP) high level of continuous availability. Follows a list of RLS
enhancements:
Extending the capacity of VSAM clusters Use of RLS coupling facility cache structure for storing
data sets with CI size bigger than 4KB.
All the following enhancements extends the capacity of Use of RLS coupling facility cache structure for all data
VSAM clusters. or some accordingly with DB manager option
Extended addressability for VSAM KSDS. It increases RLS system-managed duplexing rebuild process for
RBA from 4-byte to 8-byte allowing VSAM cache structures.
components cross the 4-GB limit capacity. For the RLS Coupling Facility structure system-managed
majority of the applications this is done with total duplexing.
compatibility. Extended addressability when Data buffer pools may be allocated above the bar
announced was not available for KSDS VSAM RLS DIAG command to find latch contentions
organization. Enhanced RLS recovery.
SMF performance data of VSAM RLS usage of 64-bit
RLS support of extended addressability of VSAM data buffers
KSDS. As stated at title, Extended Addressability is Speeding up the SMSVSAM address address space
now available for KSDS organization running with with TERMINATESERVER command
RLS buffering. Health Checks for VSAM RLS: indicating latch
Extended addressability for all VSAM data sets contention and a single point of failure (SPOF) in the
More than 255 extents for SMS-managed VSAM data SHCDS
sets. The increase of the maximum number of Support for VSAM striping in RLS mode
extents is a must to avoid frequent defrags. Follows
a list of such limits: System Managed Buffering (SMB) enhancements
o Sequential: 16 per volume.
o Extended-format sequential:123.
o PDS: 16, SMB is a function available to all VSAM data sets types
o PDSE: 123. opened for NSR buffering processing. SMB enables
o Non-system-managed VSAM up to 255
VSAM to determine the optimum number of index and
per component. If System-managed limit
data buffers, as well as the type of buffer management:
removed if DC has extent constraint
removal. LSR (LRU, lots of index buffers, lots of data buffers
o Non-system-managed VSAM can have up containing up to 20% of all data records, no look-ahead)
to 255 extents per stripe. If System- or NSR (discard the ones already processed, just one
managed VSAM the limit removed if DC index buffer, look-ahead). SMB boost the performance
has extent constraint removal specified. mainly of random NSR accesses. The late
o An HFS can have 123 extents on each enhancements are:
volume
Extended Address Volume (EAV) for VSAM data sets
SMB initially for VSAM KSDS clusters.
allowing the allocation of such data sets at cylinders
SMB retry capability for Direct Option (DO) access bias.
beyond 64K addressing at EAV volumes.
SMB support for AIX® files.
Maximum Volume Count, allows volumes be added
Limit the SMB buffer size for Direct Option (DO) at data
dynamically to a VSAM data set without manual
Class SMS construct. It avoids to have up to 20% of
intervention and taking down the application.
all data in data buffers.
Real addresses greater than 2 GB available for all
Enhancement in SMB for DO: buffers for index
VSAM data sets. Allowing pages above the bar
allocated based on SMBVSP value or data set size.
being located at frames above 2-GB.
Data Stripping enhancements
VSAM Data Sharing (RLS)
Usually, in a multi-extent, multi-volume VSAM data set
VSAM RLS is another mode of managing buffer pools,
processed in sequential access, processing does not allow for
which allows any number of applications within your
any type of parallelism for I/O operations among the volumes.
Parallel Sysplex to share your existing VSAM clusters.
This means that when an I/O operation is executed for an
It provides full data integrity (read and write). The extent in a volume, no other I/O activity from the same task or
serialization is done at record level. same data set is scheduled to the other volumes. In a situation
VSAM RLS does not introduce new types of VSAM where I/O is the major bottleneck, and there are available
clusters; rather, it is just a new way of accessing resources in the channel subsystem and controllers, it is

3. a waste of these resources.
Data striping addresses this sequential access performance
problem by adding two modifications to the traditional data
organization:
The records are not placed in key ranges along the
volumes; instead they are organized in stripes.
Parallel I/O operations are scheduled to sequential stripes
(CIs) in different volumes
The enhancements for data striping are:
Data striping for VSAM LDS through a small program
enhancement (SPE)
Data striping and multi-layering for all VSAM data set
VSAM data striping support for data sets with REUSE
attribute
Problem Determination enhancements
Here we have the features that will decrease the mean time
to repair by making the problem determination more
efficient:.
Enhancements in VSAM problem determination:
VDUMP option in F CATALOG command
Trap for data component and Trace enhancements
Dynamic trace for record management