Answer / guest

JES3 allocates datasets for all the steps before the job is
scheduled. In JES2, allocation of datasets required by a
step are done only just before the step executes.

Answer / bala

JES3 = Job Entry Subsystem 3

An MVS subsystem that receives jobs into the
system,converts them to internal format, selects them for
execution, processes their output, and purges them from the
system. In complexes that have several loosely coupled
processing units, the JES3 program manages processors so
that the global processor exercises centralized control
over the local processors and distributes jobs to them via
a common job queue.


JES2 = Job Entry Subsystem 2

An MVS subsystem that receives jobs into the system,
converts them to internal format, selects them for
execution, processes their output, and purges them from the
system. In an installation with more than one processor,
each processor's JES2 subsystem independently controls job
input, scheduling, and output processing.

Answer / shailendra

i want to add something

jes2 : it allocate resource as per step execute of the job.
jes3 : it aloocate resource before the job excution

Answer / echo

Some principle differences between the two JES systems
include:
In a mainframe installation that has only one processor,
JES3 provides tape setup, dependent job control, and
deadline scheduling for users of the system, while JES2 in
the same system would require its users to manage these
activities through other means. In an installation with a
multiprocessor configuration, there are noticeable
differences between the two, mainly in how JES2 exercises
independent control over its job processing functions. That
is, within the configuration, each JES2 processor controls
its own job input, job scheduling, and job output
processing.
In cases where multiple z/OS® systems are clustered (a
sysplex), it is possible to configure JES2 to share spool
and checkpoint data sets with other JES2 systems in the
same sysplex. This configuration is called Multi-Access
Spool (MAS). In contrast, JES3 exercises centralized
control over its processing functions through a single
global JES3 processor. This global processor provides all
job selection, scheduling, and device allocation functions
for all of the other JES3 systems.
With JES3, installations may decide whether the global JES3
or z/OS base control program will handle device allocation.
With JES2, only the z/OS base control program handles
device allocation.

Source:
http://publib.boulder.ibm.com/infocenter/zos/basics/index.js
p?topic=/com.ibm.zos.zconcepts/zconcepts_154.htm

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