ABCs of z/OS System Programming Volume 3 - IBM Redbooks

7.8 Buffering under VSAM RLS
CICS
R/W
CICS
R/W
Batch
R/O
System 1
MACRF=RLS
VSAM RLS
aka
SMSVSAM
SMSVSAM
data space
Figure 7-8 Buffering under VSAM RLS
New VSAM buffering technique MACRF=RLS
We have already discussed NSR, LSR, and GSR (refer to 7.2, “Base VSAM buffering” on
page 380). RLS is another method of buffering which you can specify in the MACRF
parameter of the ACB macro. RLS and NSR/LSR/GSR are mutually exclusive.
Bufferpools in the data space
Unlike NSR, LSR, or GSR, the VSAM buffers reside in a data space and not in private or
global storage of a user address space. Each image in the sysplex has one large local buffer
pool in the data space.
The first request for a record after data set open for RLS processing will cause an I/O
operation to read in the CI that contains this record. A copy of the CI is stored into the cache
structure of the Coupling Facility and in the buffer pool in the data space.
Buffer coherency
Buffer coherency is maintained through the use of Coupling Facility (CF) cache structures
and the XCF cross-invalidation function. For the example in Figure 7-8, that means:
1. System 1 opens the VSAM data set for read/write processing.
2. System 1 reads in CI1 and CI3 from DASD; both CIs are stored in the cache structure in
the Coupling Facility.
3. System 2 opens the data set for read processing.
388 ABCs of z/OS System Programming Volume 3
coupling facility
VSAM data set
VSAM RLS
aka
SMSVSAM
1 3 1 4
1 3 4
1 2 3 4
SMSVSAM
data space
System 2
CICS
R/W
CICS
R/W
Batch
R/O