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

4.44 VSAM: Buffering modes
User
ACB
MACRF=
(LSR,NUB)
INDEX
Data
INDEX
User ACB
MACRF=
(LSR,NUB)
Data
User ACB INDEX
MACRF=
(LSR,NUB)
Buffers and I/O related control blocks
associated with a pool (BLDVRP)
Multiple data sets share pooled resources
Figure 4-54 VSAM LSR buffering mode
Data
VSAM buffering modes
The VSAM buffering modes that you can use are:
► NSR, LSR, GSR, and RLS
Buffers and I/O related
control blocks
Non-shared resource (NSR)
Non-shared resource (NSR) is the default VSAM buffering technique. It has the following
characteristics:
► The resource pool is implicitly constructed at data set open time.
► The buffers are not shared among VSAM data sets; only one cluster has CIs in this
resource pool.
► Buffers are located in the private area.
► For sequential reads, VSAM uses the read-ahead function: when the application finishes
processing half the buffers, VSAM schedules an I/O operation for that half of the buffers.
This continues until a CA boundary is encountered; the application must wait until the last
I/O to the CA is done before proceeding to the next CA. The I/O operations are always
scheduled within CA boundaries.
► For sequential writes, VSAM postpones the writes to DASD until half the buffers are filled
by the application. Then VSAM schedules an I/O operation to write that half of the buffers
to DASD. The I/O operations are always scheduled within CA boundaries.
Chapter 4. Storage management software 177