Using Hiperspace buffers

VSAM Hiperspace™ buffers reside in MVS™ expanded storage. These buffers are backed only by expanded storage. If the system determines that a particular page of this expanded storage is to be used for another purpose, the current page’s contents are discarded rather than paged-out. If VSAM subsequently requires this page, it retrieves the data from DASD. VSAM manages the transfer of data between its Hiperspace buffers and its CICS® address space buffers. CICS file control can only work with VSAM data when it is in a CICS address space buffer. Data is transferred between Hiperspace buffers and address space buffers in blocks of pages using CREAD and CWRITE commands. See LSRpool: Hiperspace data buffer statistics for more information.

Effects

The use of a very large number of Hiperspace buffers can reduce both physical I/O and pathlength when accessing your CICS files because the chance of finding the required records already in storage is relatively high.

Limitations

Because the amount of expanded storage is limited, it is possible that the installation will overcommit its use and VSAM may be unable to allocate all of the Hiperspace buffers requested. MVS may use expanded storage pages for purposes other than those allocated to VSAM Hiperspace buffers. In this case CICS continues processing using whatever buffers are available.

If address space buffers are similarly overallocated then the system would have to page. This overallocation of address space buffers is likely to seriously degrade CICS performance whereas overallocation of Hiperspace buffers is not.

Hiperspace buffer contents are lost when an address space is swapped out. This causes increased I/O activity when the address is swapped in again. If you use Hiperspace buffers, you should consider making the CICS address space nonswappable.

Recommendations

Keeping data in memory is usually very effective in reducing the CPU costs provided adequate central and expanded storage is available. Using mostly Hiperspace rather than all address space buffers can be the most effective option especially in environments where there are more pressing demands for central storage than VSAM data.

How implemented

CICS never requests Hiperspace buffers as a result of its own resource calculations. You have to specify the size and number of virtual buffers and Hiperspace buffers that you need.

You can use the RDO parameters of HSDATA and HSINDEX, which are added to the LSRPOOL definition to specify Hiperspace buffers. Using this method you can adjust the balance between Hiperspace buffers and virtual buffers for your system.

For further details of the CEDA transaction, see the CICS Supplied Transactions.

Related tasks
VSAM and file control: improving performance
VSAM tuning: general objectives
Defining VSAM buffer allocations for NSR (INDEXBUFFERS and DATABUFFERS)
Defining VSAM buffer allocations for LSR
Defining VSAM string settings for NSR (STRINGS)
Defining VSAM string settings for LSR (STRINGS)
Specifying maximum keylength for LSR (KEYLENGTH and MAXKEYLENGTH)
Specifying resource percentile for LSR (SHARELIMIT)
Using VSAM local shared resources (LSR)
Permitting VSAM subtasking (SUBTSKS=1)
Using data tables to improve performance
Using coupling facility data tables to gain performance benefits
Performance aspects of VSAM record-level sharing (RLS)
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