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

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► Non-CICS jobs can have read-only access to recoverable data sets concurrently with<br />

CICS jobs, which can have read or write access.<br />

Coupling Facility overview<br />

The Coupling Facility (CF) is a shareable storage medium. It is licensed internal code (LIC)<br />

running in a special type <strong>of</strong> PR/SM logical partition (LPAR) in certain zSeries and S/390<br />

processors. It can be shared by the systems in one sysplex only. A CF makes data sharing<br />

possible by allowing data to be accessed throughout a sysplex with assurance that the data<br />

will not be corrupted and that the data will be consistent among all sharing users.<br />

VSAM RLS uses a Coupling Facility to perform data-set-level locking, record locking, and<br />

data caching. VSAM RLS uses the conditional write and cross-invalidate functions <strong>of</strong> the<br />

Coupling Facility cache structure, thereby avoiding the need for control interval (CI) level<br />

locking.<br />

VSAM RLS uses the Coupling Facility caches as store-through caches. When a control<br />

interval <strong>of</strong> data is written, it is written to both the Coupling Facility cache and the direct access<br />

storage device (DASD). This ensures that problems occurring with a Coupling Facility cache<br />

do not result in the loss <strong>of</strong> VSAM data.<br />

Supported data set types<br />

VSAM RLS supports access to these types <strong>of</strong> data sets:<br />

► Key-sequenced data set (KSDS)<br />

► Entry-sequenced data set (ESDS)<br />

► Relative-record data set (RRDS)<br />

► Variable-length relative-record data set cluster (VRRDS)<br />

VSAM RLS also supports access to a data set through an alternate index, but it does not<br />

support opening an alternate index directly in RLS mode. Also, VSAM RLS does not support<br />

access through an alternate index to data stored under z/<strong>OS</strong> UNIX <strong>System</strong> Services.<br />

Extended format, extended addressability, and spanned data sets are supported with VSAM<br />

RLS. Compression is also supported.<br />

VSAM RLS does not support:<br />

► Linear data sets (LDS)<br />

► Keyrange data sets<br />

► KSDS with an imbedded index (defined with IMBED option)<br />

► Temporary data sets<br />

► Striped data sets<br />

► Catalogs and VVDSs<br />

Keyrange data sets and the IMBED attribute for a KSDS are obsolete. You cannot define new<br />

data sets as keyrange or with an imbedded index anymore. However, there still might be old<br />

data sets with these attributes in your installation.<br />

386 <strong>ABCs</strong> <strong>of</strong> z/<strong>OS</strong> <strong>System</strong> <strong>Programming</strong> <strong>Volume</strong> 3

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