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VSAM for ever, Alvaro Salla, CMG Brasil 2011

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1. VSAM NEW (NOT SO) FEATURES – FROM DFSMS 1.3 TO 1.12 Journal of Computer Resource Management Prepared by the CMG Publications Committee Journal of Computer Resource…
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  • 1. VSAM NEW (NOT SO) FEATURES – FROM DFSMS 1.3 TO 1.12 Journal of Computer Resource Management Prepared by the CMG Publications Committee Journal of Computer Resource Management, a publication of CMG August, 2011 – Alvaro Guimaraes Salla This article describes briefly the new features of Virtual Sequential Access Method (VSAM) a component of DFSMS. It encompasses the most important VSAM enhancements from DFSMS 1.3 to DFSMS 1.12. The sequencing of the items is from the most important to the least one, in the author´s opinion.In the early 1970s, Virtual Sequential Access Method(VSAM) was introduced by IBM as a collection of three ENHANCEMENTS DESCRIPTIONdata set organizations — sequential, indexed, anddirect-access, together with the access method Follows the description of the VSAM enhancements.software and utilities to be used on the mainframe IBMoperating systems. General functionsThe word virtual means only that VSAM wasintroduced at approximately the same time as the Empty CA reclaim. Orphan data control areas areinitial IBM virtual storage operating systems OS/VS1 control areas with lots of free space (or totally free)and OS/VS2. Since then VSAM has been continually located below the HURBA mark. They orphans areimproved and enhanced. created through the following scenario: if at load time an specific data CA was loaded with records. belonging to a specific key range and later on the majority of these records are deleted (and the sameVSAM, AN ACCESS METHOD OF z/OS key value is not re-utilized), as the example of a timestamp key, the CA is under populated, and this free space is never reclaimed for records with keysVSAM is a z/OS access method, that is, a set of out of such specific and original key range. This set ofprograms written by IBM to help application code to keys (like the timestamp ones) are called creepingaccess data through I/O operations. keys, the ones that continues to grow in its numericThe access method runs under the application task value. Traditionally, VSAM was not designed tothat invokes it using a branch instruction (no PSW process well creeping keys. The solution is to addchange status, such as, problem mode, PSW Key)). these empty CAs to a free CA list (that originally contains the ones above HURBA) when VSAMVSAM Functionality realizes the last original record is deleted. This function applies only to KSDS clusters and to ICFThe major access method functions are: catalogs. It is important to know that for an existing Blocking by clustering logical records in one VSAM data set, by just enabling CA reclaim, does not physical block reclaim the previous internal CAs totally free. Only the Buffering by managing the buffer pool ones that will be totally freed in the future, will be efficiently reclaimed. This function will even decrease the need An access method understands the way of defrags. logical records are organized on the 3390 VSAM Partial Release on multiple volumes data set. tracks. Then, it is in charge of writing virtual Partial release was only available for sequential data sets. Later was made available only for VSAM, but channel programs for accessing such records. for single volume data sets. Invoking the I/O driver through an SVC Ignore DEFINE parameters IMBED, REPLICATE, (usually SVC 00 – also named EXCP) KEYRANGE, ORDERED options. Synchronize through the Wait function, the Health Check to detect and identify catalogs with running task with the end of the I/O operation IMBED and REPLICATE. (if needed). DFSMS Data Set Separation, to allocate data sets in Execute I/O error recovery. distinct physical DASD controllers.
  • 2. Media Manager the unique VSAM I/O driver, existing data sets. Through data sharing implemented by exception for Improved Control Interval Processing the use of coupling facility structures VSAM RLS offers a (ICIP) high level of continuous availability. Follows a list of RLS enhancements:Extending the capacity of VSAM clusters Use of RLS coupling facility cache structure for storing data sets with CI size bigger than 4KB.All the following enhancements extends the capacity of Use of RLS coupling facility cache structure for all dataVSAM clusters. or some accordingly with DB manager option Extended addressability for VSAM KSDS. It increases RLS system-managed duplexing rebuild process for RBA from 4-byte to 8-byte allowing VSAM cache structures. components cross the 4-GB limit capacity. For the RLS Coupling Facility structure system-managed majority of the applications this is done with total duplexing. compatibility. Extended addressability when Data buffer pools may be allocated above the bar announced was not available for KSDS VSAM RLS DIAG command to find latch contentions organization. Enhanced RLS recovery. SMF performance data of VSAM RLS usage of 64-bit RLS support of extended addressability of VSAM data buffers KSDS. As stated at title, Extended Addressability is Speeding up the SMSVSAM address address space now available for KSDS organization running with with TERMINATESERVER command RLS buffering. Health Checks for VSAM RLS: indicating latch Extended addressability for all VSAM data sets contention and a single point of failure (SPOF) in the More than 255 extents for SMS-managed VSAM data SHCDS sets. The increase of the maximum number of Support for VSAM striping in RLS mode extents is a must to avoid frequent defrags. Follows a list of such limits: System Managed Buffering (SMB) enhancements o Sequential: 16 per volume. o Extended-format sequential:123. o PDS: 16, SMB is a function available to all VSAM data sets types o PDSE: 123. opened for NSR buffering processing. SMB enables o Non-system-managed VSAM up to 255 VSAM to determine the optimum number of index and per component. If System-managed limit data buffers, as well as the type of buffer management: removed if DC has extent constraint removal. LSR (LRU, lots of index buffers, lots of data buffers o Non-system-managed VSAM can have up containing up to 20% of all data records, no look-ahead) to 255 extents per stripe. If System- or NSR (discard the ones already processed, just one managed VSAM the limit removed if DC index buffer, look-ahead). SMB boost the performance has extent constraint removal specified. mainly of random NSR accesses. The late o An HFS can have 123 extents on each enhancements are: volume Extended Address Volume (EAV) for VSAM data sets SMB initially for VSAM KSDS clusters. allowing the allocation of such data sets at cylinders SMB retry capability for Direct Option (DO) access bias. beyond 64K addressing at EAV volumes. SMB support for AIX® files. Maximum Volume Count, allows volumes be added Limit the SMB buffer size for Direct Option (DO) at data dynamically to a VSAM data set without manual Class SMS construct. It avoids to have up to 20% of intervention and taking down the application. all data in data buffers. Real addresses greater than 2 GB available for all Enhancement in SMB for DO: buffers for index VSAM data sets. Allowing pages above the bar allocated based on SMBVSP value or data set size. being located at frames above 2-GB. Data Stripping enhancementsVSAM Data Sharing (RLS) Usually, in a multi-extent, multi-volume VSAM data setVSAM RLS is another mode of managing buffer pools, processed in sequential access, processing does not allow forwhich allows any number of applications within your any type of parallelism for I/O operations among the volumes.Parallel Sysplex to share your existing VSAM clusters. This means that when an I/O operation is executed for anIt provides full data integrity (read and write). The extent in a volume, no other I/O activity from the same task orserialization is done at record level. same data set is scheduled to the other volumes. In a situationVSAM RLS does not introduce new types of VSAM where I/O is the major bottleneck, and there are availableclusters; rather, it is just a new way of accessing resources in the channel subsystem and controllers, it is
  • 3. a waste of these resources.Data striping addresses this sequential access performanceproblem by adding two modifications to the traditional dataorganization: The records are not placed in key ranges along thevolumes; instead they are organized in stripes. Parallel I/O operations are scheduled to sequential stripes(CIs) in different volumesThe enhancements for data striping are: Data striping for VSAM LDS through a small program enhancement (SPE) Data striping and multi-layering for all VSAM data set VSAM data striping support for data sets with REUSE attributeProblem Determination enhancementsHere we have the features that will decrease the mean timeto repair by making the problem determination moreefficient:. Enhancements in VSAM problem determination: VDUMP option in F CATALOG command Trap for data component and Trace enhancements Dynamic trace for record management
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