zHyperLink I/O

VSAM data set performance can benefit from using the IBM DS8000x zHyperLink technology, also known as synchronous I/O. zHyperLink can achieve significant I/O latency improvements for read requests when using nonlinear VSAM data sets. When zHyperLink is enabled on the system and a control interval (CI) of data is in the control unit cache, a VSAM read request is performed synchronously, which results in significantly reduced latency when compared to traditional DASD I/O. VSAM and VSAM RLS users monitor the tradeoff, including the SMF42 subtypes 5 and 6 counts that are related to zHyperLink ('syncIO').

zHyperLink I/O is disabled system-wide by default. To use zHyperLink I/O for read requests, all of the following conditions must be met for VSAM and VSAM RLS to issue a zHyperLink I/O request:
  • The user issued the SETIOS ZHYPERLINK OPER=[ALL|READ] command or the parmlib member SYS1.PARMLIB(IECIOSxx) specifies ZHYPERLINK,OPER=[ALL|READ].
  • The parmlib member SYS1.PARMLIB(IGDSMSxx) or the SETSMS command specifies VSAM_ZHYPERLINK(YES).
  • The data sets must be set to be zHyperLink eligible. This is done by specifying zHyperLink Eligible for Read: YES in the storage class for the data sets. Alternatively, V SMS,DSNAME(datasetname),ZHLREAD=YES can be issued to make read I/O for the specified data set synchronous, overriding the specification in the storage class for that data set. Default is not to override the storage class specification.
  • The RLS or VSAM-built buffer, or the VSAM user-provided buffer (such as VSAM user buffering) that receives the CI must on a quadword boundary.
  • For VSAM access, there are the additional requirements that the CI does not cross a track boundary and the request is not processing sequentially with NSR buffering.

When all of these conditions are met and VSAM or VSAM RLS initiates a zHyperLink I/O request, the I/O might still be performed asynchronously. Start of changeThis might be because the CI is larger than the size specified in the current READLIMIT setting for zHyperLink, or the CI is not found in the cache of the control unit. For more information about READLIMIT, see ZHYPERLINK in z/OS MVS Initialization and Tuning Reference.End of change

VSAM RLS accumulates the suspend elapsed time for the asynchronous DASD I/O for the data set. The counts are recorded in SMF42 subtype 6 fields S42SNAvgARDela S42SNARDelayCount to help the users understand the potential performance benefits of enabling zHyperLink reads for VSAM RLS data sets. Non-RLS VSAM does not do such counts.