The real question is, with today's disk arrays, what really is the optimal order of the CCWs in the chain? What may seem logical to you, who have apparently been around long enough to remember the days of each disk being a physical unit with cylinders and tracks being arranged sequentially, may not be optimal for disks that are striped across many physical disks.
The VSSI products, VPARS and VTAPE, use the BLOCKIO routines and, if I am not mistaken, the VSSI code "optimizes" the channel programs by sorting them into sequential order before the DIAG is issued. Regards, Richard Schuh > -----Original Message----- > From: The IBM z/VM Operating System > [mailto:[email protected]] On Behalf Of Gary M. Dennis > Sent: Monday, March 30, 2009 7:39 AM > To: [email protected] > Subject: Diagnose x'250' / z/VM I/O scheduling > > Given > > A file pool consisting of > 5 VDEV files on 5 separate real devices > 2 cylinders per device > 4096 block size > > > When: > > a block chain is given Diag x'250' (async) for either > read or write > such that 4 blocks are written to or read from each track within > the 5 files. > > Question(s): > > Does the 250 interface make any attempt to optimize I/O operations > by constructing chained channel programs for single-track or > consecutive-track multi-record writes/reads? > > If that is not the case, is such optimization achieved at > a more basic > level in z/VM real device I/O scheduling? > > Curiosity killed the.... > > In either of the above cases (that is if channel programs > are chained > based on intra-request I/O patterns), will either 250 or > VM perform > inter-request channel program chaining for multiple async requests > targeting the same real device? > > Thanks > > --. .- .-. -.-- > > Gary Dennis > > 0 ...living between the zeroes... 0 >
