KDA101@psuvm.psu.edu (KeithPetto Alexander) (04/09/90)
In article <90096.104817ESH101@psuvm.psu.edu>, Eric Hvozda <ESH101@psuvm.psu.edu> says: > >My question what is a cluster? Is is equated with the BLKSIZE on mainframes, >or is it some kind of I/O buffer size? A cluster is the smallest amout of space you can alocate to a file. On the mainframe here at PSU the smallest amout of space is called Block size as you have noted, if a file gets larger than one block by even one byte, it takes up two blocks. Petto :-> "You weren't like that before the beard." -- Q Petto is still an AI program run amuck, his opinions are the result of a deranged program from a less deranged programmer. kda101@psuvm.psu.edu
PMW1@psuvm.psu.edu (Peter Weiss) (04/09/90)
In article <90098.164830KDA101@psuvm.psu.edu>, Keith "Petto" Alexander <KDA101@psuvm.psu.edu> says: > >In article <90096.104817ESH101@psuvm.psu.edu>, Eric Hvozda ><ESH101@psuvm.psu.edu> says: >> >>My question what is a cluster? Is is equated with the BLKSIZE on mainframes, >>or is it some kind of I/O buffer size? > >A cluster is the smallest amout of space you can alocate to a file. On the >mainframe here at PSU the smallest amout of space is called Block size as >you have noted, if a file gets larger than one block by even one byte, it >takes up two blocks. A BLKSIZE in the MVS world has no direct relationship to disk/tape geometry, but is often times used as the unit of physical data transfer in Queued non-VSAM access methods. (Typically, VSAM does it's physical I/O based on Cluster Index.) The basis of queued record processing is the LRECL, which populates the BLKSIZE. Nevertheless, chosing a _good_ BLKSIZE can improved the performance of your program (sometimes, though, at the expense of system thruput in a multitasking system). On a real 3380 DASD controlled by MVS, the minimum unit of allocation is the track (47,476 bytes). Though the Channel Command Word has 16 bits for a block length (65,535 bytes), I don't recall that the standard access methods support more than 15 bits (32,767 bytes). Consequently, based on the CKD geometry, it would appear that a maximum BLKSIZE of 23,476 bytes, would give a 98.9% utilization of the track (Source - IBM 3380 Direct Access Storage: Reference Card form GX25-1678-4 (October, 1987)) assuming non-keyed data. I don't know what happens to track utilization when you start emulating another device type on a foreign geometry. -- Peter M. Weiss | (this line intentionally left blank) 31 Shields Bldg (the AIS people) | advertize here, reach Mega populi University Park, PA USA 16802 | Disclaimer -* +* applies herein