>> Great overview, but is the note above still true with modern DS8000 boxes?  
>> It's just hard for me to imagine 3390 emulation logic holding that 23K 
>> hostage.

Tom,

As other have mentioned on z/OS it is still valid.  Run this JCL and look at 
the allocation . I used the example of LRECL=500 and optimal blocksize =27500 
and max blocksize could be 32500

//STEP0100 EXEC PGM=SORT
//SYSOUT   DD SYSOUT=*
//SORTIN   DD *
ABC
//B27500   DD DSN=&SYSUID..CREATE.BLKSIZE.OF27500,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(7,2),RLSE)
//B32500   DD DSN=&SYSUID..CREATE.BLKSIZE.OF32500,
//            DISP=(NEW,CATLG,DELETE),
//            SPACE=(CYL,(7,2),RLSE),BLKSIZE=32500
//SYSIN    DD *
  OPTION COPY
  OUTFIL FNAMES=B27500,REPEAT=10000,BUILD=(500C'A')
  OUTFIL FNAMES=B32500,REPEAT=10000,BUILD=(500C'B')
/*


The result is

-------------------------------------------------------------------------------
Command - Enter "/" to select action                        Tracks %Used   XT
-------------------------------------------------------------------------------
         KOLUSU.CREATE.BLKSIZE.OF27500                         105   86     1
         KOLUSU.CREATE.BLKSIZE.OF32500                         165   93     3

Thanks,
Kolusu


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