On Sun, 15 Feb 2015 16:59:43 -0600, Barry Merrill wrote: >Except that SMF does not write normal VBS data. > >This is from Chapter 3 in my 1984 Book, member ACHAP03 in MXG Software: > >SMF has always used a RECFM=VBS, i.e., Variable, Blocked, Spanned record >format. This is because the original OS/360 implementation would only >give 1000 bytes (of its 88K nucleus!) for SMF data, and the only way you >can write long logical records with a small physical block size records >is to use VBS! > >However, the SMF Writer does not use normal VBS; in normal VBS, all data >records are spanned, so that all blocks are full, but that is not what >the SMF writer does in its "pseudo-VBS" architecture. Instead of >spanning all records, SMF records are only spanned when the LRECL of the >record to be written is larger than the BLKSIZE of the dataset. ... > This seems to me to be legal VBS, not "pseudo-VBS".
Without re-reading Using Data Sets, I'm confident that there is *no* requirement on either VB or VBS that all blocks be written to BLKSIZE maximum. In fact, I believe that for RECFM=VBS, VB, or FB, DISP=MOD can cause a record to be written starting a new block which record could have been written at the end of a short block resulting from the previous operation. (RECFM=FBS is the obvious exception.) Is the null segment provided for RECFM=VBS so all blocks can be written with uniform length? The only other rationale I can think of is to avoid unintended noise records on tape. -- gil ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
